ESTRO 2021 Abstract Book

S1564

ESTRO 2021

potentially increase the time of irradiation due to higher fraction doses. Flattening filter free (FFF) RT has been introduced in the daily clinical practice as it allows a more intense X-ray beam at the center than conventional FF photon rays, and a reduction in beam duration and a better clinical efficiency. Shortening the treatment duration is important, as it allows reducing the intrafraction variability. A feature of the pinnacle treatment planning system allows specifying the desired time of irradiation, while preserving the linac specifications (leaves and gantry rotation speed, maximum MU/degree etc.). Aim of this study is to evaluate the dosimetric performances of FF vs FFF 6MV beam, and for one or two complete arcs of a fixed duration of 60s, for hypofractionated PCa treatments. Materials and Methods We replanified 9 PCa patients originally calculated with two 6MV FF arcs. The PTV were defined as follows: PTV1 = prostate + seminal vesicles + 8 mm; PTV 2 = Prostate + 8 mm (4 mm posteriorly); PTV3 = Prostate + 4 mm (0 mm posteriorly). They were defined using the CHHIP trial volumes (Dearnaley et al, Lancet Oncol 2016), with margins slightly narrow as we performed daily IGRT. The prescribed doses were 48Gy, 57.6Gy and 60Gy in 20 fractions for PTV1, PTV2 and PTV3, respectively. The constraints used were the same of the CHHIP trial, and we also added the constraints described by Martin et al from Newcastle (BR J Radiol 2018) and by the French genito-urinary group (GETUG, Cancer Radioth 2017), as even more restrictive than those of the CHHIP trial (see Table1). For each patient, 4 plans were calculated: Plan A = 2 arc, 6MV FF (600 UM/min); Plan B = 2 arcs, 6MV FFF (1400 UM/min); Plan C = 1 arc, 6MV FFF (1400 UM/min); Plan D = 1 arc, 6MV FF (600UM/min). We used an autoplanning module to calculate all plans, which shared the same isocenter, prescription, objectives, OARs constraints, iteration number etc. PTVs covering is always obtained. After autoplanning a “start optimization” is launched for improving Rectum and Bladder dose

distribution. This plan is kept only if PTVs does not change, if not then only “autoplan” is kept. Results are given as mean dose (+/- 95%CI). The means were compared with an independent t-test

Results Table 2 summarizes the results. We underlined the differences that are statistically significant for Dmean and V15 Gy for rectum and for plan D.

Conclusion A 6 MV FFF single arc could obtain very rapid plans that are dosimetrically at least comparable to plans obtained with two FF or FFF arcs. Plans with a 6MV FF single arc need a compromise between duration of irradiation and dosimetric aspects, and it is not the case for 6 MV FFF single arc. During the congress we will present data about the possible relation between duration of irradiation and modulation index and plan quality (analyses are ongoing). PO-1836 Model based proton therapy patient selection in the context of limited slot availability J. Unkelbach 1 , D. Papp 2 1 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland; 2 North Carolina State University, Mathematics, Raleigh, USA Purpose or Objective We consider a radiotherapy clinic that has a limited number N of proton therapy slots available every day to treat head- and-neck cancer patients. The clinic's goal is to minimize the expected number of complications in the cohort of all head- and-neck patients treated at the clinic, and thereby maximize the benefit of its limited proton resources. Materials and Methods We extend the NTCP-model-based approach to proton patient selection to the situation of limited resources at a given