ESTRO 2025 - Abstract Book

S2009

Clinical - Urology

ESTRO 2025

2750

Proffered Paper Exploring predictive factors for late urinary toxicity after prostate SBRT: Findings from the PACE-B Study Ragu Ratnakumaran 1,2 , Douglas H Brand 3 , Archana Sasitharan 2 , Jonathan Mohajer 4 , Victoria Hinder 5 , Asadullah Khan 6 , Hira Mayet 2 , Andrew Loblaw 7 , Emma Hall 5 , Nicholas van As 1,2 , Alison Tree 1,2 1 Urology Unit, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 2 Radiotherapy and Imaging Division, Institute of Cancer Research, London, United Kingdom. 3 Department of Medical Physics and Bioengineering, University College London, London, United Kingdom. 4 Medical Physics, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 5 Clinical Trials and Statistics Unit, Institute of Cancer Research, London, United Kingdom. 6 Clinical Oncology, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 7 Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Canada Purpose/Objective: Stereotactic body radiotherapy (SBRT) is increasingly used for localised prostate cancer. However, there are concerns regarding the higher incidence of urinary toxicity, especially up to two years post-treatment. This study assesses patient, clinical, treatment, and dosimetric predictors for late urinary toxicity following prostate SBRT in the PACE-B cohort. Material/Methods: Patients from the PACE-B SBRT cohort with available dose volume histogram (DVH) data were analysed. The prescription dose was 36.25 Gy in 5 fractions (PTV) and 40 Gy in 5 fractions (CTV), administered daily or on alternate days. Common Terminology Criteria for Adverse Events (CTCAE) measurements were collected prospectively and available for up to 24 months post-radiotherapy. The bladder trigone and surrogate urethra were retrospectively contoured. DVHs for the surrogate urethra, contoured urethra, bladder trigone, and bladder were extracted. Univariate and cross-fit partialing-out logistic regression was used to identify predictors for grade 2+ late urinary toxicity at two years. Youden index and closest top left (CtL) index were calculated for continuous predictor variables to determine an optimum threshold that discriminates between the occurrence of late urinary toxicity. Results: Among 390 patients with late toxicity and DVH data, 357 had CTCAE urinary endpoints assessed at 2 years, with 12% (n=44) experiencing grade 2+ urinary toxicity. Multivariable analysis identified several predictive factors: higher baseline International Prostate Symptom Score (IPSS) (OR 1.12, 95% CI [1.06-1.19], p<0.0001), baseline urinary medication use (OR 2.32, 95% CI [1.09-4.97], p=0.03), and grade 2+ acute toxicity (OR 3.47, 95% CI [1.45-8.31], p<0.0001). Treatment with Conventional linac (CL) SBRT using fiducials (n=102) was associated with increased urinary toxicity at two years compared to CL-SBRT without fiducials (n=124) (OR 5.0, 95% CI [1.82-13.73], p=0.002). No significant associations were found between surrogate urethra, contoured urethra, bladder trigone, and bladder dose and grade 2+ late urinary toxicity at two years. Youden and CtL index identified a baseline IPSS of 11 ((95% CI: 7-15), Area under receiver operator curve (AUC): 0.68) as an optimal threshold to discriminate between developing grade 2+ urinary toxicity at two years following prostate SBRT. Conclusion: These findings enhance our understanding of predictors for late urinary toxicity following prostate SBRT. Baseline urinary symptoms are an important determinant for late urinary toxicity and can help inform treatment selection. No association was seen between dose to urinary sub-structures and toxicity, possibly due to homogenous dose delivered. Further validation is required to confirm the association between fiducial use in the CL-SBRT cohort and urinary toxicity.

Keywords: Stereotactic body radiotherapy; urinary toxicity