ESTRO 2025 - Abstract Book

S4369

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

[2] De Hertogh O, Le Bihan G, et al. Consensus Delineation Guidelines for Pelvic Lymph Node Radiation Therapy of Prostate Cancer: On Behalf of the Francophone Group of Urological Radiation Therapy (GFRU). Int J Radiation Oncol Biol Phys, Vol. 118, No. 1, pp. 29−40, 2024 [3] Sheng Y, Li T, et al. Exploring the Margin Recipe for Online Adaptive Radiation Therapy for Intermediate-Risk Prostate Cancer: An Intrafractional Seminal Vesicles Motion Analysis. Int J Radiation Oncol Biol Phys, Vol. 98, No. 2, pp. 473-480

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Digital Poster Volume changes in prostate cancer patients during the course of MRI-guided radiotherapy

Sanne Conijn, Karolina Menne Guricova, Tomas Janssen, Uulke A van der Heide Radiotherapy, THE NETHERLANDS CANCER INSTITUTE, Amsterdam, Netherlands

Purpose/Objective: During MRI-guided radiotherapy (MRIgRT) volume change and deformation of the prostate are visible [1]. Volume change can be due to for example dose, number of fractions and hormonal therapy. The aim of this study was to determine factors with a significant influence on the prostate volume during MRIgRT and whether longitudinal volume changes require online adaptation. Material/Methods: Patients treated on the Unity MR-linac (Elekta, Stockholm, Sweden) between January and September 2024 were retrospectively included. The CTV was delineated on the planning MRI by a radiation oncologist. During each fraction, RTTs could adapt the CTV based on the daily anatomy. The prostate volume over the course of treatment was studied as a fraction of the prostate volume pre-treatment. Linear mixed effect modeling was used (Rstudio) to determine if number of fractions, dose to the GTV and initial prostate volume associate with relative volume changes. The standard deviation of the volume over the first 5 treatment fractions was considered as a metric for contouring variability and intrinsic radial uncertainty. Results: We included 189 patients, 52-85 years old. 36 patients received 20x3.10 Gy, all with hormonal therapy. The remaining 153 patients received 5 fractions with a dose of 7-7.25 Gy to the CTV, and 62 patients received a GTV boost of up to 10 Gy, 38 patients received hormonal therapy. The mean prostate volume was 44 cm 3 (range 17 - 95 cm 3 ), with a standard deviation of 1.6 cm 3 over the first 5 fractions, corresponding to a standard deviation in radius of about 0.2 mm. Volume changes were small on average, but hormonal therapy and fraction number had a significant influence on the volume change of the prostate (Figure 1, Table 1).