Abstract Book

S1010

ESTRO 37

homogeneous, conformal dose distribution with improved target coverage while sparing dose to OAR. For this aim, intensity-modulated radiation therapy (IMRT) is a proven technique. The enhanced volumetric-modulated arc therapy (VMAT) performs equal or better compared to IMRT. Material and Methods Treatment plans of 228 HNC and 29 prostate cancer patients who received curative-intent IMRT or VMAT between 10/2014 and 09/2016 at the Dept. of Radiation Oncology of the Martin Luther University Halle- Wittenberg were evaluated. The homogeneity (HI), conformity index (CI) and target volume coverage (CVI) was calculated (ICRU report 83) for each treatment plan and compared between IMRT and VMAT. Results A highly significant (p< 0.001) improvement of plan quality (HI, CVI) was determined for head-neck cancer treated with VMAT. A relative HI reduction of 30% was determined for VMAT plans (HI=0.085) compared to IMRT plans (HI=0.12). The CVI was significantly (p=0.031) improved by 2.6% (CVI IMRT: 0.954 vs. CVI VMAT: 0.98). In prostate cancer, statistics showed a significantly improved HI (p=0.031) with the HI being 0.068 for VMAT and 0.087 for IMRT plans, equivalent of a 22% reduced HI with the VMAT technique (p< 0.001). However, the CVI was non-significantly superior (p=0.117) in VMAT plans (CVI=0.987) compared to IMRT (CVI=0.978) in the same tumor entity. Furthermore, VMAT reduced delivery time by 50% compared to IMRT. Conclusion Our results for HNC indicate a significantly superior plan quality (HI, CVI, CI) and a reduced delivery time in VMAT compared to IMRT, minimizing the probability of intra- fractional organ/patient motion. In prostate cancer however, besides superior plan homogeneity in VMAT, further CVI optimization by VMAT is rather difficult due to the fact that CVI is already very high in IMRT prostate plans. An increased patient number in this group is necessary to increase the validity of the comparison of VMAT vs. IMRT. EP-1870 Monitor Units influence on RT/SBRT plans for prostate cancer. N. Cavalli 1 , E. Bonanno 1 , G. Pisasale 2 , N. Ricottone 2 , A. D'Agostino 2 , A. Girlando 2 , C. Marino 1 1 HUMANITAS - CENTRO CATANESE DI ONCOLOGIA, Medical Physics, Catania, Italy 2 HUMANITAS - CENTRO CATANESE DI ONCOLOGIA, Radiation Oncology, Catania, Italy Purpose or Objective In this work the correlation between total monitor units (MUs) constraints, dosimetric findings and pre-treatment verifications has been studied for prostate (SBRT and conventionally fractionated regimen) VMAT plans. Material and Methods A retrospective analysis on 10 conventionally fractionated and 5 SBRT prostate treatments has been performed. Plans were been accomplished with Varian Eclipse treatment planning system (version 13.6.23) and optimization was performed using the Photon Optimizer (PO 13.6.23) on TrueBeam TM 2.5. The so called MU Objective tool implemented in RapidArc optimization engine was used. This tool allows setting the range and the relative priority to the minimum and maximum number of MU employed for a plan optimization. Dose prescription was 76 Gy in 38 fractions and 35 Gy in 5 fractions for SBRT plans, using respectively 10MV and 10MV-FFF photon beams. Plan objectives were set according with QUANTEC and AAPM TG 101. All plans were calculated with and without constraints on MUs. For each case the 2 plans were compared in terms of DVH objectives, MUs, modulation complexity coefficient

(MU/Gy) and pre-treatment quality assurance performed with the EPID and Epiqa software. Results For conventionally fractionated plans average MUs were 562.4 and 587.55, respectively for plans with and without MUs constraints; modulation complexity coefficient were 281.7 MU/Gy and 293.8 MU/Gy. Similarly, for SBRT plans average MUs were 973 and 1120; modulation coefficient were 277.5 MU/Gy and 320 MU/Gy, respectively for plans with and without forced MUs. For 2 Gy/fraction calculated plans, no relevant discrepancies have been found in terms of DVHs (for PTV and OARs) between plans with and without MU constraints. Differences in plans quality have been found in terms of DVH for SBRT plans. In fact, for 3 of the 5 analyzed case, the use of “MU objective” involve a better PTV coverage and a reduced dose to OARs. Pre-treatments verifications showed a better agreement between calculation and delivery for all SBRT plans obtained forcing MUs. Tab.1 shows pre-treatment quality assurance results.

Tab. 1 Conclusion

Any relevant difference has been detected for conventionally fractionated plans in terms of DVH and pre-treatments verification. Results suggest that the use of MU Objective doesn’t invalidate plan quality. However, important discrepancies have been found (for both SBRT and conventional fractionation) in terms of total MUs and of modulation complexity coefficient, that are considerably reduced for plans obtained forcing the MUs. SBRT plans obtained using forced MUs reveals an increasing of GAI value associated with MUs reduction. In our opinion, for SBRT treatments further studies are necessaries on a wider sample of cases to understand if minimizing modulation (i.e. decreasing total MUs) could be a good solution for all SBRT treatment plans. EP-1871 Development of a class solution for prostate VMAT planning using Pareto Front comparison M. Pirotta 1 1 Sir Anthony Mamo Oncology Center, Radiotherapy Department, Msida, Malta Purpose or Objective The purpose of this planning study was to determine optimal VMAT planning parameters values for a prostate