breast - conserving therapy ( bct ) , including breast - conserving surgery ( bcs ) and postoperative radiotherapy ( rt ) , has become a standard treatment in early stages ( t1 , t2 ) breast cancer for the last 30 years [ 1 , 2 ] . breast - conserving therapy was determined as primary treatment at the national institutes for health consensus development conference in 1990 , for tumours up to 4 cm . many long - term studies confirm that breast - conserving surgery followed by radiotherapy results in outcomes comparable to mastectomy , allowing women to preserve their breast [ 1 , 48 ] . the standard postoperative rt after bcs consists of external beam radiotherapy of the whole breast ( whole breast irradiation  wbi ) using two tangential fields up to a total dose of 45 - 50 gy . a significant role in reducing a risk of local recurrence rate is performed by an additional boost treatment to the tumour bed up to a dose of 10 - 25 gy [ 913 ] , although choice of an optimal technique of boost treatment between brachytherapy , photons beams or electrons remains controversial . this issue is a case of many studies , but none of them has definitely shown the superiority of any of these techniques [ 9 , 10 , 1419 ] . an advantage of brachytherapy is the delivery of a high dose in limited volume , due to a large dose gradient , minimizing the treated volume to the tumour bed and it s immediate surroundings , resulting in limitation of post radiation injuries . moreover , in case of brachytherapy , a dose intensity measured in minutes is higher than that of external radiotherapy measured in days , what increased a biological effectiveness against resistant tumour cells . a role of brachytherapy as a boost increases especially in case of lesions deeply seated in breast [ 2224 ] and modern , image - based brachytherapy planning systems allow for an assessment not only of a dose at a certain point , but doses received by respective volumes of structures . it is a huge advantage enabling to avoid a single point dose estimate uncertainty caused by a large dose gradient of brachytherapy sources . according to american brachytherapy society ( abs ) recommendations it is required to monitor maximum dose received by a skin , however , there is no unambiguous way of skin contouring provided [ 26 , 27 ] . there are two most often ways used to define maximum skin dose . the first way represents it as a dose at a point on the body surface ( determined by external patient contour ,  external  ) [ 2835 ] . the second one defines maximum skin dose as the maximum dose to a relatively small volume ( 0.1 - 1 cc ) in the structure created by expanding an external patient contour by 5 - 10 mm [ 3638 ] . the structure defined this way is in fact placed outside the body and doses calculated should be treated as a dose at surface rather than a dose in volume . more corresponding to the anatomical model of skin is a structure made by reduction of external patient contour by some value representing breast skin thickness [ 39 , 40 ] . there are many studies focused on measuring breast skin thickness , both healthy and changed by disease or after radiotherapy [ 4147 ] . it has been shown that in case of tumour or when radiotherapy was applied , the skin is thickening [ 4143 , 46 , 47 ] . according to different studies , mean skin thickness in normal breast varies in range of 1.44 - 2.05 mm , while in breasts after radiotherapy  1.44 - 2.68 mm [ 4143 , 46 , 47 ] . this study compares doses received by different models of skin of 20 patients , who underwent interstitial brachytherapy as a boost in breast - conserving therapy . twenty patients with recognized breast cancer and treated with radiotherapy after breast - conserving surgery were examined . all patients underwent external beam radiotherapy ( wbi ) up to a dose of 45 - 54 gy and interstitial brachytherapy hdr as a boost . patients were treated with remote afterloading microselectron v2 hdr ( nucletron , an elekta company , elekta ab , stockholm , sweden ) using an ir stepping source with nominal activity of 10 ci . interstitial brachytherapy procedure were performed under local anaesthesia . from 4 to 9 metal needles in 1 or 2 planes ( 3 patients  1 plane , 17 patients  2 planes ) all patients with interstitial implant were scanned using ct scanner somatom sensation open ( siemens medical solutions inc . the ct images were sent to the brachytherapy planning system oncentra masterplan ( version 4.3 , nucletron , an elekta company , elekta ab , stockholm , sweden ) , where structures were outlined and treatment plan was made . planning target volume ( ptv ) and organ at risk : skin , most exposed rib , and ipsilateral lung were contoured . for this examination , ptv was defined by interstitial implant reaching 5 mm from needles in orthogonal plane and separated by 5 mm from the body surface , because the purpose of this study is not the evaluation of optimal treatment plan , but the examination of differences between doses received by skin based on the way of contouring of this structure . application of the standard 2d planning paris dosimetry system was perform to all patients , enabled invariable conditions for the entire analysed group . the skin was outlined in three different ways . the first model of skin , made by expansion of external patient contour by 4 mm , was named skin ext ( ext = external ) for the purpose of this study . it was created by an addition of a 4 mm bolus to the external contour . the second model , skin 2 mm , corresponds to the dermatological breast skin thickness . it was reaching 2 mm into an external patient contour and was created by contouring 2 mm inner margin of an external contour . the third model of skin , skin 4 mm , was made by inserting a 4 mm bolus onto the same contours as skin 2 mm ( external contour reduced by 2 mm margin ) . all models of skin were spatially limited to the treated breast in order to avoid a shift in dose - volume histograms ( dvh ) , caused by a large volume of this structure receiving only a insignificant dose . an evaluation of doses received by skin 2 mm was not performed in this study , because of an incorrect estimation of dose distribution for this structure , due to computing limitations of the planning system . during ct scanning , breasts of all patients were very tightly compressed by templates , causing a large contour gradient in proximity of templates . during the outlining of skin 2 mm , a certain discontinuity occurred in this structure ( despite using the biggest available accuracy  structure set grid  1 mm ) in some particular configurations of templates relative to ct scans ( parallel or slightly sloped ) . the dose distribution was not calculated in these parts , so dose parameters for skin 2 mm did not correspond with the actual dose . in this connection , the analysis was done only for skin ext and skin 4 mm , both 4 mm wide . figure 2 shows the problem with skin 2 mm . a presentation of different skin models . skin ext ( orange ) = external + 4 mm , skin 2 mm ( yellow ) = external  2 mm , skin 4 mm ( green ) = external  2 mm a comparison of skin 2 mm ( yellow ) and skin 4 mm ( green ) in the same case . skin 2 mm with a certain discontinuity in places where template compressing the breast caused high contour gradient . in case of skin 4 mm , this problem does not occur all treatment plans were done according to paris dosimetry system . usage of a breast template enables an obtainment of a perfect implant , composed of parallel and equidistant needles ( distance between needles dose is specified at basal points , defined in the central plane of an implant , and placed between needles at points of lowest dose . in case of a single plane implant , basal points are placed midway between each pair of needles . in case of two or more planes , needles create set of equilateral triangles and basal points are in the center of gravity of each triangle . a reference dose is defined as 85% of basal dose in order to gain a reference isodose 5 mm from external needles . all treatment plans were optimized by geometrical distance or volume optimization , depending on type of interstitial implant ( one or two planes ) . a normality of the dose distribution was checked for all volumes of both skin models using the shapiro - wilk test . these variables were compared using the wilcoxon signed - rank test , while normally distributed variables ( skin d0.1cc , d1cc , d2cc ) were compared using the student s t - test for dependent samples . a difference between two variables was considered statistically significant when the p - value was less or equal than 0.05 . in all cases , minimal coverage of 90% of the prescription dose v90 exceeded 95.98% in all cases ( mean  97.94% ) . mean coverage of 100% of the prescription dose v100 was 91.83% ( 85.26 - 97.41% ) . mean volume of ptv encompassed by 150% and 200% ( v150 , v200 ) of the reference isodose was 30.98% ptv ( 8.10 cc ) and 15.47% ptv ( 3.99 cc ) , respectively . mean minimum dose at ptv ( d100 ) was 76.46% ( 62.83 - 87.65% ) , while mean value of a parameter more relevant for bt mean dose at 0.1 cc of the most exposed rib was 17.18% of the reference dose ( 9.86 - 33.10% ) , and mean dose at 2 cc of ipsilateral lung was 13.45% ( 9.28 - 22.88% ) . mean , median , minimum , maximum , and standard deviation of dose at 0.1 cc for skin ext and skin 4 mm were , respectively : 63.01% , 62.59% , 51.79% , 79.42% , 6.76% for skin ext , and 81.02% , 78.79% , 65.40% , 98.62% , 9.49% for skin 4 mm . parameters of target and oar dose distributions according to the abs recommendations , the values of ptv volumes encompassed by high doses v150 and v200 are given both as percentage and in absolute values . table 2 contains doses received by most exposed 0.1 cc , 1 cc , 2 cc , and the maximum dose for both models of skin . table 3 contains mean percentage differences between doses received by particular volumes of both models of skin , skin ext and skin 4 mm . a bold line on every figure points at the value of mean difference between doses received by both models of skin and equals , respectively : 50.35% ( 20.09 - 127.47% ) for maximum dose , 18.01% ( 13.27 - 27.84% ) for d0.1cc , 11.22% ( 6.69 - 14.35% ) for d1cc , and 9.78% ( 5.33 - 11.75% ) for d2cc . a bold , horizontal line at 18.01% indicates a mean percent difference percentage difference in d1cc of skin between skin ext and skin 4 mm . a bold , horizontal line at 11.22% indicates a mean percent difference percentage difference in d2cc of skin between skin ext and skin 4 mm . a bold , horizontal line at 9.78% indicates a mean percent difference percentage difference in dmax of skin between skin ext and skin 4 mm . a bold , horizontal line at 50.35% indicates a mean percent difference doses received by different skin volumes for two skin models percentage difference in doses received by skin ext and skin 4 mm for particular skin volumes the development of brachytherapy planning systems based on a three - dimensional imaging brings in many previously unavailable possibilities . image - based planning allows for an outlining of the planning target volume , and creation of a treatment plan based on the ptv instead of implementing standard dose distributions related to the geometry of applicators or interstitial implant . 3d - planning enables also a precise dose evaluation in defined volumes of organs at risk instead of an evaluation of a point dose . doses for the most important oars in the breast brachytherapy ( rib , lung , and skin ) have to be monitored according to abs recommendations . for a skin a few millimetre shifts may result in dose difference of a dozen up to several dozen percent , due to a large dose gradient in brachytherapy . huang et al . used breast ct acquisition techniques , combined with algorithms based on monte carlo method , designed for determining specific breast skin thickness metrics . he has examined 51 patients , determining mean breast skin thickness as 1.45 ( 1.0 - 2.2 mm ) among the cancer - free breasts , 1.53 mm ( 1.2 - 1.9 mm ) among those with benign findings , and 1.46 ( 0.9 - 2.3 mm ) among breasts with biopsy - confirmed breast cancer . , analysed 137 tomograms of breasts , both healthy and changed by a disease , determined breast skin thickness as 1.44 mm ( 0.87 - 2.34 mm ) and also no statistically significant difference between these groups . liu s ultrasound scanner research has shown a significant breast skin thickness difference between healthy and treated by radiotherapy breasts . he has noted mean skin thickness increased by 27.3% : from 2.05 mm ( 1.66 - 2.38 mm ) in untreated breasts to 2.61 mm ( 1.53 - 3.65 mm ) in treated breasts . a comparison of skin thickness in healthy breasts and those after radiotherapy was also made libshitz et al . according to his research , skin thickness in breasts after radiation treatment returned to normal ( defined as equal to the thickness of the healthy breast ) in 2 years for 60% of the patients , in 3 years for 80% of the patients , and in 4 years for all the patients . warszawski et al . also used ultrasonography for measurements comparing healthy breasts to breasts that undergone radiotherapy . their measurements of corium ( the deep vascular inner layer of the skin ) thickness for non - irradiated breast was on average 1.68  0.3 mm ) , increasing to the maximum value of 2.68  0.7 mm ) after 3 months ( early reactions ) , and then decreased to 2.31 mm (  0.9 mm ) for late reactions ( 6 - 8 months later ) . basing on these studies , it seems reasonable to define mean breast skin thickness for irradiated patients as 2 mm . this thickness was assumed by gifford and his co - workers in their study comparing the doses received by skin in differently defined models in case of apbi brachytherapy provided by contura and savi balloons . gifford analysed a group of 70 patients and compared the doses received by the most exposed 0.1 cc ( d0.1cc ) of skin defined by two models . the first model , 2 mm thick , was created by 2 mm contraction of an external patient contour . the second one , named  ext + 5 mm  was also based on the external patient contour , but created by expanding this structure 5 mm outside . mean , median , maximum , and standard deviation of percentage difference between these two models for the whole examined group were 12.5% , 12.5% , 23.8% , and 3.6% , respectively . taking into consideration the treatment delivery method , smaller differences were recorded for contura balloon ( senorx , inc . , aliso viejo , ca , usa ) : 10.1% , 10.9% , 14.1% and 2.6% , respectively . the differences between doses for patients treated by savi applicator ( cianna medical , aliso viejo , ca ) were 14.4% , 14.4% , 23.8% , and 3.1% , respectively . our study compares the doses received by skin 4 mm , defined as 2 mm inside and 2 mm outside of the external contour , and skin ext , defined as a structure based on the external contour and expanding 4 mm outside . skin 4 mm replaced skin 2 mm , created earlier and representing actual breast skin thickness , which was not representative , because of discontinuities in its structure . however , skin 4 mm may successfully constitute an equivalent of two - millimetre skin used by gifford , because both of them were based on the same contour . d0.1cc , d1cc , d2cc , and dmax compares exactly the same volume inside the external patient contour both for our model and gifford s . moreover , skin ext can be an equivalent for  ext + 5 mm  from gifford s study , so the percentage difference between skin 4 mm and skin ext can be compared . mean , median , maximum , and standard deviation of percentage dose difference for the most exposed 0.1cc ( d0.1cc ) of skin in our study were 18.01% , 17.20% , 27.84% , and 4.01% , respectively . it must be remembered that the group examined by us underwent interstitial brachytherapy with needles stabilized by a template compressing the breast . the planning target volume was not limited to the tumour bed , but was defined by the needle implant on the full length of the needles preserving the 5 mm margin from external patient contour . probably extreme active source positions in needles were placed closer to patient s skin than in the case of balloons . it means that in our case both models of skin were in a region of higher dose gradient . this study compares also the maximum dose ( dmax ) and the doses for the most exposed 1 cc ( d1cc ) and 2 cc ( d2cc ) of skin . mean percentage difference between skin 4 mm and skin ext for dmax , d0.1cc , d1cc , and d2cc decreases with the increase of measured volume and was 50.35% , 18.01% , 11.22% , and 9.78% , respectively . calculations of dmax is based on single voxels , so this might not be authoritative . hence , the dose for the most exposed 0.1 cc ( d0.1cc ) is often treated as the maximum dose in brachytherapy . a comparison of different breast skin contouring methods was made by berger using a self - made phantom by inserting 7 needles in two planes . in his study he created three skin models contoured from the external contour inwards to the depth of 1 , 2 , and 3 mm , respectively , and three skin models based on the same contour , but extended 5 , 10 , and 15 mm outside of the phantom surface . all these structures were outlined in oncentra masterplan by means of an automatic contouring and transferred to the plato system . the values from both planning systems were compared to the real volumes of all six structures calculated , based on the dimensions of the phantom . in case of the inside structures , only the 5 mm one was within the tolerable 5% accuracy , whereas the models defined outside the phantom showed high accuracy of contouring . a similar tendency of discontinuities in structures of small thickness was found in our study ( skin 2 mm ) . in order to avoid unstable and uncertain parameters of histogram for structures inside the phantom , berger suggested to choose the structures outlined outside the phantom . moreover , berger   considered the analysis of dose - surface histograms ( dsh ) , a better way to report skin dose than the dose - volume histogram ( dvh ) . only the position of the contour directed on the external patient contour is relevant in case of skin models outlined to outside , if an appropriate thickness is chosen and a histogram parameters for these skin models can be a good approximation of surface dose . berger analysed dmax , d0.1cc , d1cc , and d10cc for these skin models , but skipped the evaluation of doses received by the inside skin models . according to turesson and notter van limbergen noticed that 97% of the skin blood vessels , that are concerned by this complication , are located within the first 5 mm below the surface of the skin . these vessels receive about 40 - 80% dose from external beam radiotherapy ( 20 - 40 gy ) . the dose received by skin as the result of an additional brachytherapy , the results of this study suggest that it should be considered to introduce an additional skin model inside patient body and 5 mm thick . we have to remember that dose calculation algorithms in commercially available treatment planning software are still performing the calculations of the dose based on the tg-43 report , assuming that the radiation source is in the middle of a homogeneous phantom with a radius of 15 cm and full backscatter . in practice , the difference in density is especially observable near the body limits ( tissue - air ) . karaiskos et al . analyzed the difference in doses estimated theoretically and experimentally for points near the border of a water phantom containing a radiation source  192ir . compared breast doses computed by a treatment planning system with measurements based on the monte carlo method , and noticed that the doses for volumes encompassed by dose larger than 60% do not change , because of the proximity of a skin border or the presence of a lung . however , treatment planning systems overestimate the skin dose by 5 - 10% at points near body surface or lung and relatively far from implant compared to a monte carlo calculation . monitoring of doses received by this structure is necessary to avoid complications and to obtain a satisfactory cosmetic effect . it is difficult to assess the compatibility of skin doses of treatment plans with recommendations , while there is no unambiguous way of skin contouring provided . especially , if a mean difference of doses between two models of skin contouring is 18% ( 1.8 gy ) for the most exposed 0.1 cc and can reach almost 28% ( 2.8 gy ) in some cases . in order to evaluate the clinical usefulness of both skin models it would be necessary to analyze the actually used treatment plans for a larger group of patients and a much longer observation time is essential to assess the cosmetic effect after the radiotherapy .