S1293
Clinical - Urology
ESTRO 2026
A, Naismith O, Mayles H, Snelson N, Hassan S, Brown S, Porta N, Griffin C, Hall E. PIVOTALboost: A phase III randomised controlled trial of prostate and pelvis versus prostate alone radiotherapy with or without prostate boost (CRUK/16/018). Clin Transl Radiat Oncol. 2020 Sep 1;25:22-28. doi:10.1016/j.ctro.2020.08.003. 2. Groen VH, Haustermans K, Pos FJ, Draulans C, Isebaert S, Monninkhof EM, et al. Patterns of Failure Following External Beam Radiotherapy With or Without an Additional Focal Boost in the Randomized Controlled FLAME Trial for Localized Prostate Cancer. Eur Urol. 2022 Sep 1;82(3):252–7. Keywords: Prostate, Boost, Contouring Digital Poster 4226 Ultrahypofractionation in prostate radiotherapy - does bladder size matter? Yee Pei Song 1 , Zhu Chuen Oong 1 , Hitesh Mistry 2 , Vodathi Bamunuarachchi 1 , Ereny Saad 1 , Ruth Conroy 1 , Hanis Hanafi 1 , Andrew Hudson 1 , John Logue 1 , Maria Serra 1 , Martin Swinton 1 , Anna Tran 1 , James Wylie 1 , Ananya Choudhury 2,1 1 Clinical Oncology, The Christie Hospital NHS Foundation Trust, Manchester, United Kingdom. 2 Division of Cancer Sciences, The University of Manchester, Manchester, United Kingdom Purpose/Objective: Ultrahypofractionation in prostate radiotherapy leads to higher acute genitourinary (GU) toxicity rates compared to the 20 fractions hypofractionation. The PACE trials recommend a partially filled bladder approach (Ref 1). An empty bladder protocol has better reproducibility but risks a larger proportion of bladder volume in the high-dose area. Our study investigated real-world outcomes of genitourinary (GU) toxicity with this approach. Material/Methods: This is a retrospective study of patients treated with 36.25Gy/5 fractions. Radiotherapy plan parameters including bladder V30Gy were manually retrieved from dose volume histograms. GU toxicity was graded with CTCAE v5.1 at 6 weeks, 3 months, 6 months and 12 months post radiotherapy. Associations between clinical and dosimetric predictors, and GU toxicity were evaluated with multivariable logistic regression model. This was adjusted for baseline IPSS, number of dose levels, and treatment platforms. Odds ratio and 95% confidence intervals were estimated and forest plot generated with R studio v4.2.1. Results: 195 patients were included in this study between July 2020 and August 2024. Patients were treated on standard CT-Linac (n=45) or MR-Linac (n=150). 43
radiotherapy plans were generated based on submitted contour sets, and the Dmin to the GS_CTVb and GS_PTV were calculated. Plans were normalised for Dmean of 67Gy to CTVb. Pearson correlation analysis was used to determine association between DSC to GS and Dmin. Results: Table 1 shows the geometric variability compared to GS of the submitted contours. Mean DSC for GTV contours was 0.5, however median GTV contour was similar to GS contour but varied most in the superior/ inferior extent (Figure 1). CTVp DSC to GS was 0.85 but the volume was 57cc to 92cc. Table 1 also shows Dmin from VMAT plans generated from 45 of the 61 submitted contours. The Dmin in GS CTV plan was 63.5Gy. The Dmin mean from submitted contours in CTVb was 61.2Gy, range 55.4- 64.44. There was no significant association between DSC (compared to GS) and Dmin (CTVb) There were 6 plans with low Dmin (<60Gy) to the CTVb, these were due to failure to extend CTVp to cover GTVb in the seminal vesicle.
Conclusion: Discussion: This project demonstrates interobserver variation of GTVb contouring across UK centres, and more difficulties to define the superior and inferior extent. The GTV volumes are small; as a result, the geometric metrics overestimate the variations and don’t give any spatial information. Contour sets with low Dmins did not extend the prostate CTV to include the whole of the outlined tumour. References: 1. Syndikus I, Cruickshank C, Staffurth J, Tree A, Henry
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