ESTRO 2020 Abstract book

S664 ESTRO 2020

Hospital Heidelberg, Department of radiation Oncology, Heidelberg, Germany Purpose or Objective During radiotherapy for prostate cancer, interfractional variability results from changes in bladder and rectal position and filling. As a result, the delivered dose may deviate from the planned dose, resulting in reduced dose coverage of the target volumes and increased doses to the organs-at-risk. However, the effects of interfractional variability on dose deviations are largely unknown especially for hypofractionated radiation therapy concepts. Material and Methods For ten consecutive patients receiving intensity- modulated radiotherapy for low- or intermediate-risk prostate carcinoma hypofractionated radiation treatment was planned to atotal dose of 60 Gy in 3 Gy fractions using the commercial TPS RayStation (RaySearch). Patients were instructed to present to treatment with an empty rectum and a comfortably filled bladder; all patients received daily in-room diagnostic CT imaging (fxCT) in treatment position for position verification (total of 200 fxCTs). GTV, CTV, PTV as well as bladder, rectum, sigma and femoral heads were contoured on all fxCTs, and the pelvic anatomy was compared to that on the respective planning CT scans. Applied fractional doses were calculated based on each fxCT, and doses were mapped to the planning CT and accumulated over the course of treatment using deformable registrationPlanned and delivered doses were compared on a voxel-by-voxel basis regarding dose volume parameters and and γ-analysis to a level of 3mm/3%. Results The mean CTV volume was 74 ± 41 ml; the mean volume for the PTV was 140 ± 90 ml. The delivered dose D 50 to the CTV was 0.13 ± 0.57 Gy higher than prescribed with a p- value of 0,13941, while both the D 2 (mean 1,45 ± 3,33 Gy, p-value 0,28450) and the D 98 (mean -0,26 ± 0,57 Gy, p- value 0,72127) were lower. For the PTV the delivered dose D 50 was 0.03 ± 0.47 Gy lower than prescribed with a p-value of 0,72127 and the D 2 (mean -0,43 ± 0,54 Gy, p-value 0,05933) and the D 98 (mean -10.34 ± 7,96 Gy. p-value 0,00506 ) were also lower. The bladder volume showed the biggest interfractional volumetrical variability. On average, the bladder volume ranged at 286 ± 168 ml, resulting in a dose deviation of 1,96 ± 4,8 Gy (D 50 ) with a p-value of 0,11412. The average rectal volume was 76 ± 33 ml, leading to an increase in the median delivered dose of 0.29 ± 2,76 Gy (p-value 0,57506). The γ-analysis to a tolerance level of 3mm and 3 % dose difference resulted in 96,08 ± 3,4 %.

Results With a mean follow-up of 20 months, we estimated that the progression-free survival was 98,7%. Cancer-specific global survival was 100%, while actuarial overall survival was 95,8% None of our patients experienced acute or late grade 3 toxicity. 53,36% had acute genitourinary grade 1 (GU) and 5,53% grade 2 GU. 11,86% reported acute gastrointestinal toxicity (GI) and 1.01% grade 2 GI. Late toxicity grade 1 GU 3,56% and GI 11,86%.

Conclusion Stereotactic body radiation therapy administered with linear accelerator, in patients with low and intermediate risk prostate cancer is effective, achieving good control of the disease and toxicity profile. PO-1172 dose effects due to interfractional variability for hypofractionated radiotherapy of prostate cancer M. Splinter 1,2,3 , I. Sachpazidis 4 , T. Bostel 3,5 , P. Häring 1,2,3 , C. Lang 1,2,3 , A. Grosu 4 , J. Debus 2,3,6 , D. Baltas 4 , N.H. Nicolay 3,4 1 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology- E040, Heidelberg, Germany ; 2 Heidelberg Institute for Radiooncology, National Center for Radiation Research in Oncology, Heidelberg, Germany ; 3 German Cancer Research Center DKFZ, Clinical Cooperation Unit "Radiation Oncology", Heidelberg, Germany ; 4 Freiburg University Medical Center, Department of radiation Oncology, Freiburg, Germany ; 5 Mainz University Medical Center, Department of radiation Oncology, Mainz, Germany ; 6 University

Conclusion The observed anatomical variabilities of bladder, rectum and target volumes resulted in significant dose deviations only of the PTV (p-value 0,00506) at low doses, while both the D 50 and D 2 of the PTV were comparable. For the bladder and the rectum only non-significant dose deviations occurred. Thus in case of daily CT-based repositioning, regular adaptive re-planning may not be necessary in the majority of cases.