a computed tomography ( ct ) incidentaloma is an incidentally detected and previously unsuspected finding or abnormality that is not related to the indication for obtaining the ct examination . chest ct incidentalomas are pandemic in modern health care due to many factors , including the explosion in the use of chest ct scanning , particularly in the emergency department , in oncology centers , and in screening for coronary artery disease ( cad ) and lung cancer . multidetector ct results in huge numbers of thin ct sections with excellent spatial resolution , leading to the detection of many incidental findings . incidentalomas on chest ct scans may represent normal findings , normal variants , or abnormal findings . abnormal findings may be of little or no clinical significance ( e.g. a calcified lung granuloma ) , requiring no further evaluation , or they may be highly significant , requiring immediate treatment ( e.g. a large , central pulmonary embolus ) . commonly , however , incidentalomas are of questionable clinical significance ( e.g. a thyroid nodule ) , necessitating further evaluation via clinical correlation , follow - up and/or additional testing . a systematic review of 11 chest ct studies performed for cad and lung cancer screening revealed that the proportion of patients with at least one incidental imaging abnormality requiring follow - up varied from 3% to 41.5% ; the wide variation was largely due to considerable differences in follow - up recommendations for incidental findings across studies . overall , 7.7% and 14.2% of participants in cad or lung cancer screening studies , respectively , had further investigations of incidental findings . other individual cad screening studies found major extracardiac incidentalomas in 516% of patients . a lung cancer screening study involving 5200 participants discovered incidental findings in 3% , leading to 12 invasive procedures ; however , only 8 of these were clinically relevant . the nelson lung cancer screening trial found that 129/1929 ( 7% ) ct examinations revealed clinically relevant incidental findings , resulting in 118 additional imaging tests ; furthermore , 8 of these showed new incidental findings . eleven patients underwent more than one additional imaging study , and ultimately , only 15 patients needed clinical treatment or further follow - up of their incidental findings . only one incidental cancer was detected ( pancreatic cancer ) , which was incurable due to the presence of metastatic disease . based on these results , the authors advised against systematically searching for and reporting of incidental findings on lung cancer screening studies , as this practice may lead to additional costs , patient anxiety , and risk of iatrogenic injury . despite the conclusions of the authors of the nelson study , most radiologists feel obligated to report incidental findings and give suggestions regarding the clinical significance and appropriate work - up of such findings . for some findings , there are well established clinical guidelines , whereas , for others , no specific guidelines exist . the following sections give suggested guidelines for reporting incidental findings , based on the best evidence that is currently available in the medical literature . thyroid abnormalities , such as nodules ( solitary or multiple ) , cystic lesions , calcifications and diffuse glandular enlargement , are often detected on chest ct examinations ( fig . thyroid nodules are found in approximately 16% of chest ct scans , more commonly in women . most incidentally found thyroid lesions are benign , with an approximately 911% prevalence of malignant lesions . the differential diagnosis for an incidental thyroid abnormality in euthyroid individuals includes nontoxic multinodular goiter , follicular adenoma , thyroiditis , simple cyst , and carcinoma ( primary or secondary ) . clinical features that correlate with malignancy include : age ( younger than 35 years or older than 70 years ) ; size ( > 2.5  cm ) ; presence of dysphagia or hoarseness ; history of external neck radiation in childhood or adolescence ; firm , irregular and/or fixed nodule at physical examination ; presence of cervical lymph node enlargement ; multiple endocrine neoplastic syndromes ; personal or family history of thyroid malignancy ; and possibly male gender . 
 figure 1incidental nodule in the left lobe of the thyroid gland ( arrow ) . furthermore , it has been reported that ct is unreliable in distinguishing simple cysts , complex cysts , and solid nodules . ct underestimates the number of nodules relative to sonography , suggesting that sonography is a useful adjunctive test after the incidental detection of a thyroid abnormality on ct ; furthermore , current management guidelines for thyroid nodules are based on morphologic sonographic characteristics . although sonography is sensitive in detecting thyroid nodules , it is not accurate in diagnosing malignancy ; its major usefulness is in guiding thyroid biopsies . once an incidental thyroid nodule is found , referral to a thyroid specialist may be the most cost - effective option for further evaluation , although ultrasonography may also be suggested . according to the guidelines of the american thyroid association , generally , only nodules > 1 cm need further evaluation , because they have a greater potential to represent clinically significant cancers compared with lesions < 1  cm , and because of cost / benefit considerations regarding the work - up of all small nodules . however , it is probably prudent to correlate even small nodules with physical examination and assessment of individual risk factors . mildly enlarged mediastinal lymph nodes are commonly seen in patients with congestive heart failure ( chf ) ( fig . 2 ) , and they should be recognized as a manifestation of this disease process , rather than as a marker of potential malignancy . in one study of 31 patients with subacute chf , 42% ( 13/31 ) showed enlarged mediastinal lymph nodes ; in 8/13 patients the lymph node enlargement regressed within 1 week after treatment of the heart failure . the lymph nodes were mildly to moderately enlarged , measuring 2  cm in short axis diameter , and were generally located in the right and left paratracheal , subcarinal , anterior mediastinal and precarinal regions . the severity of left heart decompensation correlated with the frequency of lymph node enlargement . other correlative findings included inhomogeneity of the mediastinal fat , consistent with edema , and ill - defined lymph node margins . lymph node enlargement has also been reported for acute chf ; ct findings of acute failure such as ground glass opacities , smooth septal thickening , fissural and peribronchovascular thickening , air space consolidation , perihilar parenchymal disease , pleural and pericardial effusions , and cardiomegaly may help to establish the diagnosis of chf , and concomitant , enlarged mediastinal lymph nodes are to be expected . enlarged lymph nodes are also seen in association with chronic chf ; in these patients the myriad manifestations of acute failure may not be present , and therefore the cause of the nodal enlargement may not be immediately obvious without clinical information regarding cardiac function . a proposed mechanism for nodal enlargement postulates that elevated left ventricular filling pressure leads to decreased pulmonary drainage and the accumulation of excessive interstitial fluid , with dilatation of lymphatics draining the lungs and heart , resulting in lymph node enlargement . ct shows ground glass pulmonary opacities , parenchymal consolidation and smooth septal thickening ( a ) , as well as bilateral pleural effusions , consistent with chf . there are transvenous pacemaker leads in the superior vena cava and enlarged mediastinal lymph nodes ( b , arrows ) . ct shows ground glass pulmonary opacities , parenchymal consolidation and smooth septal thickening ( a ) , as well as bilateral pleural effusions , consistent with chf . there are transvenous pacemaker leads in the superior vena cava and enlarged mediastinal lymph nodes ( b , arrows ) . if incidental , mild mediastinal lymph node enlargement is seen in the setting of chf , no further work - up is necessary , unless there are other clinical or imaging features to suggest an underlying malignancy . mediastinal lymph node enlargement is common in various types of interstitial lung disease ( ild ) , including usual interstitial pneumonitis ( uip ) , nonspecific interstitial pneumonitis ( nsip ) , collagen vascular disease ( cvd ) , sarcoidosis , extrinsic allergic alveolitis ( eaa ) , respiratory bronchiolitis , and desquamative interstitial pneumonitis ( dip ) ( fig . the prevalence of enlarged nodes was 84% ( 46/55 ) in sarcoidosis , 67% ( 41/61 ) in idiopathic pulmonary fibrosis ( ipf ) , 70% ( 14/20 ) in cvd , 53% ( 9/17 ) in eaa , and 36% ( 8/22 ) in cryptogenic organizing pneumonia ( cop ) , previously known as idiopathic bronchiolitis obliterans organizing pneumonia ( boop ) . involved nodes typically occur in the right lower paratracheal and subcarinal regions , and enlargement is typically mild ( the mean size of the largest lymph node was 1.4  cm in one study ) . similar to the situation for incidental , mild mediastinal lymph node enlargement in chf , no further work - up is indicated in the absence of other clinical or imaging features that are suggestive of a coexistent malignancy . figure 3incidental mediastinal lymph node enlargement ( b , arrow ) in a patient with pulmonary interstital fibrosis ( a ) . incidental mediastinal lymph node enlargement ( b , arrow ) in a patient with pulmonary interstital fibrosis ( a ) . the cisterna chyli ( cc ) is a bulbous convergence of lymphatic channels formed by the right and left lumbar trunks and the intestinal trunk ; it is often mistaken for an enlarged lymph node on cross - sectional imaging , either as an incidental finding , or in the setting of cancer staging ( fig . 4 ) . the key to correct identification includes knowledge of the anatomy and morphology of the structure . the cc is located in the retrocrural region at the level of t11l2 , usually on the right , between the aorta and azygos vein . it is continuous with the thoracic duct , which ascends in the right side of the mediastinum medial to the azygos vein , eventually crossing to the left and emptying into the confluence of the left subclavian and jugular veins . it is generally approximately 7  mm in axial cross - sectional diameter and 1530  mm long ; it has been suggested that diameter larger than 10  mm correlates with the presence of an underlying malignancy . although the cc usually shows the attenuation of water ( average 5 hu ) , the attenuation may be elevated due to increased protein content or after the injection of intravenous contrast ( particularly in patients with renal failure ) . fatty attenuation may occasionally be seen , most likely due to dietary fat intake ; a fat - fluid level may rarely be detected . the cc may vary in size depending on the phase of respiration , degree of hydration , and the peristalsis of the muscular lower one - third of the thoracic duct . the structure is visible on approximately 16% of ct scans and can usually be differentiated from an abnormal lymph node by its characteristic appearance and location . figure 4the cc ( a , arrow ) may simulate an abnormal retrocrural lymph node . the cc is continuous with the thoracic duct ( b and c , arrows ) . the cc is continuous with the thoracic duct ( b and c , arrows ) . results of cad and lung cancer screening studies have shown a high prevalence ( 2553% ) of lung nodules on chest ct examinations . nodules representing granulomas are particularly common in endemic areas for histoplasmosis , such as the central united states . approximately half of smokers older than age 50 years have small nodules on thin - section ct , and most of these nodules are benign . when assessing an indeterminate nodule for possible malignant potential , the first rule of thumb is to look for comparison examinations . a solid nodule that is stable for at least 2 years the likelihood of malignancy is directly correlated with nodule size ; < 1% of nodules < 4  mm in diameter in smokers turn into lethal cancers , compared with approximately 1020% of nodules that are about 8 mm . benign morphologic features include certain types of calcification ( laminated , concentric , dense central nidus , popcorn ) and the presence of fat ( consistent with a hamartoma ) . smooth nodule margins , polygonal shape , and subpleural location are also suggestive of benignancy . incidental perifissural nodules are extremely common ; these are generally well circumscribed , smooth , triangular or oval , contacting or closely related to a pleural fissure , occurring below the level of the carina and often showing a septal attachment to the pleural surface ( figs . 5 and 6 ) . these nodules probably represent lymph nodes , and it has been reported that they have low or no chance of malignancy in the setting of lung cancer screening . figure 5small , incidental perifissural nodules ( arrow ) associated with the left major fissure . figure 6small , incidental perifissural nodule associated with the minor fissure and showing a septal attachment ( arrow ) to the anterior pleural surface . small , incidental perifissural nodule associated with the minor fissure and showing a septal attachment ( arrow ) to the anterior pleural surface . when an incidental , indeterminate lung nodule is encountered ( without established long - term stability or clearly benign morphologic features ) , the fleischner society guidelines should be utilized in recommending further work - up or imaging follow - up . these guidelines stratify patients according to clinical risk for developing lung cancer and nodule size . although most radiologists are familiar with the guidelines , the latter are often applied inappropriately or incorrectly . for example , the guidelines only apply to incidental nodules , meaning that the nodules are unrelated to any known underlying disease . it is inappropriate to use the guidelines for patients with a known primary neoplasm that may metastasize to the lungs or for patients with an unexplained fever , in whom a nodule may represent infection . in addition , the guidelines only apply to patients 35 years of age or older , because primary lung cancer is rare in young patients , and this population is at greater risk from radiation exposure compared with older patients . according to the guidelines , no follow - up is recommended for a small nodule ( 4  mm ) in a low - risk patient , because the risk of malignancy is less than 1% ; yet many radiologists incorrectly recommend follow - up for such a nodule , perhaps due to disagreement with the guidelines , medicolegal concerns , or , more likely , lack of familiarity with the details of the guidelines . furthermore , it should be remembered that conservative management of incidentally found lung nodules is often appropriate for very elderly patients and patients with major co - morbid disease and shorter life expectancy . pure ground glass nodules should be approached differently than solid nodules . most small ( < 5  mm ) ground glass nodules probably represent foci of atypical adenomatous hyperplasia ( aah ) , although the differential diagnosis includes infection , inflammation , fibrosis , organizing pneumonia , and hemorrhage ( fig . 7 ) . although aah lesions are felt to be pre - malignant , they tend to grow extremely slowly , and therefore follow - up is not recommended for such small lesions ( < 5  mm ) . on the other hand , a 3- to 6-month follow - up ct may be recommended for a pure ground glass nodule that measures 510  mm ; a persistent nodule likely represents aah or adenocarcinoma in situ ( previously called bronchioloalveolar cell carcinoma ) . these nodules should be followed with serial ct examinations for at least 3 years in order to document long - term stability . for ground glass nodules > 10  mm , a 3-month follow - up examination may be optimal ; persistence suggests the presence of adenocarcinoma in situ or invasive adenocarcinoma . these nodules can be followed to assess for long - term stability , although a more aggressive approach , including biopsy or resection , may be chosen . features that correlate with malignancy include increase in nodule size , increase in overall attenuation , and development of a solid component ( figs . 8 and 9 ) . figure 7small , incidental ground glass nodule ( arrow ) that likely represents a focus of aah . figure 9incidental ground glass nodule ( a , arrow ) grew into a mixed solid and ground glass nodule 4 years later ( b , arrow ) . small , incidental ground glass nodule ( arrow ) that likely represents a focus of aah incidental ground glass nodule ( a , arrow ) grew into a mixed solid and ground glass nodule 4 years later ( b , arrow ) . incidental findings are frequently found on chest ct examinations . although most of these findings are not clinically significant , some may represent incidentally discovered malignancies . detection of incidentalomas may lead to a large number of expensive follow - up examinations , extra radiation in healthy people , unnecessary invasive procedures and patient anxiety . when reporting an incidental finding , the radiologist should indicate the probability of the importance of the finding , and should offer evidence - based guidelines for further work - up or follow - up when necessary .