ESTRO 36 Abstract Book

S183 ESTRO 36 2017 _______________________________________________________________________________________________

of 41.4Gy (preoperative RT) or 50.4Gy (definitive RT). For IMRT/VMAT, a DO strategy (i.e., assigning mass density to gas pockets in the pCT) with three settings was used: no DO (denoted as DO=0), DO=0.5, and DO=1 (equivalent to an adipose-muscle mixture), resulting in 6 plans per patient. Next, by copying the gas pockets derived from the available CBCT to the pCT, a fractional CT was simulated to calculate the fractional doses using all 6 plans. DVH parameters of the CTV and organs at risk (OARs) were compared between 1) the three DO settings, 2) IMRT and VMAT, and 3) fractional and planned dose. Dose distribution difference in the CTV between fractional and planned dose was also compared. Results The range of initial gas volume measured in the pCT was 56–732ml. The gas volume fluctuated over the treatment course with no time trend (range of mean: 33–519ml, range of standard deviation: 20–162ml). For the fractional dose, V 95% of the CTV was always >98% for VMAT but not for IMRT with DO=0 (Fig.1). For both IMRT and VMAT, DVH parameters of the CTV were significantly larger for DO=1 than for DO=0 and 0.5 ( p <0.05, Wilcoxon signed-rank test). For an increasing gas volume, an overdose (>3.5% higher than the planned dose) in the CTV was found in 72–88%/64– 77% cases for IMRT/VMAT with all three DO settings. The amount of overdose increased as the gas volume increased relative to the initial volume and was >5% when the increase was >100ml (Fig.2). For a decreasing gas volume, an underdose (>3.5% lower than the planned dose) in the CTV was found for IMRT/VMAT in 34%/23% cases with DO=0, 7%/0% cases with DO=0.5, and 0%/0% cases with DO=1. The underdose became more severe as the gas volume decreased for DO=0 and 0.5. An overdose (>3.5%) still existed in up to 28% cases for DO=1 when the gas volume decreased. DVH parameters of OARs in the fractional dose were almost the same as in the planned dose and below the clinical constraints for all scenarios. Conclusion For esophageal cancer RT, the use of VMAT with DO=0.5 in treatment planning is preferable to avoid an overdose/underdose in the CTV when the abdominal gas volume decreases during treatment. However, when the gas volume increases with >100ml, a DO strategy would result in an overdose >5%. Therefore, in that case re- planning may be a better solution.

OC-0349 Prediction of GTV median dose differences benefit Monte Carlo re-prescription in lung SBRT D. Dechambre 1 , Z.L. Janvary 1 , N. Jansen 1 , C. Mievis 1 , P. Berkovic 1 , S. Cucchiaro 1 , V. Baart 1 , C. Ernst 1 , P. Coucke 1 , A. Gulyban 1 1 C.H.U. - Sart Tilman, Radiotherapy department, Liège, Belgium Purpose or Objective The use of Monte Carlo (MC) dose calculation algorithm for lung patients treated with stereotactic body radiotherapy (SBRT) can be challenging. Prescription in low density media and time-consuming optimization conducted CyberKnife centers to propose an equivalent path length (EPL)-to-Monte Carlo re-prescription method, for example on GTV median dose (Lacornerie T, et al. Radiat Oncol 2014;9:223). The aim of this study was to evaluate the differences between the two calculation algorithms and their impact on organs at risk (OAR) and to create a predictive model for the re-prescription. Material and Methods One hundred and twenty seven patients (with 149 lesions) were treated with CyberKnife (CK; Accuray, Sunnyvale, US) between 2010 and 2012. A high-resolution grid (512³) was used for the EPL and MC calculations (2% variance). All re-calculation from EPL to MC maintained the number of beams and their monitor units. Relative differences in GTV D50 between the two algorithms were assessed and uni/multivariate linear regression was performed using prescription dose (Gy), tracking (ITV concept if not available), location (peripheral or central) and volume (in cc) of the lesion as input parameters. Statistical significance was determined using F-test at p-value<0.05. OARs volumetric dose constraints were applied from Timmerman RD et al. (Semin Radiat Oncol 2008;18:215- 22). As tolerance limits were defined based on simple heterogeneity correction algorithm (e.g. EPL), correlation between EPL and MC OARs dose values was assessed following the work from the Rotterdam team (van der Voort van Zyp NC , et al. Radiother Oncol 2010;96:55–60). Results The observed difference (MC compared to EPL) varied from 0 % to 48% (median = 10%, standard deviation = 9%).