ESTRO 2024 - Abstract Book

S2375

Clinical - Urology

ESTRO 2024

Fabian Funer 1,2,3 , Esther G.C. Troost 1,2,4 , Luise A. Künzel 1,2,5

1 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany. 2 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany. 3 National Center for Tumor Diseases (NCT/UCC), Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany, Medizinische Fakultät and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany. 4 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany. 5 Department of Radiation Oncology and Radiotherapy, Charité Universitätsmedizin Berlin (Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health), Berlin, Germany Ultrahypofractionated radiotherapy (RT) is an alternative to normo- or hypo-fractionated RT of localized prostate cancer. With increased dose per fraction, the need of accurate dose delivery is of high importance to guarantee tumor control probability with minimum side-effects. The commonly used endorectal balloon (RB) is considered to be obsolete in magnetic resonance image guided radiotherapy (MRIgRT), since MRIgRT allows for daily treatment plan adaptation. However, the interval between the MRI for daily treatment plan adaptation and the actual dose delivery may be too long to rule out intrafactional changes. Therefore, this study aimed to assess, whether a RB stabilized the position of the prostate and seminal vesicles (SV) during ultrahypofractioned MRIgRT of prostate adenocarcinoma patients. After approval by the Ethics Committee of the Technische Universität Dresden, data of ten intermediate or high risk prostate cancer patients having undergone MRIgRT with an RB at the University Hospital Carl Gustav Carus Dresden on a 1.5T MR-Linac between September 2022 and May 2023 were included in this single institution analysis. The dose to the prostate and SV was 39.20 Gy in 7 fractions with a simultaneous integrated boost to 42.60 Gy on the prostate. Only treatment fractions with three anatomical T2-weighted MR scans consisting of the daily planning (PL), position verification after plan adaptation (PV), and post treatment (PT) were included. After revision of the contours on the PL-MRI, contours for PV- and PT-MRIs were deformably propagated and manually adjusted in the treatment planning system Monaco 5.51.11 (Elekta, Stockholm, Sweden). All session MRs were registered by isocenter. The centre of mass was evaluated for the prostate and SV, and the position deviations between PL-MRI and PV-MRI or PT-MRI were computed. Deviations were evaluated in the left-right (LR, x), superior-inferior (SI, y), anterior posterior (AP, z) directions, and the Euclidean distance (d) was calculated. Cases with d>0.4 cm were visually inspected for potential patient movement, and excluded, if observed. To classify random and systematic errors, the group mean error M and standard deviations (SD) of random, σ, and systematic, ∑, errors were computed. Purpose/Objective: Material/Methods:

Results: