S1249
Clinical - Urology
ESTRO 2026
demonstrated a clear and consistent increase in prostate volume across the course of five treatment fractions, suggesting a measurable and progressive interfraction anatomical change during therapy (Table 1). The prostate volume increased from 49.25 cc at baseline to 64.16 cc by fraction 5, corresponding to a total relative increase of approximately 30% over the treatment period.
Conclusion: Preliminary results from the PRO-SPEED trial confirm that CyberKnife-based UH-SBRT with a DIL boost is a safe, feasible, and well-tolerated option for localized prostate cancer, achieving excellent early safety and biochemical outcomes. However,longer follow-up and further evaluation of patient-reported outcomes are needed to confirm its long-term efficacy and safety. Keywords: Prostate cancer, SBRT, CyberKnife Prostate contour variability during MR-guided Radiotherapy and its dosimetric consequences Jayna Chauhan 1 , Robert Stevenson 1 , Asad Khan 1 , Ben George 2 1 MRLinac Radiotherapy, GenesisCare UK, Guildford, United Kingdom. 2 MRLinac Radiotherapy, GenesisCare UK, Oxford, United Kingdom Purpose/Objective: Stereotactic body radiotherapy (SBRT) for prostate cancer delivers high-dose, hypofractionated treatment with steep dose gradients. It is well established that the prostate gland can increase in volume during the course of SBRT [1, 2, 3], potentially due to treatment- related oedema and inflammation. However, the dosimetric impact of these volumetric changes has not been fully characterised, particularly in the setting of adaptive SBRT delivered on a magnetic resonance– Digital Poster 3095 linac (MRL). Our purpose was to investigate how prostate volume changes during the course of adaptive SBRT on the MRL and to evaluate the dosimetric implications of these changes on target coverage. Material/Methods: Daily MR-guided adaptive plans from 20 patients treated with 5-fraction prostate SBRT were retrospectively analysed. Changes in prostate volume were quantified, and their effect on target dose coverage will be assessed. To minimise contouring variability, all prostate structures were re-outlined by a single consultant radiation oncologist ensuring consistency and reliability across all cases. Results: Preliminary analysis of one representative patient
When analysing the data it was evident that as prostate volume increases, the Prostate+3mm coverage (D98%) decreases significantly. For this index patient, a 30% increase in the prostate volume resulted in 17% decrease in Prostate+3mm D98%; The coverage to the prostate gland itself decreased by 10%. Conclusion: In conclusion, we have found that without re-planning or adaptive radiotherapy, parts of the prostate— especially the peripheral gland may be underdosed, potentially reducing treatment effectiveness. There appears to be a direct link between swelling during radiotherapy and its impact on the target coverage. References: 1. Antti Vanhanen, Petri Reinikainen & Mika Kapanen (2022) Radiationinduced prostate swelling during SBRT of the prostate, Acta Oncologica, 61:6, 698-704, DOI: 10.1080/0284186X.2022.20626822. S.E. Alexander, H.A. McNair, U. Oelfke et al (2022) Prostate volume changes during extreme and moderately hypofractionated magnetic resonance image-guided radiotherapy, Clin Onc 34 e383-3913. J.J. Shi, A. Yen, S. Wang et al (2024) dosimetric Comparison between Adaptation Strategies for MRI-Guided Dose-Intensified Prostate Stereotactic Ablative Radiotherapy, Int J Rad Onc, Biol, Phys. 120(2)S1 e584-585 Keywords: Dosimetery, Contouring, MR-Linac
Digital Poster 3138 Five-Fraction Linac-based Stereotactic
Radiotherapy (SBRT) for Localized Prostate Cancer: Results from an Observational Prospective Study Claudia Grondelli, Federico Colombo, Marco Galaverni, Francesco Salaroli, Cristina Dell'Anna, Ilaria Renna, Maria Luisa Bergamini, Giovanni Ceccon, Elisabetta Lattanzi, Stella Gianni, Caterina Ghetti, Livia Ruffini, Umberto Maestroni, Nunziata D'abbiero, Nicola Simoni Department of Radiation Oncology and Radiosurgery, University Hospital of Parma, Parma, Italy
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