high - dose - rate ( hdr ) brachytherapy ( bt ) is a successful treatment option for many head and neck ( h&n ) cancer diseases [ 14 ] . in general , plastic tubes are implanted into the target volume followed by computed tomography ( ct ) in order to prepare a treatment plan . an important question in modern h&n bt is the accuracy of dose calculation . nowadays , dose calculation is performed usually using the tg-43 formalism . nevertheless , it is known that results of the tg-43 formalism can be suboptimal in several scenarios , in particular when tissue inhomogeneities , like bony structures and air cavities , are present [ 6 , 7 ] , or finite patient dimensions are taken into account . thus , modern treatment planning systems ( tpss ) might be used to account for these inhomogeneities and provide the user with a more accurate calculated dose . .   gave an overview for which treatment sites and dose rates ( high or low ) vary between tg-43 formalism   and modern algorithms , like monte - carlo ( mc ) , and model - based dose calculation algorithms ( mbdcas ) are to be expected . since h&n treatments are not included in discussion , a retrospective analysis was performed and is presented here . data of h&n bt patients treated by a hdr ir bt source were evaluated , and dosimetry of a commercially available mbdca grid - based boltzmann solver ( gbbs ) [ 913 ] was compared with tg-43 based dosimetry . the patient cohort ( 15 female , 34 male ) were treated with a hdr ir bt afterloader of type varisource ( varian medical systems , palo alto , ca , usa ) between 2001 and 2009 , applying a single fraction dose of 2.5 gy . overall doses varied between 25 and 40 gy depending on the patient . the 2.5 gy single fraction , with 2 fractions per day , dose was chosen to be radiobiologically equivalent with a pulse - dose - rate ( pdr ) regimen of 5 fractions of 1 gy single fraction dose each 2 hours . the mean age of the patients was 57.1 years , standard deviation ( sd ) 12 years , maximum age 86 years , and minimum age 36 years . the diagnoses comprised floor of mouth carcinoma , larynx carcinoma , and parotid carcinoma with varying stages and grading . plastic catheters ( median : 5.2 , sd : 2 , min : 2 , max : 12 ) were implanted . all patients underwent a 3d computed tomography ( ct ) for treatment planning purposes using a picker pq 2000 ct ( philips healthcare , eindhoven , the netherlands ) with 2 mm slice thickness . the patient 's treatment plans were calculated with the brachyvision v8.8 ( varian medical system , palo alto , ca , usa ) software , using the commercial gbbs acuros module ( v1.3.1 ) of brachyvision without reoptimization of dwell weights as well as the tg-43 formalism . for commissioning of the gbbs , level 1 recommendations of the tg-186 report were followed . for calculation in water of a single dwell position , constant dwell time of 10 s , the gbbs algorithm was compared in defined dose points against tg-43 dose distribution of the used varisource 2000 source in 1 and 5 cm distance . the vendor anisotropy function for the vs2000 hdr ir - source was used ; relevant data were published by mikell et al . . low densities ( < 0.001 g / cm ) as well as high densities ( > 8 g / cm ) remained leveled at minimum / maximum values ( brachyvision - acuros algorithm reference guide , varian medical systems , march 2009 ) . dosimetric indices d90 , v100 , and v150 of the ctvs , and the dose homogeneity index ( dhi )   were evaluated and compared between two dose calculation methods . because target delineation was not performed in all cases , for all patients the 100% prescription dose outline of the tg-43 dose calculation was converted into a 3d contour , and used as an artificial clinical target volume ( ctv ) . statistical significance ( p - value < 0.05 ) was analyzed by the two - sided mann - whitney rank sum test . differences between gbbs and tg-43 based doses in orthogonal and semi - orthogonal ( 45 degree ) directions from the source direct axis are in mean 2.1% in 1 cm distance , and 2% in 5 cm distance . in direct axis directions of the source dose differences up to 89.1% were found . the negative signs indicate that doses from the tg-43 were lower than computed by gbbs algorithm . this dose difference was found at a test point 1 cm in distal direction of the source ( direction to the source cable ) . 0.987 gy ( gbbs ) were calculated by the tps in a reference point for a dwell time of 10 s. median ctv volume was 30.0 ml ( sd : 26.3 ml , minimum : 4.1 ml , maximum : 132.1 ml ) . median d90 and v100 for tg-43 calculation are about 3% higher , and statistically significant ( p < 0.01 ) than for gbbs . the v150 dose parameter showed a 1.6% increase from gbbs to tg-43 , also statistically significant ( p < 0.01 ) . nevertheless , these are median parameters and clinical assessment must be performed individually for each patient . the dose parameters d90 , v100 , and v150 showed larger variations for the gbbs than for the tg-43 calculations . dose parameters for the investigated h&n patient cohort showing results for the tg-43 and gbbs based dose calculation in fig . 1 , a typical isodose distribution for both calculation methods is demonstrated and illustrates the influence of the medium to the dose calculation results . it is obvious that the gbbs resulted in a lower dose , in particular in the right anterior region of the head of this patient . this is mainly due to the consideration of air cavities and air outside the head , producing less backscatter in these regions . transversal ct slice of a patient with mandibular cancer with infiltration of the floor - of - mouth and tongue using a single prescription dose of 2.5 gy . dotted isodose lines represent doses of tg-43 formalism , whereas straight lines show isodose lines of computations of the gbbs algorithm dose values of the gbbs algorithm were subtracted from corresponding tg-43 values and are depicted against the volume of the individual ctvs , as it is show in fig . 2 . there seems to be a trend for differences in d90 and v100 to increase when treating larger target volumes . the pearson correlation coefficients are 0.62 for d90 , and 0.64 for v100 . in the v150 this trend is not so clear , with a pearson correlation coefficient of 0.47 . dose differences in percent between tg-43 formalism and the gbbs ( tg-43 minus gbbs ) for d , v , and v of the ctv versus volume of the ctv . dose differences that were measured in our commissioning process were partly discussed in . in particular , high dose differences near the source delivery cable are described by mikell et al . . they observed dose differences of > 20% in between tg-43 and acuros . for analysis they exported the doses and analysed it separately , whereas in our study we only used the tps itself . this as well as slightly different positioning of the source center , with respect to the voxel data , might have caused variations in the results of the dose differences . this study of hdr ir bt for h&n patients showed small variations between doses from gbbs and tg-43 algorithm . however , the differences were about 3% for d90 , and v100 of the ctv between gbbs and tg-43 calculations . there was a trend for larger target volumes to provide higher dose differences than for smaller volumes . poon and verhaegen   compared results from tg-43 calculations with an analytical approach for one h&n bt patient case with two catheters implanted , and found that the isodose lines > 50% of reference dose ( no reference dose was provided ) for tg-43 and their analytical dose calculation algorithm were almost the same . hyer et al .   reported for cervical cancer patients treated with hdr ir bt for the d90 of the ctv a decrease of 1.9% when accounting for heterogeneities . calculated dose differences due to tissue inhomogeneities for oesopharingeal hdr ir bt patients , and showed that presence tissue inhomogeneities do not significantly change the planning target volume ( ptv ) dose distribution . investigations for breast hdr ir bt patients between tg-43 and mc were presented by pantelis et al . , and found isodose contours > 60% of the prescribed dose were not affected by the presence of the lung or finite breast dimensions . the trend for higher dose differences in larger volumes should be assessed with care , because uncertainties in dvh dose calculation are higher for smaller volumes than for larger ones , as was shown by kirisits et al . . the impact on this to volume computation is larger for small structures than for large ones , and might be a reason for the decrease of the v150 differences presented in fig . 2 . in particular , when the higher doses at the distal and proximal end of the source calculated with the gbbs is taken into account ; the shown but as demonstrated in other publications , dose differences are higher when further away from the implant [ 16 , 19 ] . the reason for this is that with increasing distance from the source , the scattered dose proportion is growing . thus , in larger target volumes this effect might be considered as presented in our work . the presented h&n dosimetry study demonstrate complementary data to existing literature on dose distributions of modern treatment planning algorithms for various treatment sites in hdr bt . dose differences between gbbs and tg-43 for hdr h&n bt patients to the target volume were found . differences of about 3% for d90 , and v100 of the ctv between gbbs and tg-43 dose computations were observed . in our clinical practice , prescription doses remain unchanged for high - dose - rate head and neck brachytherapy for the time being .
purposeto compare dosimetry for head and neck cancer patients , calculated with tg-43 formalism and a commercially available grid - based boltzmann solver.material and methodsthis study included 3d - dosimetry of 49 consecutive brachytherapy head and neck cancer patients , computed by a grid - based boltzmann solver that takes into account tissue inhomogeneities as well as tg-43 formalism . 
 3d - treatment planning was carried out by using computed tomography.resultsdosimetric indices d90 and v100 for target volume were about 3% lower ( median value ) for the grid - based boltzmann solver relative to tg-43-based computation ( p < 0.01 ) . 
 the v150 dose parameter showed 1.6% increase from grid - based boltzmann solver to tg-43 ( p < 0.01).conclusionsdose differences between results of a grid - based boltzmann solver and tg-43 formalism for high - dose - rate head and neck brachytherapy patients to the target volume were found . 
 distinctions in d90 of ctv were low ( 2.63 gy for grid - based boltzmann solver vs. 2.71 gy tg-43 in mean ) . in our clinical practice , 
 prescription doses remain unchanged for high - dose - rate head and neck brachytherapy for the time being .