ESTRO 38 Abstract book

S995 ESTRO 38

1 UMCG University Medical Center Groningen, Radiotherapy, Groningen, The Netherlands

Purpose or Objective Extreme hypofractionated radiotherapy is an efficient and convenient treatment option for prostate cancer (PCa) patients, mostly delivered by stereotactic body radiotherapy (SBRT). Recent improvements in proton planning techniques like robust proton pencil beam scanning (PBS) has the potential to deliver the same quality hypofractionated plans as SBRT with much lower exposure dose to the surrounding tissues. The purpose of this study is to investigate the feasibility of delivering clinically acceptable extreme hypofractionated proton plans. Material and Methods Planning CT scans of 4 representative PCa patients treated at our department were used in this study. The organs at risk (OARs) (rectum, anal canal, urethra and bladder) and the CTV (prostate) were delineated by a radiation oncologist according to our SBRT protocol. For each CT scan, 4 plans were created: two fractionation regimen (4x9.5 Gy and 5x7.5 Gy) each having two different setup margins (3 and 5 mm). Treatment plans were robustly evaluated for the corresponding setup and ±3% density changes. The considered protocol for target coverage and OAR constraints is listed in Table 1. For proton planning robust optimization and Monte Carlo based dose calculation in Raystation v6.99 was used. Target coverage was optimized without violating the OAR dose constraints. Clinically acceptable plans were those which met the protocol constraints with minor violation of max 5%. The endpoint of the study was percentage of acceptable plans for each fractionation regimen and each robustness setup margin.

Conclusion Both fractionation regimen of 4 and 5 fractions met the dose constraints and are considered clinically feasible if 3- mm setup margin are used. The target coverage was suboptimal in most plans for 5 mm setup margin. A larger cohort of patients is needed to confirm our results. EP-1835 Use of the gEUD in modern TPSs for prostate radiotherapy with VMAT tecnique L. Grimaldi 1 , V. Morandini 1 , S. Berlinghieri 1 , E. Castrezzati 1 , P. Frata 1 , N. Pasinetti 1 1 Esine Hospital, Radiation Oncology Service, Esine, Italy Purpose or Objective Aim of this study is to find a starting point for gEUD parameters in the Philips Pinnacle 3 TPS to reach a comparable dose distribution to target and to maximaze the OARSs sparing irradiation in a normo-fractionated prostate RT schedule (74 Gy-2 Gy/fr). Material and Methods For 10 patients treated with prostate radiotherapy, two RT plans were optimized using VMAT technique. For the first plan only physical parameter were used, instead for the other one, the biological gEUD parameter were replaced to all OARs. The TPS was Philips Pinnacle 3 v9.10. Results For the targets volumes the two plans show an equivalent dosimetry in term of Conformity Index (COIN=(V T,pi ) 2 /(V T *V pi ), Homogeneity Index (HI=100*(D 2% - D 98% )/D pi ) and Gradient Index (GI=V 50% /V 100% ). Instead for the principals OARs: the femoral heads don't put in evidence substantial differences, at absorbed doses of 30- 40 Gy, the rectum volumes involved are halved and for bladder the reduction of absorbed medium doses are of about 30%. There isn't a significant reduction for absorbed dose of 65-70 Gy. To optimize a VMAT plan by using the Pinnacle 3 radiobiological function and obtain a dose distribution comparable to that based on a physical optimization, the following parameters have been set:

Results The target coverage was acceptable in 7/8 plans with 3- mm margin, while only 3/8 plans (two in 4-fraction and one in 5-fraction regimen) with 5 mm margin were acceptable. Bladder constraints were acceptable in all plans with 3-mm margin, and in only 4/8 when 5-mm plans were used. In two patients, dose constraints were met in not any plan. All rectum constraints met the protocol constraints in both setup margin groups. There were no significant differences in target coverage and OAR constraints between 4 and 5 fractions regimen.