Non-Invasive Techniques for Oral PMDs Detection and Diagnosis165Tohoku J. Exp. Med., 2016, 238, 165-177
Received December 8, 2015; revised and accepted January 18, 2016.    Published online February 16, 2016; doi: 10.1620/tjem.238.165.Correspondence: Hongxia Dan, Ph.D., DMD, State Key Laboratory of Oral Diseases, West China College of Stomatology, Sichuan Uni-versity, No. 14, Section 3, Renmin South Road, Chengdu, Sichuan 610041, China.e-mail: hxdan@foxmail.com
Review
Non-Invasive Techniques for Detection and Diagnosis of Oral Potentially Malignant Disorders
Dongjuan Liu,1 Xin Zhao,1 Xin Zeng,1 Hongxia Dan1,* and Qianming Chen1
1State Key Laboratory of Oral Diseases, West China College of Stomatology, Sichuan University, Chengdu, Sichuan, China
Oral squamous cell carcinoma (OSCC) is the most common oral and maxillofacial malignancy, and its morbidity and mortality rates are still high in most countries.  Oral potentially malignant disorders (PMDs) are used to refer to a heterogeneous group of conditions that are characterized by increased risk for malignant transformation to OSCC.  Currently identified oral PMDs include leukoplakia, erythroplakia, palatal lesions associated with reverse smoking, oral lichen planus, oral submucous fibrosis, actinic keratosis, and discoid lupus erythematosus.  The early detection and diagnosis of these lesions are important for cancer prevention and disease management.  In recent years, there has been a growing and persistent demand for new non-invasive, practical diagnostic techniques that might facilitate the early detection of oral PMDs.  The non-invasive detection techniques evaluated in this review are divided into four categories: vital staining with a solution that can be used as a mouth rinse or applied onto a suspected area of the mouth, light-based detection systems, optical diagnostic technologies that employ returned optical signals to reflect structural and morphological changes within tissues, and salivary biomarkers.  Most of these techniques have shown great potential for screening and monitoring oral PMDs.  In this review article, the authors critically assess these non-invasive detection techniques for oral PMDs.  We also provide a summary of the sensitivity and specificity of each technique in detecting oral PMDs and oral cancer, as well as their advantages, disadvantages, clinical applications, and indications.
Keywords: early diagnosis; non-invasive detection technique; oral potentially malignant disorders; sensitivity; specificityTohoku J. Exp. Med., 2016 February, 238 (2), 165-177. © 2016 Tohoku University Medical Press
IntroductionOral squamous cell carcinoma (OSCC) is the most common oral and maxillofacial malignancy (Lambert et al. 2011).  Currently, the morbidity and mortality rates of OSCC are still high in most countries.  The reported 5-year survival rate is around 50-63% (Mehrotra et al. 2012; Messadi 2013).  OSCC is usually preceded by premalignant lesions (Warnakulasuriya et al. 2007).  In 2005, the World Health Organization recommended the use of the term oral “potentially malignant disorders” (PMDs) instead of pre-cancerous lesions/disorders.  However, a definition of oral PMDs had not been proposed at that time.  Sarode et al. (2012) attempted to propose a definition for oral PMDs; namely, it is a group of disorders of varying etiologies, usu-ally tobacco, characterized by mutagen associated, sponta-neous or hereditary alterations or mutations in the genetic material of oral epithelial cells with or without clinical and histomorphological alterations that may lead to oral squa-
mous cell carcinoma transformation.  Currently identified oral PMDs include leukoplakia, erythroplakia, palatal lesions associated with reverse smoking, oral lichen planus, oral submucous fibrosis, actinic keratosis and discoid lupus erythematosus (Table 1) (Warnakulasuriya et al. 2007).  These lesions have greater potential for malignant transfor-mation than other oral lesions.  The early detection and diagnosis of oral PMDs allow clinicians to monitor and treat oral carcinogenesis at intraepithelial stages, including mild, moderate, and severe dysplasia, as well as carcinoma in situ.  This is crucial for improving the survival rate and reducing morbidity, disfigurement, function loss, treatment duration, and hospital costs of OSCC patients.Conventional oral examinations, including visual inspection and palpation, are the routine methods for the screening of oral lesions.  However, subtle lesions may pass undetected, and it is difficult to make a distinction among benign, premalignant, and malignant lesions.  It has been reported that dysplasia or micro-invasive carcinoma can 
D. Liu et al.166
occur in clinically normal-appearing mucosa (Thomson 2002).Currently, oral biopsy with histological assessment remains the gold standard for the diagnosis of oral PMDs.  Nevertheless, as it is invasive, some patients may not accept this test, especially when the lesion appears “normal”.  Moreover, subjectivity and observer variability (intra- and interobserver variability) are very common in the histologic diagnosis of oral PMDs and oral cancer (Karabulut et al. 1995; Brothwell et al. 2003; Fischer et al. 2004).  The results could be affected by the size and depth of the biopsy, the quality of the specimen, fixation and freezing tech-niques, and the pathologist’s experience (Seoane et al. 2002, 2004; Mercadante et al. 2012).Accordingly, non-invasive detection techniques for oral PMDs are needed.  In the last few decades, wide ranges of non-invasive techniques have been developed for the detection of oral PMDs.  However, it is difficult for the cli-nicians to decide which diagnostic tool is most appropriate and useful for oral PMD screening.  The aim of this review is to critically assess these non-invasive techniques and to present the available evidence concerning their efficacy in the early detection and diagnosis of oral PMDs.In this review, the non-invasive detection techniques employed for oral PMD detection are divided into four cat-egories: vital staining with the solution that can be used as a mouth rinse or applied directly onto a suspected area, light-based detection systems, optical diagnostic technologies that employ returned optical signals to reflect structural and morphological changes within tissues, and salivary bio-markers (Table 2).
Vital stainingVital staining is usually simple, cheap, sensitive, and efficient.  It can be used and interpreted by clinicians by the chair.  Vital staining can enhance lesion characteristics, identify satellite or clinically non-apparent lesion sites, and assist in the choice of site and the timing of a biopsy.
Toluidine blue stainingToluidine blue (TB) is a basic thiazine metachromatic dye with a high affinity for acidic tissue components; there-fore, it stains tissues rich in nucleic acids.  Dysplastic and neoplastic cells contain more nucleic acids than normal cells.  Moreover, malignant epithelium may contain intra-cellular canals that are wider than those of normal epithe-lium, facilitating penetration of the dye (Epstein et al. 1992).  TB-stained tissue may appear dark or pale royal blue.  Dark royal blue stain is considered positive (Fig. 1), pale royal blue stain is doubtful, and when no color is observed it is interpreted as negative.  It is still controver-sial whether pale blue staining should be considered as pos-itive.  Several studies suggested that only dark royal blue staining should be regarded as positive (Gandolfo et al. 2006; Pallagatti et al. 2013).  Digital color analysis may be helpful to observers in the color interpretation of TB-stained oral lesions, especially when the blue tone is intermediate (Guneri et al. 2013).TB staining has been used for many years as an aid for the screening and post-surgical management of oral PMDs and oral cancer (Epstein et al. 2007; Patton et al. 2008; Epstein and Guneri 2009; Awan et al. 2012).  It has been claimed that TB staining has a higher detection rate of oral PMDs and could further reduce the incidence of oral cancer compared to conventional visual inspection (Su et al. 2010). However, TB staining can yield a high percentage of false positive results.  Some benign hyperplasia and inflamma-tory lesions can be stained, as they contain large quantities of nucleic acids (Driemel et al. 2007).  Traumatic benign lesions can also be stained; however, they differ from malignant lesions in that the coloration often surrounds the ulceration and does not last long.  In order to reduce the false positive rate, further assessment of a non-healing wound after at least 15 days is highly recommended (Mercadante et al. 2012).Most studies of TB staining had significant limitations and biases (Lingen et al. 2008): (1) very few were random-ized controlled trials; (2) histological diagnosis was not 
Table 1.  Oral potentially malignant disorders and their clinical features.
LesionClinical features
LeukoplakiaWhite plaque of questionable risk having excluded (other) known diseases or disorders that carry no increased risk for cancer (Warnakulasuriya et al. 2007).
ErythroplakiaA fiery red patch that cannot be characterized clinically or pathologically as any other definable disease (Warnakulasuriya et al. 2007).
Palatal lesions associated with reverse smokingRed, white, or mixed lesions of the palate in patients who smoke with the lighted end of the tobacco (Warnakulasuriya et al. 2007).
Oral lichen planusWhite striations, white papules, white plaques, erythema, erosions, or blisters affecting predominantly the buccal mucosa, tongue and gingivae (Sugerman and Savage 2002).
Oral submucous fibrosisBlanching and stiffening of the oral mucosa leading to limitation in opening of the mouth with fibrous bands in different sites of the mouth (Warnakulasuriya et al. 2007).
Actinic keratosisSolar keratosis of the lip secondary to sun exposure with dry and whitish gray scaly ero-sions on the lip mucosa (Zide 2008).
Discoid lupus erythematosusWhite papules, central erythema, a border zone of irradiating white striae and peripheral telangiectasia (Schiodt and Pindborg 1984).
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis167
used as gold standard; and (3) there was variability in the staining techniques and color interpretation.
Methylene blue stainingMethylene blue was first used in detecting oral mucosa lesions in 2007 (Chen et al. 2007a, b).  Similarly to TB, methylene blue also stains tissue with large quantities of nucleic acids (Riaz et al. 2013).  Methylene blue staining is useful for screening oral cancer in high-risk individuals, and shows high sensitivity in detecting oral PMDs (see Table 1). 
Rose bengal stainingRose bengal (RB) is the 4,5,6,7-tetrachloro-2′,4′,5′,7′-tetraiodo-derivative of fluorescein.  RB staining is widely used to diagnose ocular surface disorders (Rodrigues et al. 2009; Murube 2014).  It stains desquamated ocular epithe-lial cells, dead or degenerated cells, but not healthy epithe-lial cells (Mittal et al. 2012).  Two studies demonstrated that RB staining may be a valuable diagnostic technique in the detection of oral PMDs and oral cancer (Du et al. 2007; Mittal et al. 2012).  In one study, authors investigated the efficacy of RB staining in the detection of oral PMDs and 
Table 2.  Non-invasive techniques for the clinical detection and diagnosis of oral PMDs and oral cancer.
CategoriesTechniquesSensitivity (%)Specificity (%)Citations
Vital stainingToluidine blue staining 38-1009-100(Warnakulasuriya and Johnson 1996; Epstein et al. 1997; Ram and Siar 2005; Epstein et al. 2008; Lingen et al. 2008; Patton et al. 2008; Balevi 2011; Awan et al. 2012; Mojsa et al. 2012; Rajmohan et al. 2012)
Methylene blue staining  90-91.466.6-69(Chen et al. 2007a; Chen et al. 2007b; Riaz et al. 2013)
Rose bengal staining 90-100 73.7-89.09(Du et al. 2007; Zhang et al. 2013)
Lugol’s iodine staining87.5-94.783.8-84.2(Epstein et al. 1992; Chaudhari et al. 2013)
Light-based detection systems
Chemilu-minescenceViziLite 71-100 0-84.6(Ram and Siar 2005; Farah and McCullough 2007;  Patton et al. 2008; Trullenque-Eriksson et al. 2009; Awan et al. 2011b; Rajmohan et al. 2012; Vashisht et al. 2014; Rashid and Warnakulasuriya 2015)
ViziLite Plus77.327.8(Sambandham et al. 2013)
Microlux/DL77.8-94.370.7-99.6(McIntosh et al. 2009; Ibrahim et al. 2014)
Orascoptic DKn/an/a(Patton et al. 2008)
VELscope 30-100 15-100(Poh et al. 2006; Balevi 2007; Patton et al. 2008;  Trullenque-Eriksson et al. 2009; Mehrotra et al. 2010; Moro et al. 2010; Awan et al. 2011a; Balevi 2011;  Fricain 2011; Lopez-Jornet and De la Mano-Espinosa 2011; Scheer et al. 2011; Farah et al. 2012; Marzouki et al. 2012; Bhatia et al. 2013)
Photodynamic diagnosis 79-10050-99(Leunig et al. 2000; Betz et al. 2002; Zheng et al. 2002; Chang and Wilder-Smith 2005; Sharwani et al. 2006a; Driemel et al. 2007; Sieron et al. 2008)
Optical diagnostic technologies
Raman spectroscopy97.44-100 77-100(Li et al. 2010; Guze et al. 2015)
Elastic scattering spectroscopy72-9868-75(Jerjes et al. 2004; Sharwani et al. 2006b; Green et al. 2014)
Diffuse reflectance spectroscopy 76-10076-97(Mallia et al. 2010; Jayanthi et al. 2011; Stephen et al. 2013)
Narrow-band imaging84.62-9688.2-100(Piazza et al. 2010; Yang et al. 2012b; Yang et al. 2013; Shibahara et al. 2014; Vu and Farah 2014; Yang et al. 2014)
Optical coherence tomography 73-10078-98(Matheny et al. 2004; Wilder-Smith et al. 2004; Wilder-Smith et al. 2005; Wilder-Smith et al. 2009; Olivo et al. 2011; Hamdoon et al. 2013; Green et al. 2014)
Confocal laser endomicroscopy80100(Nathan et al. 2014)
Confocal reflectance microscopy73 88(Olivo et al. 2011)
Salivary biomarkermiRNA-184n/an/a(Zahran et al. 2015)
IL-6n/an/a(Sharma et al. 2011)
D. Liu et al.168
oral cancer in 132 patients, and concluded that RB staining seemed promising for the detection of epithelial dysplasia in oral leukoplakia, lichen planus, and leukokeratosis (Du et al. 2007).  Zhang et al. (2013) combined fluorescence spectroscopic techniques with RB staining to detect oral PMDs in an animal model in vivo and reported that the method showed excellent sensitivity.  Recently, a rose-ben-gal-conjugated gold nanorod (RB-GNR) platform was developed for the optical detection of cancer cells (Wang et al. 2014).  Studies have demonstrated that the RB-GNR platform has positive significance in detecting cancer cells, but it has not yet been used in a clinical setting. 
Lugol’s iodine stainingThe mechanism of Lugol’s iodine staining is that iodine reacts with glycogen in the cytoplasm.  The reaction, known as the iodine-starch reaction, is visualized by a color change.  The loss of cellular differentiation and the enhanced glycolysis in cancer cells do not promote the iodine-starch reaction.  When it is applied on the suspicious lesions, normal mucosa stains brown or mahogany due to its high glycogen content, while dysplastic and cancer lesions do not stain, and appear pale compared with the sur-rounding tissue (Petruzzi et al. 2010).Chaudhari et al. (2013) conducted a study on male inmates, and suggested that Lugol’s iodine was highly effective as a screening tool for oral PMDs and oral cancer in inmate populations.  Lugol’s iodine staining during sur-
gery can determine surgical resection margins, reduce local recurrence and improve survival in patients with epithelial dysphasia or malignant lesions (Umeda et al. 2011; Kurita et al. 2012).  Maeda et al. (2010) applied colorimetric anal-ysis to analyze the color of unstained lesions in relation to the histopathologic findings, they made a color chart and proposed a possible histological diagnosis based on the measured colors of the unstained lesions.  Epstein et al. (1992) conducted a comparative study, in which TB and Lugol’s iodine were used both in combination and sepa-rately in 59 patients.  The sensitivity and specificity of TB were 0.925 and 0.632, respectively, while the sensitivity and specificity of Lugol’s iodine were 0.875 and 0.842, respectively.  When combined, the two stains showed a sen-sitivity of 0.850 and a specificity of 0.895.  The authors concluded that Lugol’s iodine had less sensitivity in identi-fying oral malignant and dysplastic lesions but it had greater specificity.  Moreover, in another study, 30 subjects with clinically suspicious oral PMDs and 30 subjects with clinically suspicious malignant lesions were stained consec-utively with TB and Lugol’s iodine, the overall sensitivity in detecting dysplasia or malignant lesions was 92.7% while the specificity was 60% (Nagaraju et al. 2010).  Most of the studies consulted found the Lugol’s iodine might have great potential for oral PMDs and oral cancer screen-ing.In a recent meta-analysis on the application of vital staining in detecting oral PMD and oral cancer (Macey et 
Fig. 1.  TB staining and VELscope images of an erythroplakia lesion.  (a) Erythroplakia lesion on the buccal mucosa of a 62-year-old man.  (b) Same lesion with TB staining.  TB was retained in some part of the lesion.  (c) Same lesion viewed through VELscope.  The arrows indicate areas with loss of fluorescence.  (d) The lesion was histologically diag-nosed as erythroplakia with severe dysplasia (H&E staining, 100×). 
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis169
al. 2015), 14 studies were included (12 using TB, one using TB and/or Lugol’s iodine and one using RB).  The esti-mated sensitivity was 0.84 (95% CI 0.74 to 0.90), and the specificity was 0.70 (0.59 to 0.79).
Light-based detection systemsA number of light-based detection systems have been developed for the identification of oral PMDs and oral can-cer at their earliest stage.  Mucosal tissues undergoing abnormal metabolic or structural changes have different absorbance and reflectance profiles when exposed to vari-ous forms of light sources, enabling the identification of oral mucosal abnormalities (Awan et al. 2011a).
ChemiluminescenceCommercially available chemiluminescence tech-niques include ViziLite, ViziLite Plus, Microlux/DL and Orascoptic DK.  The main difference between these tech-niques is that ViziLite and ViziLite Plus involve a single-use chemiluminescent stick, while Microlux/DL and Orascoptic DX provide a blue-white light-emitting diode (LED) fiber-optic light. The ViziLite kit contains a vial of 1% acetic acid solu-tion, a capsule, a retractor and the manufacturer’s instruc-tions (Sambandham et al. 2013).  The capsule has an outer shell of flexible plastic and an inner vial of fragile glass.  To activate it, the capsule is bent to break the glass vial, so that the chemical products react with each other and produce bluish-white light with a wave length of 430-580 nm that lasts for around 10 min.  The specific wavelength is absorbed by normal cells and reflected by abnormal cells that have a higher nuclear-cytoplasmic ratio.  The presence of an “ace to white” lesion after a one-minute rinse with 1% acetic acid solution is considered as positive.  The absence of such findings is considered as negative (Rajmohan et al. 2012).  ViziLite positive lesions are considered to be abnor-mal oral mucosa. ViziLite shows high sensitivity in detecting oral PMDs and oral cancer.  However, these studies often involved only patients with mucosal lesions that were previously visual-ized under conventional light (Patton et al. 2008).  Some studies found that ViziLite could not differentiate between keratotic or inflammatory oral PMDs and oral cancer (Farah and McCullough 2007; Ram and Siar 2005).  It preferen-tially detected leukoplakia but might fail to spot red patches (Epstein et al. 2006; Kerr et al. 2006; Awan et al. 2011b; Rashid and Warnakulasuriya 2015).  In short, the evidence supporting the use of ViziLite for the early detection of oral PMDs is actually quite insufficient.  Some studies com-pared the efficacies of ViziLite and TB staining for the diagnosis of oral PMDs and oral cancer, reporting that ViziLite was relatively more reliable compared to TB stain-ing (Rajmohan et al. 2012; Vashisht et al. 2014).The manufacturer of ViziLite is now marketing an updated product called ViziLite Plus.  The only difference between the Plus and the earlier version is that the latter 
contains a TB staining solution (Patton et al. 2008).  TB can further delineate ViziLite-positive lesions, thus improving the specificity (Rashid and Warnakulasuriya 2015).Microlux/DL shares the basic principles of ViziLite.  The oral cavity is examined with a battery-powered LED fiber-optic source that emits a blue-white light (Patton et al. 2008; Ibrahim et al. 2014).  Few studies have been pub-lished on its effectiveness in oral PMD diagnosis (McIntosh et al. 2009).  In a study of 599 tobacco users, using biopsy as the gold standard, the sensitivity and specificity of Microlux/DL in detecting oral PMDs were 100% and 32.4%, which indicated that it seemed useful for enhancing lesion visibility, but couldn’t discriminate between benign and malignant lesions (Ibrahim et al. 2014).  Microlux/DL is also a poor discriminator for inflammatory, traumatic, and malignant lesions (McIntosh et al. 2009).The Orascoptic DK system also requires an acetic acid rinse and a three-in-one, battery-operated, hand-held LED instrument to improve the visualization of oral lesions (Patton et al. 2008).  There is no published evidence regard-ing its utility in oral PMD detection.
VELscopeThe VELscope is a hand held device that can enhance the visibility of oral mucosal abnormalities by activating tissue autofluorescence.  Autofluorescence is due to the presence of endogenous fluorophores in cells, which pro-duce a fluorescent emission when exposed to light of a spe-cific wavelength (Farah et al. 2012).  Within the oral mucosa, the most relevant fluorophores are nicotinamide adenine dinucleotide and flavin adenine dinucleotide in the epithelium, and collagen cross-links in the stroma (Pavlova et al. 2008).  Mucosal abnormalities can alter the absorption and scattering properties of light as a result of changes in tissue architecture and concentrations of fluorophores (Bhatia et al. 2013).  At these excitation wavelengths, nor-mal cells show a pale green fluorescence when viewed through a filter, while abnormal cells show a loss of auto-fluorescence and appear dark (Fig. 1) (Balevi 2007).Some studies found that the VELscope can assist in screening for oral PMDs and oral cancer in populations at risk (Moro et al. 2010; Scheer et al. 2011).  However, there is no evidence that it can distinguish between them (Balevi 2007), and no published studies have assessed VELscope as a diagnostic adjunct in general populations.  In addition, several commonly occurring conditions, such as mucosal pigmentations, ulcerations, irritations, and gingivitis showed a loss of fluorescence under VELscope (Huber 2009).  Blood hemoglobin can also reduce fluorescence.  If a dark area appears during direct fluorescence visualization, the oral lesion must be considered as suspicious and clinical examination should be repeated by applying some pressure to remove blood from the target area.  If normal green fluo-rescence returns after this pressure, the lesion is likely to have an inflammatory component (Mercadante et al. 2012). Moreover, Kois and Truelove (2006) proposed that positive 
D. Liu et al.170
lesions need to be followed up with great caution; if they do not resolve within two weeks, further assessment and biopsy are generally recommended.  In short, the results should be interpreted with caution, bearing in mind the fre-quent occurrence of false positive results.
Photodynamic diagnosisPhotodynamic diagnosis (PDD) is based on the fluo-rescence generated by administration of an exogenous pho-toactivated compound that accumulates in cells with malig-nant potential, followed by appropriate photoirradiation (Uekusa et al. 2010).  One of the most promising photosen-sitizers for oral PMDs and oral cancer diagnosis is 5-ami-nolevulinic acid (ALA), which does not fluoresce itself, but can induce protoporphyrin IX (PPIX) fluorescence in tissue. PDD in the oral cavity can be performed simply by rinsing with a 0.4% ALA solution for 20 min (Zenk et al. 1999). Topical application of excessive 5-ALA stimulates the pro-duction and intracellular accumulation of highly fluorescent PPIX in dysplastic and cancerous tissues, which is excited by brief exposure to light with a wavelength of 405 nm (Chang and Wilder-Smith 2005).  Fluorescent tissues are considered suspicious for malignant transformation and biopsy should be considered (Driemel et al. 2007). A high sensitivity but limited specificity of PDD in the detection of oral PMDs and oral cancer has been shown in several studies (Leunig et al. 2000; Sharwani et al. 2006b). The false-positive rate is especially high in patients who have a history of radiotherapy (Zenk et al. 1999).  Given that various bacterial strains in the oral cavity can also pro-duce PPIX under incubation of 5-ALA (Zenk et al. 1999), professional hygiene measures must be employed before PDD in order to increase the specificity.  The need for an interval of about one hour between hygiene measures and the application of photosensitizing 5-ALA, as well as about three hours between the application of 5-ALA and the for-mation of a sufficient level of PPIX for fluorescence mea-surement, limits the use of this method in outpatient cancer prevention and aftercare.  Moreover, even low levels of glucose can influence PPIX production; thus, patients must fast during the four hours necessary for examination, which requires strict patient management. To improve the diagnostic specificity, a 5-ALA-mediated digitized fluorescence endoscopic imaging system has been built (Zheng et al. 2002).  When PPIX fluores-cence endoscopic images are quantified, the red-to-green and red-to-blue intensity ratios of dysplasia and malignancy are larger than those of benign tissues (Zheng et al. 2002; Bigio and Bown 2004).  Zheng et al. (2004) quantified the red-to-blue intensity ratio in the ALA-induced PPIX fluo-rescence endoscopic images, and showed that it could pro-vide good differentiation between benign dysplasia and cancer.  Wang et al. (2009) found that oral submucous fibro-sis mucosa had the lowest red-to-blue intensity ratio, and their results indicated that ALA-induced PPIX fluorescence spectroscopy could be used to identify oral submucous 
fibrosis.  Moreover, in a study of 71 patients with clinically suspicious oral leukoplakia, Sharwani et al. (2006b) found that ALA-induced PPIX fluorescence spectroscopy had great potential for identifying dysplastic changes at an early stage and might be useful as a screening tool for the detec-tion of subsequent primary lesions. In a recent meta-analysis on the application of light-based detection systems in detecting oral PMD and oral cancer, 11 studies were included.  The estimated sensitivity was 0.91 (95% CI 0.77 to 0.97) and the specificity was 0.58 (0.22 to 0.87) (Macey et al.  2015).
Optical diagnostic technologiesRaman spectroscopyRaman spectroscopy is a vibrational spectroscopic technique that relies on the inelastic scattering of light, usu-ally from a laser in the visible, near-infrared, or near-ultra-violet range.  The vibrational changes in tissue parallel the variations in chemical characterization and molecular struc-ture in the sample (Krishnakumar et al. 2013).  Raman spectroscopy performs vibrational spectroscopy of the tis-sue content, thus providing immediate real-time histology. Raman spectroscopy has been successful in differenti-ating normal tissue from premalignant and malignant tissue in a range of non-oral tumor types, including brain, breast, lower gastrointestinal tract, nasopharynx, skin, lung, and cervix (Guze et al. 2015).  In vivo Raman spectroscopy has shown efficacy in the detection of normal tissue, PMDs, cancer, and even of early changes such as cancer field-effects or malignancy-associated changes in the oral cavity (Singh et al. 2012; Krishna et al. 2014).  However, the clini-cal applications of Raman spectroscopy have been limited by both the difficulty of capturing inherently weak tissue Raman signals and the relatively slow speed of spectrum acquisitions (Guze et al. 2009).  Because of the technologi-cal limitations, research using Raman spectroscopy has mostly been confined to either ex vivo studies of tissue sam-ples or in vivo studies in animal models. 
Elastic scattering spectroscopyElastic scattering spectroscopy (ESS) is an emerging technique for generating a wavelength spectrum that reflects structural and morphological changes within tissues (Bigio and Mourant 1997; Green et al. 2014).  ESS has been shown to be sensitive to nuclear size, chromatin content, and nuclear/cytoplasmic ratio—all of which characteristics are of interest to the histopathologist who is screening for malignancy within tissue (Wallace and Van Dam 2000). ESS has rarely been used in studies of premalignant and malignant oral tissue.  Sharwani et al. (2006b) compared the findings of ESS with the histopathology of oral leuko-plakia in 25 patients, reporting a sensitivity of 72% and a specificity of 75%.
Diffuse reflectance spectroscopyIn diffuse reflectance spectroscopy (DRS), white light 
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis171
entering the tissue undergoes a combination of multiple elastic scattering and absorption.  The diffusely reflected emanating light is greatly affected by tissue morphology, such as nuclear size, distribution, epithelial thickness, colla-gen content, and the amount of oxy- and deoxy-hemoglobin in the exposed tissue, all of which can vary during carcino-genesis of epithelial tissue (Schwarz et al. 2008; Jayanthi et al. 2011).  A current DRS system typically consists of an imaging device for recording diffuse reflectance images and a fiber-optic probe for relaying light to and from the instru-ment (Yu et al. 2014). Some studies have showed that information about tis-sue transformation, obtained in vivo with the aid of the oxy-genated hemoglobin spectral ratio (R545/R575) algorithm, was able to differentiate normal mucosa, hyperplasia, dys-plasia, and OSCC with high sensitivity and specificity (Table 1) (Mallia et al.  2008, 2010; Jayanthi et al. 2011). Moreover, Stephen et al. (2013) suggested that DRS could be very effective as a screening tool for the location of oral PMDs. 
Narrow-band imagingNarrow-band imaging (NBI) is based on the depth of light penetration.  The narrow-band blue light with a short wavelength (415 nm) penetrates into the mucosa and high-lights the superficial capillaries as brown in color, while another wavelength (540 nm) identifies prominent vessels in the submucosal layer as cyan (Tan et al. 2012).  The reflected light is captured by a monochromatic charge-cou-pled device located at the tip of the endoscope; a colored composite image is then created by the image processor, which is displayed on a high-definition video screen (Piazza et al. 2008).  Potentially malignant and malignant lesions have distinct microvascular morphologies, as angiogenesis occurs at an early stage of carcinogenesis (Fujii et al. 2010). Under NBI, these lesions appear as scattered spots with well-demarcated borders.  Moreover, the regular capillary arrangement is lost (Tan et al. 2012; Green et al. 2014). NBI highlights abnormalities in the superficial vasculature of mucosal lesions so that precancerous or cancerous lesions can be identified more easily (Tan et al. 2012).Yang et al. (2012a, b; 2013) showed that NBI is a promising non-invasive tool for the evaluation and manage-ment of oral leukoplakia.  NBI can capture the twisted elon-gation of intrapapillary capillary loops (IPCL).  IPCL pat-tern destruction can be a useful indicator of the severity of oral leukoplakia.  Nguyen et al. (2013) conducted a pro-spective study involving 73 patients with head and neck cancer and found that the sensitivity for detecting moderate dysplasia or worse was 96% with NBI—better than white light, which achieved only 38%.  However, chronic inflam-mation and postoperative radiotherapy may lead to false positivity, and this may be exacerbated if the operator lacks experience in recognizing the different IPCL changes asso-ciated with inflammatory and neoplastic lesions (Bhatia et al. 2013). 
Optical coherence tomographyOptical coherence tomography (OCT) is an evolving optical technology that produces cross-sectional images of tissue with a high spatial resolution of 10-20 µm (Fercher et al. 1993).  It has most often been compared with ultrasonic imaging.  Both technologies employ back-scattered signals reflected from different layers within the tissue to recon-struct structural images, with the latter measuring sound rather than light.  The high spatial resolution of OCT enables “optical biopsy” and provides immediate and local-ized diagnostic information. This technique is capable of imaging tissue depths of up to 1-2 mm and is thus considered suitable for imaging oral mucosal lesions (Fujimoto et al. 1995), as the normal human oral mucosa is very thin, ranging from 0.2-1 mm (Wilder-Smith et al. 2004, 2009).  In a study of 50 patients with suspicious lesions, including oral leukoplakia and erythroplakia, the sensitivity and specificity of OCT were 93-97%, showing the excellent capability of in vivo OCT for the detection and diagnosis of oral PMDs and oral can-cer (Wilder-Smith et al. 2009).  In another ex vivo study, Jerjes et al. (2010) confirmed the feasibility of using OCT to identify architectural changes in an abnormal lesion area; unfortunately, it was unable to provide a diagnosis or to dif-ferentiate between lesions.  Moreover, there are still limita-tions regarding the application of OCT: (1) a histopatholo-gist is needed to interpret the result, which is subjective, as OCT does not provide quantitative information; (2) only a small area can be examined at a time, because of the small size of the OCT probe (Olivo et al. 2011). Several types of particulate contrast agent, such as air-filled microbubbles, engineered microspheres, and gold nanostructures, have been developed to improve the OCT image by enhancing the intensity of backscattered light from the tissue (Barton et al. 2002).  In addition, an OCT system can be readily combined with nonlinear optical modalities, such as two-photon excited fluorescence and second-harmonic generation.  These combined techniques have been found to yield increased sensitivity and specific-ity in the diagnosis of oral dysplasia and cancer (Wilder-Smith et al. 2005).
Confocal laser endomicroscopyConfocal laser endomicroscopy (CLE) is an adaptation of white-light microscopy in which tissue and cellular structures are visualized by stimulating the emission of light using a low-power laser beam after the application of fluorescent contrast agents (Wright and Wright 2002).  The image of the scanned region can be constructed and digi-tized by measuring the light returning to the detector.  A series of confocal images can be used to observe fine cellu-lar or subcellular structures, and three-dimensional struc-tures in the specimen can also be reconstructed—thus CLE provides a tool for “optical biopsy” in the clinic (De Palma 2009). CLE holds promise for image-guided surgery by aid-
D. Liu et al.172
ing the assessment of lesion margins during surgical proce-dures (Nguyen et al. 2009; Pogorzelski et al. 2012). Recently, Nathan et al. (2014) conducted a prospective study of 21 patients with leukoplakia and cancerous head and neck lesions; the results showed that the diagnosis based on CLE correlated well with histopathology. 
Confocal reflectance microscopyConfocal reflectance microscopy (CRM) is a new opti-cal imaging method that can provide detailed images of the tissue structure and cellular morphology of living tissue in real time.  CRM is based on the mechanism of backscatter-ing of light.  This is primarily determined by natural differ-ences in the refractive indices of organelles and other sub-cellular structures within the tissues (White et al. 2004).  Basically, the CRM set consists of a light source, a con-denser, an objective lens, and a detector.  A tightly focused 
beam of light is focused into a small spot at a chosen depth within a sample.  Only the light from the chosen spot at the chosen depth is in focus at the pinhole and is able to pass through it unimpeded (Gerger et al. 2006).  Theoretically, in vivo confocal imaging resembles histological tissue evalua-tion, except that three-dimensional subcellular resolution can be achieved non-invasively without stains (Maitland et al. 2008). Parameters extracted from confocal images were used to discriminate high-grade cervical pre-cancers with a sen-sitivity of 100% and a specificity of 91% in a study of 25 samples (Collier et al. 2002).  In vivo CRM has already been applied to oral mucosa.  The researchers suggested the potential of CRM in the clinical evaluation of oral lesions, early diagnosis of oral PMDs and oral cancer, real-time identification of tumor margins, and monitoring of the response to therapeutic treatment (Clark et al. 2003; 
Table 3.  Summary of the advantages, disadvantages, and clinical applications/indications of non-invasive detection techniques in the detection and diagnosis of oral PMDs.
TechniquesAdvantagesDisadvantagesClinical applications/Indications
TB stainingSensitive; chair-side; rapid; low-cost
High false-positive ratesHas been used for many years.  May help dental care providers with clinical assessment of oral PMDs and selection of the optimal biopsy site.
Methylene blue stainingUnknownVery few studies have been conducted so far to assess the efficacy of this method in detecting oral PMDs.RB staining
Chemilu-minescence
Effective; rapid; chair-sideLow specificityCould help improve the visualization of white lesions.
VELscopeRapid; chair-side; easy to operateModerate false-positive ratesMay help identify oral PMDs that are not visible to the naked eye.
PDDReal-time; cost-effectiveStrict patient management; high false-positive ratesThe cumbersome process, strict patient management requirements and high false-positive rates limit the use of this method in the clinic.
Raman spectroscopyReal-timeWeak signal; time consumingLimited by the difficulty of capturing weak tissue Raman signals and the relatively slow speed of spectrum acquisitions.
ESSReal-time
Unknown
Seldom used in studies of premalignant and malignant oral tissue.
DRSSimple; low-cost; real-time
May help the practitioner to detect abnormal tissue morphology of the oral mucosa.
NBIReal-time; easy to manageModerate false-positive ratesMay enhance the visualization of the mucosa surface and microvascu-lature.
OCTReal-time; high sensitivity; high specificityNeed a histopathologist to interpret and assess; examines only a very small area at a time
Has been used to assess suspicious oral lesions ex vivo or in vivo.  May help the practitioner to image and assess oral mucosa lesions.
CLEReal-time; “optical biopsy”May help the practitioner to image three-dimensional structures of the mucosa.CRM
miRNA-184Rapid; easy to operate
UnknownMay be a helpful indicator for early detection and dynamic monitoring of oral PMDs.IL-6Moderate false-positive rates 
TB staining, Toluidine blue staining; RB staining, Rose bengal staining; PDD, Photodynamic diagnosis; ESS, Elastic scattering spec-troscopy; DRS, Diffuse reflectance spectroscopy; NBI, Narrow-band imaging; OCT, Optical coherence tomography; CLE, Confocal laser endomicroscopy; CRM, Confocal reflectance microscopy.
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis173
Anuthama et al. 2010).  However, in an pilot study, when the researchers examined the microscopic anatomy of freshly excised head and neck surgical specimens using CRM, and compared the findings with those generated by conventional histologic analysis, they found that there were fewer microscopic details visible in the CRM images than in paraffin-embedded histologic sections (White et al. 2004).
Salivary biomarkerAdvanced research exploring molecular markers for diagnosis and risk predictions appears to be promis-ing. Salivary biomarker test might be a non-invasive, rapid adjunctive aid for revealing malignant transformation in oral PMDs.  Zahran et al. (2015) found that miRNA-184 could distinguish OSCC from oral PMD with dysplasia.  Sharma et al. (2011) assessed salivary interleukin-6 (IL-6) as a marker for malignant transformation of leukoplakia.  IL-6 level was found to increase with the severity of dys-plasia, but periodontitis and tobacco use also contributed to an elevated IL-6 level.
DiscussionIn this review article, the authors have critically described the non-invasive detection techniques for oral PMDs and summarized the range of sensitivity and speci-ficity of each technique in detecting oral PMDs and oral cancer (Table 2).  A summary of the advantages, disadvan-tages and clinical applications/indications of each technique is also provided (Table 3).  Latest meta-analysis showed that vital staining had less sensitivity but greater specificity in identifying oral PMDs and oral cancer compared with light-based detection systems.  However, given that the studies evaluating optical diagnostic technologies and sali-vary biomarker were too few, the integral evaluation was not done.Physicians or dental practitioners at local community medical centers are often encountered with patients with oral PMDs.  Early identification of these patients is neces-
sary for cancer prevention and disease management.  As oral medicine specialists, we believe that it would be help-ful if the doctors at community medical centers could do some screening of these patients.  Currently, there is no widely accepted strategy for the application of non-invasive techniques in the detection and diagnosis of oral PMDs. Considering sensitivity, specificity, expenses and feasibility of the techniques mentioned above, we propose a protocol for the detection and diagnosis of oral PMDs at local com-munity hospitals/medical centers (Fig. 2).
ConclusionMost of the non-invasive detection techniques evalu-ated in this review showed great potential for screening and monitoring oral PMDs.  But there is insufficient evidence to recommend for or against the standalone use of any method for identifying oral PMDs.  Besides, there is no site-specific study and few studies about the combination of plural tech-niques for the clinical detection and diagnosis of oral PMDs, which could be further explored in the future.  Also, well-designed prospective studies with large sample sizes are still needed.
Acknowledgments
This work was supported by grants from the National Natural Science Foundation of China (No. 81321002, 81472533, 81200791), the Fok Ying Tung Education Foundation (No. 132028) and the Doctoral Program of the Ministry of Education of China (No. 20120181120011).
Conflict of Interest
The authors declare no conflict of interest.
References
Anuthama, K., Sherlin, H.J., Anuja, N., Ramani, P., Premkumar, P. & Chandrasekar, T. (2010) Characterization of different tissue changes in normal, betel chewers, potentially malignant lesions, conditions and oral squamous cell carcinoma using reflectance confocal microscopy: correlation with routine histopathology.  Oral Oncol., 46, 232-248.
Fig. 2.  Diagnostic protocol of oral potentially malignant disorders (PMDs) proposed for community hospitals/medical cen-ters.
D. Liu et al.174
Awan, K.H., Morgan, P.R. & Warnakulasuriya, S. (2011a)  Evalua-tion of an autofluorescence based imaging system (VELscope™) in the detection of oral potentially malignant disorders and benign keratoses.  Oral Oncol., 47, 274-277.Awan, K.H., Morgan, P.R. & Warnakulasuriya, S. (2011b)  Utility of chemiluminescence (ViziLite™) in the detection of oral potentially malignant disorders and benign keratoses.  J. Oral Pathol. Med., 40, 541-544.Awan, K.H., Yang, Y.H., Morgan, P.R. & Warnakulasuriya, S. (2012)  Utility of toluidine blue as a diagnostic adjunct in the detection of potentially malignant disorders of the oral cavity—a clinical and histological assessment.  Oral Dis., 18, 728-733.Balevi, B. (2007)  Evidence-based decision making: should the general dentist adopt the use of the VELscope for routine screening for oral cancer?  J. Can. Dent. Assoc., 73, 603-606.Balevi, B. (2011)  Assessing the usefulness of three adjunctive diagnostic devices for oral cancer screening: a probabilistic approach.  Community Dent. Oral Epidemiol., 39, 171-176.Barton, J.K., Hoying, J.B. & Sullivan, C.J. (2002)  Use of micro-bubbles as an optical coherence tomography contrast agent.  Acad. Radiol., 9 Suppl 1, S52-S55.Betz, C.S., Stepp, H., Janda, P., Arbogast, S., Grevers, G., Baumgartner, R. & Leunig, A. (2002)  A comparative study of normal inspection, autofluorescence and 5-ALA-induced PPIX fluorescence for oral cancer diagnosis.  Int. J. Cancer, 97, 245-252.Bhatia, N., Lalla, Y., Vu, A.N. & Farah, C.S. (2013)  Advances in optical adjunctive AIDS for visualisation and detection of oral malignant and potentially malignant lesions.  Int. J. Dent., 2013, 194029.Bigio, I.J. & Bown, S.G. (2004)  Spectroscopic sensing of cancer and cancer therapy: current status of translational research.  Cancer Biol. Ther., 3, 259-267.Bigio, I.J. & Mourant, J.R. (1997)  Ultraviolet and visible spectros-copies for tissue diagnostics: fluorescence spectroscopy and elastic-scattering spectroscopy.  Phys. Med. Biol., 42, 803-814.Brothwell, D.J., Lewis, D.W., Bradley, G., Leong, I., Jordan, R.C., Mock, D. & Leake, J.L. (2003) Observer agreement in the grading of oral epithelial dysplasia.  Community Dent. Oral Epidemiol., 31, 300-305.Chang, C.J. & Wilder-Smith, P. (2005)  Topical application of photofrin for photodynamic diagnosis of oral neoplasms.  Plast. Reconstr. Surg., 115, 1877-1886.Chaudhari, A., Hegde-Shetiya, S., Shirahatti, R. & Agrawal, D. (2013)  Comparison of different screening methods in esti-mating the prevalence of precancer and cancer amongst male inmates of a jail in maharashtra, India.  Asian Pac. J. Cancer Prev., 14, 859-864.Chen, Y.W., Lin, J.S., Fong, J.H., Wang, I.K., Chou, S.J., Wu, C.H., Lui, M.T., Chang, C.S. & Kao, S.Y. (2007a)  Use of methylene blue as a diagnostic aid in early detection of oral cancer and precancerous lesions.  Br. J. Oral Maxillofac. Surg., 45, 590-591.Chen, Y.W., Lin, J.S., Wu, C.H., Lui, M.T., Kao, S.Y. & Fong, Y. (2007b)  Application of in vivo stain of methylene blue as a diagnostic aid in the early detection and screening of oral squamous cell carcinoma and precancer lesions.  J. Chin. Med. Assoc., 70, 497-503.Clark, A.L., Gillenwater, A.M., Collier, T.G., Alizadeh-Naderi, R., El-Naggar, A.K. & Richards-Kortum, R.R. (2003)  Confocal microscopy for real-time detection of oral cavity neoplasia.  Clin. Cancer Res., 9, 4714-4721.Collier, T., Lacy, A., Richards-Kortum, R., Malpica, A. & Follen, M. (2002)  Near real-time confocal microscopy of amelanotic tissue: detection of dysplasia in ex vivo cervical tissue.  Acad. Radiol., 9, 504-512.De Palma, G.D. (2009)  Confocal laser endomicroscopy in the “in vivo” histological diagnosis of the gastrointestinal tract.  
World J. Gastroenterol., 15, 5770-5775.Driemel, O., Kunkel, M., Hullmann, M., von Eggeling, F., Muller-Richter, U., Kosmehl, H. & Reichert, T.E. (2007)  Diagnosis of oral squamous cell carcinoma and its precursor lesions.  J. Dtsch. Dermatol. Ges., 5, 1095-1100.Du, G.F., Li, C.Z., Chen, H.Z., Chen, X.M., Xiao, Q., Cao, Z.G., Shang, S.H. & Cai, X. (2007)  Rose bengal staining in detec-tion of oral precancerous and malignant lesions with colori-metric evaluation: a pilot study.  Int. J. Cancer, 120, 1958-1963.Epstein, J.B., Gorsky, M., Lonky, S., Silverman, S. Jr., Epstein, J.D. & Bride, M. (2006)  The efficacy of oral lumenoscopy (ViziLite) in visualizing oral mucosal lesions.  Spec. Care Dentist., 26, 171-174.Epstein, J.B. & Guneri, P. (2009)  The adjunctive role of toluidine blue in detection of oral premalignant and malignant lesions.  Curr. Opin. Otolaryngol. Head Neck Surg., 17, 79-87.Epstein, J.B., Oakley, C., Millner, A., Emerton, S., van der Meij, E. & Le, N. (1997)  The utility of toluidine blue application as a diagnostic aid in patients previously treated for upper oropha-ryngeal carcinoma.  Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod., 83, 537-547.Epstein, J.B., Sciubba, J., Silverman, S. Jr. & Sroussi, H.Y. (2007)  Utility of toluidine blue in oral premalignant lesions and squa-mous cell carcinoma: continuing research and implications for clinical practice.  Head Neck, 29, 948-958.Epstein, J.B., Scully, C. & Spinelli, J. (1992)  Toluidine blue and Lugol’s iodine application in the assessment of oral malignant disease and lesions at risk of malignancy.  J. Oral Pathol. Med., 21, 160-163.Epstein, J.B., Silverman, S. Jr., Epstein, J.D., Lonky, S.A. & Bride, M.A. (2008)  Analysis of oral lesion biopsies identified and evaluated by visual examination, chemiluminescence and tolu-idine blue.  Oral Oncol., 44, 538-544.Farah, C.S. & McCullough, M.J. (2007)  A pilot case control study on the efficacy of acetic acid wash and chemiluminescent illu-mination (ViziLite) in the visualisation of oral mucosal white lesions.  Oral Oncol., 43, 820-824.Farah, C.S., McIntosh, L., Georgiou, A. & McCullough, M.J. (2012)  Efficacy of tissue autofluorescence imaging (VELScope) in the visualization of oral mucosal lesions.  Head Neck, 34, 856-862.Fercher, A.F., Hitzenberger, C.K., Drexler, W., Kamp, G. &  Sattmann, H. (1993)  In vivo optical coherence tomography.  Am. J. Ophthalmol., 116, 113-114.Fischer, D.J., Epstein, J.B., Morton, T.H. & Schwartz, S.M. (2004)  Interobserver reliability in the histopathologic diagnosis of oral pre-malignant and malignant lesions.  J. Oral Pathol. Med., 33, 65-70.Fricain, J.C. (2011)  Autofluorescence for the detection of poten-tially malignant and malignant lesions of the oral cavity lining.  Rev. Stomatol. Chir. Maxillofac., 112, 16-21.Fujii, S., Yamazaki, M., Muto, M. & Ochiai, A. (2010)  Microvas-cular irregularities are associated with composition of squa-mous epithelial lesions and correlate with subepithelial inva-sion of superficial-type pharyngeal squamous cell carcinoma.  Histopathology, 56, 510-522.Fujimoto, J.G., Brezinski, M.E., Tearney, G.J., Boppart, S.A., Bouma, B., Hee, M.R., Southern, J.F. & Swanson, E.A. (1995)  Optical biopsy and imaging using optical coherence tomog-raphy.  Nat. Med., 1, 970-972.Gandolfo, S., Pentenero, M., Broccoletti, R., Pagano, M., Carrozzo, M. & Scully, C. (2006)  Toluidine blue uptake in potentially malignant oral lesions in vivo: clinical and histological assess-ment.  Oral Oncol., 42, 89-95.Gerger, A., Koller, S., Weger, W., Richtig, E., Kerl, H., Samonigg, H., Krippl, P. & Smolle, J. (2006) Sensitivity and specificity of confocal laser-scanning microscopy for in vivo diagnosis of malignant skin tumors.  Cancer, 107, 193-200.
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis175
Green, B., Cobb, A.R., Brennan, P.A. & Hopper, C. (2014)  Optical diagnostic techniques for use in lesions of the head and neck: review of the latest developments.  Br. J. Oral Maxillofac. Surg., 52, 675-680.Guneri, P., Epstein, J.B., Ilhan, B., Kaya, A. & Boyacioglu, H. (2013)  Clinical application of a digital method to improve the accuracy of color perception in toluidine blue stained oral mucosal lesions.  Quintessence Int., 44, 619-627.Guze, K., Pawluk, H.C., Short, M., Zeng, H., Lorch, J., Norris, C. & Sonis, S. (2015)  Pilot study: raman spectroscopy in differ-entiating premalignant and malignant oral lesions from normal mucosa and benign lesions in humans.  Head Neck, 37, 511-517.Guze, K., Short, M., Sonis, S., Karimbux, N., Chan, J. & Zeng, H. (2009)  Parameters defining the potential applicability of Raman spectroscopy as a diagnostic tool for oral disease.  J. Biomed. Opt., 14, 014016.Hamdoon, Z., Jerjes, W., Upile, T., McKenzie, G., Jay, A. & Hopper, C. (2013)  Optical coherence tomography in the assessment of suspicious oral lesions: an immediate ex vivo study.  Photodiagnosis Photodyn. Ther., 10, 17-27.Huber, M.A. (2009)  Assessment of the VELscope as an adjunctive examination tool.  Tex. Dent. J., 126, 528-535.Ibrahim, S.S., Al-Attas, S.A., Darwish, Z.E., Amer, H.A. & Hassan, M.H. (2014)  Effectiveness of the Microlux/DLTM chemiluminescence device in screening of potentially malig-nant and malignant oral lesions.  Asian Pac. J. Cancer Prev., 15, 6081-6086.Jayanthi, J.L., Nisha, G.U., Manju, S., Philip, E.K., Jeemon, P., Baiju, K.V., Beena, V.T. & Subhash, N. (2011)  Diffuse reflec-tance spectroscopy: diagnostic accuracy of a non-invasive screening technique for early detection of malignant changes in the oral cavity.  BMJ Open, 1, e000071.Jerjes, W., Swinson, B., Pickard, D., Thomas, G.J. & Hopper, C. (2004)  Detection of cervical intranodal metastasis in oral cancer using elastic scattering spectroscopy.  Oral Oncol., 40, 673-678.Jerjes, W., Upile, T., Conn, B., Hamdoon, Z., Betz, C.S., McKenzie, G., Radhi, H., Vourvachis, M., El Maaytah, M., Sandison, A., Jay, A. & Hopper, C. (2010)  In vitro examina-tion of suspicious oral lesions using optical coherence tomog-raphy.  Br. J. Oral Maxillofac. Surg., 48, 18-25.Karabulut, A., Reibel, J., Therkildsen, M.H., Praetorius, F., Nielsen, H.W. & Dabelsteen, E. (1995)  Observer variability in the histologic assessment of oral premalignant lesions.  J. Oral Pathol. Med., 24, 198-200.Kerr, A.R., Sirois, D.A. & Epstein, J.B. (2006)  Clinical evaluation of chemiluminescent lighting: an adjunct for oral mucosal examinations.  J. Clin. Dent., 17, 59-63.Kois, J.C. & Truelove, E. (2006) Detecting oral cancer: a new technique and case reports.  Dent. Today, 25, 94, 96-97.Krishna, H., Majumder, S.K., Chaturvedi, P., Sidramesh, M. & Gupta, P.K. (2014)  In vivo Raman spectroscopy for detection of oral neoplasia: a pilot clinical study.  J. Biophotonics, 7, 690-702.Krishnakumar, N., Sulfikkarali, N.K., Manoharan, S. & Venkatachalam, P. (2013)  Raman spectroscopic investigation of the chemopreventive response of naringenin and its nanoparticles in DMBA-induced oral carcinogenesis. Spectro-chim.  Acta A Mol. Biomol. Spectrosc., 115, 648-653.Kurita, H., Kamata, T., Li, X., Nakanishi, Y., Shimane, T. & Koike, T. (2012)  Effectiveness of vital staining with iodine solution in reducing local recurrence after resection of dysplastic or malignant oral mucosa.  Br. J. Oral Maxillofac. Surg., 50, 109-112.Lambert, R., Sauvaget, C., de Camargo Cancela, M. &  Sankaranarayanan, R. (2011)  Epidemiology of cancer from the oral cavity and oropharynx.  Eur. J. Gastroenterol. Hepatol., 23, 633-641.
Leunig, A., Betz, C.S., Mehlmann, M., Stepp, H., Arbogast, S., Grevers, G. & Baumgartner, R. (2000) Detection of squamous cell carcinoma of the oral cavity by imaging 5-aminolevulinic acid-induced protoporphyrin IX fluorescence.  Laryngoscope, 110, 78-83.Li, Y., Wen, Z.N., Li, L.J., Li, M.L., Zhang, Z. & Gao, N. (2010)  Near infrared Raman spectral character and diagnostic value of squamous cell carcinoma of oral mucosa.  Hua Xi Kou Qiang Yi Xue Za Zhi, 28, 61-64.Lingen, M.W., Kalmar, J.R., Karrison, T. & Speight, P.M. (2008)  Critical evaluation of diagnostic aids for the detection of oral cancer.  Oral Oncol., 44, 10-22.Lopez-Jornet, P. & De la Mano-Espinosa, T. (2011)  The efficacy of direct tissue fluorescence visualization in screening for oral premalignant lesions in general practice: an update.  Int. J. Dent. Hyg., 9, 97-100.Macey, R., Walsh, T., Brocklehurst, P., Kerr, A.R., Liu, J.L., Lingen, M.W., Ogden, G.R., Warnakulasuriya, S. & Scully, C. (2015)  Diagnostic tests for oral cancer and potentially malig-nant disorders in patients presenting with clinically evident lesions.  Cochrane Database Syst. Rev., 5, CD010276.Maeda, K., Suzuki, T., Ooyama, Y., Nakakuki, K., Yamashiro, M., Okada, N. & Amagasa, T. (2010) Colorimetric analysis of unstained lesions surrounding oral squamous cell carcinomas and oral potentially malignant disorders using iodine.  Int. J. Oral Maxillofac. Surg., 39, 486-492.Maitland, K.C., Gillenwater, A.M., Williams, M.D., El-Naggar, A.K., Descour, M.R. & Richards-Kortum, R.R. (2008)  In vivo imaging of oral neoplasia using a miniaturized fiber optic confocal reflectance microscope.  Oral Oncol., 44, 1059-1066.Mallia, R., Thomas, S.S., Mathews, A., Kumar, R., Sebastian, P., Madhavan, J. & Subhash, N. (2008) Oxygenated hemoglobin diffuse reflectance ratio for in vivo detection of oral pre-cancer.  J. Biomed. Opt., 13, 041306.Mallia, R.J., Narayanan, S., Madhavan, J., Sebastian, P., Kumar, R., Mathews, A., Thomas, G. & Radhakrishnan, J. (2010)  Diffuse reflection spectroscopy: an alternative to autofluorescence spectroscopy in tongue cancer detection.  Appl. Spectrosc., 64, 409-418.Marzouki, H.Z., Tuong Vi Vu, T., Ywakim, R., Chauvin, P., Hanley, J. & Kost, K.M. (2012)  Use of fluorescent light in detecting malignant and premalignant lesions in the oral cavity: a prospective, single-blind study.  J. Otolaryngol. Head Neck Surg., 41, 164-168.Matheny, E.S., Hanna, N.M., Jung, W.G., Chen, Z., Wilder-Smith, P., Mina-Araghi, R. & Brenner, M. (2004)  Optical coherence tomography of malignancy in hamster cheek pouches.  J. Biomed. Opt., 9, 978-981.McIntosh, L., McCullough, M.J. & Farah, C.S. (2009)  The assess-ment of diffused light illumination and acetic acid rinse (Microlux/DL) in the visualisation of oral mucosal lesions.  Oral Oncol., 45, e227-e231.Mehrotra, R., Sharma, N., Umudum, H., Ceyhan, K. & Rezanko, T. (2012)  The role of cytopathology in diagnosing HPV induced oropharyngeal lesions.  Diagn. Cytopathol., 40, 839-843.Mehrotra, R., Singh, M., Thomas, S., Nair, P., Pandya, S., Nigam, N.S. & Shukla, P. (2010)  A cross-sectional study evaluating chemiluminescence and autofluorescence in the detection of clinically innocuous precancerous and cancerous oral lesions.  J. Am. Dent. Assoc., 141, 151-156.Mercadante, V., Paderni, C. & Campisi, G. (2012)  Novel non-invasive adjunctive techniques for early oral cancer diagnosis and oral lesions examination.  Curr. Pharm. Des., 18, 5442-5451.Messadi, D.V. (2013)  Diagnostic aids for detection of oral precan-cerous conditions.  Int. J. Oral Sci., 5, 59-65.Mittal, N., Palaskar, S. & Shankari, M. (2012)  Rose Bengal staining - diagnostic aid for potentially malignant and malig-nant disorders: a pilot study.  Indian J. Dent. Res., 23, 
D. Liu et al.176
561-564.Mojsa, I., Kaczmarzyk, T., Zaleska, M., Stypulkowska, J., Zapala-Pospiech, A. & Sadecki, D. (2012) Value of the ViziLite Plus System as a diagnostic aid in the early detection of oral cancer/premalignant epithelial lesions.  J. Craniofac. Surg., 23, e162-e164.Moro, A., Di Nardo, F., Boniello, R., Marianetti, T.M., Cervelli, D., Gasparini, G. & Pelo, S. (2010) Autofluorescence and early detection of mucosal lesions in patients at risk for oral cancer.  J. Craniofac. Surg., 21, 1899-1903.Murube, J. (2014)  Rose bengal: the second most commonly used surfocular vital stain.  Ocul. Surf., 12, 14-22.Nagaraju, K., Prasad, S. & Ashok, L. (2010)  Diagnostic efficiency of toluidine blue with Lugol’s iodine in oral premalignant and malignant lesions.  Indian J. Dent. Res., 21, 218-223.Nathan, C.A., Kaskas, N.M., Ma, X., Chaudhery, S., Lian, T., Moore-Medlin, T., Shi, R. & Mehta, V. (2014)  Confocal laser endomicroscopy in the detection of head and neck precan-cerous lesions. Otolaryngol.  Head Neck Surg., 151, 73-80.Nguyen, N.Q., Biankin, A.V., Leong, R.W., Chang, D.K., Cosman, P.H., Delaney, P., Kench, J.G. & Merrett, N.D. (2009)  Real time intraoperative confocal laser microscopy-guided surgery.  Ann. Surg., 249, 735-737.Nguyen, P., Bashirzadeh, F., Hodge, R., Agnew, J., Farah, C.S., Duhig, E., Clarke, B., Perry-Keene, J., Botros, D., Masters, I.B. & Fielding, D. (2013)  High specificity of combined narrow band imaging and autofluorescence mucosal assess-ment of patients with head and neck cancer.  Head Neck, 35, 619-625.Olivo, M., Bhuvaneswari, R. & Keogh, I. (2011)  Advances in bio-optical imaging for the diagnosis of early oral cancer.  Phar-maceutics, 3, 354-378.Pallagatti, S., Sheikh, S., Aggarwal, A., Gupta, D., Singh, R., Handa, R., Kaur, S. & Mago, J. (2013) Toluidine blue staining as an adjunctive tool for early diagnosis of dysplastic changes in the oral mucosa.  J. Clin. Exp. Dent., 5, e187-e191.Patton, L.L., Epstein, J.B. & Kerr, A.R. (2008)  Adjunctive tech-niques for oral cancer examination and lesion diagnosis: a systematic review of the literature.  J. Am. Dent. Assoc., 139, 896-905; quiz 993-894.Pavlova, I., Williams, M., El-Naggar, A., Richards-Kortum, R. & Gillenwater, A. (2008)  Understanding the biological basis of autofluorescence imaging for oral cancer detection: high-reso-lution fluorescence microscopy in viable tissue.  Clin. Cancer Res., 14, 2396-2404.Petruzzi, M., Lucchese, A., Baldoni, E., Grassi, F.R. & Serpico, R. (2010)  Use of Lugol’s iodine in oral cancer diagnosis: an overview.  Oral Oncol., 46, 811-813.Piazza, C., Cocco, D., Del Bon, F., Mangili, S., Nicolai, P., Majorana, A., Bolzoni Villaret, A. & Peretti, G. (2010)  Narrow band imaging and high definition television in evaluation of oral and oropharyngeal squamous cell cancer: a prospective study.  Oral Oncol., 46, 307-310.Piazza, C., Dessouky, O., Peretti, G., Cocco, D., De Benedetto, L. & Nicolai, P. (2008)  Narrow-band imaging: a new tool for evaluation of head and neck squamous cell carcinomas.  Review of the literature.  Acta Otorhinolaryngol. Ital., 28, 49-54.Pogorzelski, B., Hanenkamp, U., Goetz, M., Kiesslich, R. & Gosepath, J. (2012)  Systematic intraoperative application of confocal endomicroscopy for early detection and resection of squamous cell carcinoma of the head and neck: a preliminary report.  Arch. Otolaryngol. Head Neck Surg., 138, 404-411.Poh, C.F., Zhang, L., Anderson, D.W., Durham, J.S., Williams, P.M., Priddy, R.W., Berean, K.W., Ng, S., Tseng, O.L., MacAulay, C. & Rosin, M.P. (2006)  Fluorescence visualiza-tion detection of field alterations in tumor margins of oral cancer patients.  Clin. Cancer Res., 12, 6716-6722.Rajmohan, M., Rao, U.K., Joshua, E., Rajasekaran, S.T. & Kannan, 
R. (2012)  Assessment of oral mucosa in normal, precancer and cancer using chemiluminescent illumination, toluidine blue supravital staining and oral exfoliative cytology.  J. Oral Maxillofac. Pathol., 16, 325-329.Ram, S. & Siar, C.H. (2005)  Chemiluminescence as a diagnostic aid in the detection of oral cancer and potentially malignant epithelial lesions.  Int. J. Oral Maxillofac. Surg., 34, 521-527.Rashid, A. & Warnakulasuriya, S. (2015)  The use of light-based (optical) detection systems as adjuncts in the detection of oral cancer and oral potentially malignant disorders: a systematic review.  J. Oral Pathol. Med., 44, 307-328.Riaz, A., Shreedhar, B., Kamboj, M. & Natarajan, S. (2013)  Meth-ylene blue as an early diagnostic marker for oral precancer and cancer. Springerplus, 2, 95.Rodrigues, E.B., Costa, E.F., Penha, F.M., Melo, G.B., Bottos, J., Dib, E., Furlani, B., Lima, V.C., Maia, M., Meyer, C.H., Hofling-Lima, A.L. & Farah, M.E. (2009)  The use of vital dyes in ocular surgery.  Surv. Ophthalmol., 54, 576-617.Sambandham, T., Masthan, K.M., Kumar, M.S. & Jha, A. (2013)  The application of vizilite in oral cancer.  J. Clin. Diagn. Res., 7, 185-186.Sarode, S.C., Sarode, G.S. & Tupkari, J.V. (2012)  Oral potentially malignant disorders: precising the definition.  Oral Oncol., 48, 759-760.Scheer, M., Neugebauer, J., Derman, A., Fuss, J., Drebber, U. & Zoeller, J.E. (2011)  Autofluorescence imaging of potentially malignant mucosa lesions.  Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod., 111, 568-577.Schiodt, M. & Pindborg, J.J. (1984)  Oral discoid lupus erythema-tosus. I. The validity of previous histopathologic diagnostic criteria.  Oral Surg. Oral Med. Oral Pathol., 57, 46-51.Schwarz, R.A., Gao, W., Daye, D., Williams, M.D., Richards-Kortum, R. & Gillenwater, A.M. (2008)  Autofluorescence and diffuse reflectance spectroscopy of oral epithelial tissue using a depth-sensitive fiber-optic probe.  Appl. Opt., 47, 825-834.Seoane, J., Varela-Centelles, P., Ramirez, J.R., Romero, M.A. & De La Cruz, A. (2002)  Artefacts produced by suture traction during incisional biopsy of oral lesions.  Clin. Otolaryngol. Allied Sci., 27, 549-553.Seoane, J., Varela-Centelles, P.I., Ramirez, J.R., Cameselle-Teijeiro, J. & Romero, M.A. (2004) Artefacts in oral incisional biopsies in general dental practice: a pathology audit.  Oral Dis., 10, 113-117.Sharma, M., Bairy, I., Pai, K., Satyamoorthy, K., Prasad, S., Berkovitz, B. & Radhakrishnan, R. (2011) Salivary IL-6 levels in oral leukoplakia with dysplasia and its clinical relevance to tobacco habits and periodontitis.  Clin. Oral Investig., 15, 705-714.Sharwani, A., Jerjes, W., Salih, V., MacRobert, A.J., El-Maaytah, M., Khalil, H.S. & Hopper, C. (2006a) Fluorescence spectros-copy combined with 5-aminolevulinic acid-induced protopor-phyrin IX fluorescence in detecting oral premalignancy.  J. Photochem. Photobiol. B, 83, 27-33.Sharwani, A., Jerjes, W., Salih, V., Swinson, B., Bigio, I.J., El-Maaytah, M. & Hopper, C. (2006b) Assessment of oral premalignancy using elastic scattering spectroscopy.  Oral Oncol., 42, 343-349.Shibahara, T., Yamamoto, N., Yakushiji, T., Nomura, T., Sekine, R., Muramatsu, K. & Ohata, H. (2014) Narrow-band imaging system with magnifying endoscopy for early oral cancer.  Bull. Tokyo Dent. Coll., 55, 87-94.Sieron, A., Gibinski, P., Pustelny, T., Kwiatek, S., Opilski, Z., Kawczyk-Krupka, A., Woznica, T., Maciak, E., Kubica, W., Urbanczyk, M. & Latos, W. (2008)  Optical biopsy using spectral camera in BCC and oral leukoplakia.  Photodiagnosis Photodyn. Ther., 5, 271-275.Singh, S.P., Deshmukh, A., Chaturvedi, P. & Murali Krishna, C. (2012)  In vivo Raman spectroscopic identification of prema-lignant lesions in oral buccal mucosa.  J. Biomed. Opt., 17, 
Non-Invasive Techniques for Oral PMDs Detection and Diagnosis177
105002.Stephen, M.M., Jayanthi, J.L., Unni, N.G., Kolady, P.E., Beena, V.T., Jeemon, P. & Subhash, N. (2013) Diagnostic accuracy of diffuse reflectance imaging for early detection of pre-malig-nant and malignant changes in the oral cavity: a feasibility study.  BMC Cancer, 13, 278.Su, W.W., Yen, A.M., Chiu, S.Y. & Chen, T.H. (2010)  A commu-nity-based RCT for oral cancer screening with toluidine blue.  J. Dent. Res., 89, 933-937.Sugerman, P.B. & Savage, N.W. (2002)  Oral lichen planus: causes, diagnosis and management.  Aust. Dent. J., 47, 290-297.Tan, N.C., Herd, M.K., Brennan, P.A. & Puxeddu, R. (2012)  The role of narrow band imaging in early detection of head and neck cancer.  Br. J. Oral Maxillofac. Surg., 50, 132-136.Thomson, P.J. (2002)  Field change and oral cancer: new evidence for widespread carcinogenesis?  Int. J. Oral Maxillofac. Surg., 31, 262-266.Trullenque-Eriksson, A., Munoz-Corcuera, M., Campo-Trapero, J., Cano-Sanchez, J. & Bascones-Martinez, A. (2009)  Analysis of new diagnostic methods in suspicious lesions of the oral mucosa.  Med. Oral Patol. Oral Cir. Bucal, 14, E210-E216.Uekusa, M., Omura, K., Nakajima, Y., Hasegawa, S., Harada, H., Morita, K.I. & Tsuda, H. (2010) Uptake and kinetics of 5-aminolevulinic acid in oral squamous cell carcinoma.  Int. J. Oral Maxillofac. Surg., 39, 802-805.Umeda, M., Shigeta, T., Takahashi, H., Minamikawa, T.,  Komatsubara, H., Oguni, A., Shibuya, Y. & Komori, T. (2011)  Clinical evaluation of Lugol’s iodine staining in the treatment of stage I-II squamous cell carcinoma of the tongue.  Int. J. Oral Maxillofac. Surg., 40, 593-596.Vashisht, N., Ravikiran, A., Samatha, Y., Rao, P.C., Naik, R. & Vashisht, D. (2014)  Chemiluminescence and toluidine blue as diagnostic tools for detecting early stages of oral cancer: an invivo study.  J. Clin. Diagn. Res., 8, ZC35-38.Vu, A.N. & Farah, C.S. (2014)  Efficacy of narrow band imaging for detection and surveillance of potentially malignant and malignant lesions in the oral cavity and oropharynx: a system-atic review.  Oral Oncol., 50, 413-420.Wallace, M. & Van Dam, J. (2000)  Enhanced gastrointestinal diagnosis: light-scattering spectroscopy and optical coherence tomography.  Gastrointest. Endosc. Clin. N. Am., 10, 71-80, vi.Wang, B., Wang, J.H., Liu, Q., Huang, H., Chen, M., Li, K., Li, C., Yu, X.F. & Chu, P.K. (2014) Rose-bengal-conjugated gold nanorods for in vivo photodynamic and photothermal oral cancer therapies.  Biomaterials, 35, 1954-1966.Wang, C.Y., Tsai, T., Chiang, C.P., Chen, H.M. & Chen, C.T. (2009)  Improved diagnosis of oral premalignant lesions in submucous fibrosis patients with 5-aminolevulinic acid induced PpIX fluorescence.  J. Biomed. Opt., 14, 044026.Warnakulasuriya, K.A. & Johnson, N.W. (1996)  Sensitivity and specificity of OraScan (R) toluidine blue mouthrinse in the detection of oral cancer and precancer.  J. Oral Pathol. Med., 25, 97-103.Warnakulasuriya, S., Johnson, N.W. & van der Waal, I. (2007)  Nomenclature and classification of potentially malignant disorders of the oral mucosa.  J. Oral Pathol. Med., 36, 575-580.White, W.M., Baldassano, M., Rajadhyaksha, M., Gonzalez, S., Tearney, G.J., Anderson, R.R. & Fabian, R.L. (2004)  
Confocal reflectance imaging of head and neck surgical speci-mens. A comparison with histologic analysis.  Arch. Otolar-yngol. Head Neck Surg., 130, 923-928.Wilder-Smith, P., Jung, W.G., Brenner, M., Osann, K., Beydoun, H., Messadi, D. & Chen, Z. (2004)  In vivo optical coherence tomography for the diagnosis of oral malignancy.  Lasers Surg. Med., 35, 269-275.Wilder-Smith, P., Krasieva, T., Jung, W.G., Zhang, J., Chen, Z., Osann, K. & Tromberg, B. (2005) Noninvasive imaging of oral premalignancy and malignancy.  J. Biomed. Opt., 10, 051601.Wilder-Smith, P., Lee, K., Guo, S., Zhang, J., Osann, K., Chen, Z. & Messadi, D. (2009)  In vivo diagnosis of oral dysplasia and malignancy using optical coherence tomography: preliminary studies in 50 patients.  Lasers Surg. Med., 41, 353-357.Wright, S.J. & Wright, D.J. (2002)  Introduction to confocal microscopy.  Methods Cell Biol., 70, 1-85.Yang, S.W., Lee, Y.S., Chang, L.C., Chien, H.P. & Chen, T.A. (2012a)  Clinical appraisal of endoscopy with narrow-band imaging system in the evaluation and management of homo-geneous oral leukoplakia.  ORL J. Otorhinolaryngol. Relat. Spec., 74, 102-109.Yang, S.W., Lee, Y.S., Chang, L.C., Chien, H.P. & Chen, T.A. (2013)  Light sources used in evaluating oral leukoplakia: broadband white light versus narrowband imaging.  Int. J. Oral Maxillofac. Surg., 42, 693-701.Yang, S.W., Lee, Y.S., Chang, L.C., Hwang, C.C. & Chen, T.A. (2012b)  Diagnostic significance of narrow-band imaging for detecting high-grade dysplasia, carcinoma in situ, and carci-noma in oral leukoplakia.  Laryngoscope, 122, 2754-2761.Yang, S.W., Lee, Y.S., Chang, L.C., Hwang, C.C. & Chen, T.A. (2014)  Use of endoscopy with narrow-band imaging system in detecting squamous cell carcinoma in oral chronic non-healing ulcers.  Clin. Oral Investig., 18, 949-959.Yu, B., Shah, A., Nagarajan, V.K. & Ferris, D.G. (2014)  Diffuse reflectance spectroscopy of epithelial tissue with a smart fiber-optic probe.  Biomed. Opt. Express, 5, 675-689.Zahran, F., Ghalwash, D., Shaker, O., Al-Johani, K. & Scully, C. (2015)  Salivary microRNAs in oral cancer.  Oral Dis., 21, 739-747.Zenk, W., Dietel, W., Schleier, P. & Gunzel, S. (1999)  Visualizing carcinomas of the mouth cavity by stimulating synthesis of fluorescent protoporphyrin IX.  Mund Kiefer Gesichtschir., 3, 205-209.Zhang, L., Bi, L., Shi, J., Zhang, Z., Cao, W., Lin, J., Li, C., Bi, J. & Yu, Y. (2013)  A quantitative diagnostic method for oral mucous precancerosis by Rose Bengal fluorescence spectros-copy.  Lasers Med. Sci., 28, 241-246.Zheng, W., Olivo, M. & Soo, K.C. (2004)  The use of digitized endoscopic imaging of 5-ALA-induced PPIX fluorescence to detect and diagnose oral premalignant and malignant lesions in vivo.  Int. J. Cancer, 110, 295-300.Zheng, W., Soo, K.C., Sivanandan, R. & Olivo, M. (2002)  Detec-tion of squamous cell carcinomas and pre-cancerous lesions in the oral cavity by quantification of 5-aminolevulinic acid induced fluorescence endoscopic images.  Lasers Surg. Med., 31, 151-157.Zide, M.F. (2008)  Actinic keratosis: from the skin to the lip.  J. Oral Maxillofac. Surg., 66, 1162-1176.