ESTRO 2025 - Abstract Book

S4192

RTT - Patient experience and quality of life

ESTRO 2025

3715

Mini-Oral The impact of margin reduction on acute GU toxicity in prostate SBRT on a 1.5T MR-linac E. N. de Groot - van Breugel, L. M.W. Snoeren, J. C.J. de Boer, T. A. Lalmahomed, T. Willigenburg, J. R.N. van der Voort van Zijp Radiation Oncology, UMC Utrecht, Utrecht, Netherlands Purpose/Objective: A sub-fractionation workflow was implemented in our center to correct for intrafraction motion during MRI-guided prostate cancer radiotherapy, allowing for reduced PTV margins from 5 mm (standard) to 2 mm (LR-CC) and 3 mm (AP) (tight) 1 . We investigated if the group with tight margins showed less acute genitourinary (GU) toxicity and if toxicity was related to bladder wall dose. Previous work 2 showed that bladder wall dose, specifically V25Gy > 9 cc, was correlated with acute grade ≥2 urinary toxicity (G2+). Material/Methods: Included patients were treated with 36.25 Gy in five fractions on a 1.5T MR-linac (Elekta AB), initially with PTV margins of 5 mm and after introduction of sub-fractionation 2-3 mm. 282 eligible patients were identified, of whom 40 experienced acute grade≥2 GU toxicity (≤90 days post-RT). We investigated the differences in the dose to the bladder wall between the ATS (Adapt-to-Shape) treatment plans (200 individual fractions) of the 40 patients with acute GU toxicity and the bladder wall dose of the ATS treatment plans (200 fractions) of 40 patients without acute GU toxicity. For practical reasons, the bladder wall was created by 4 mm contraction of the outer bladder wall 2, 3 . Results: Interestingly, no significant difference in GU toxicity patterns was observed between the tight and standard margin groups (Lalmahomed et al, submitted ). (Table 1). The median V25Gy dose in the individual plans for the bladder wall for the group with acute GU toxicity was 7.2 ± 3.0 (SD) cc, while in the non-toxicity group it was 6.3 ± 2.1 cc. This was a statistically significant difference in both median (p = 0.0053, Wilcoxon Ranksum test). The distribution of V25Gy was skewed toward higher values in the toxicity group (p = 0.0057, KS-test) (Table 2). Additionally, 9 out of 40 patients (22.5%) in the toxicity group had an average V25Gy > 9 cc, compared to 3 out of 40 (7.5%) in the non-toxicity group (p = 0.04).