ESTRO 2025 - Abstract Book

S2104

Clinical - Urology

ESTRO 2025

undergoing online Adaptive Radiation Therapy (oART). Our study explored whether stable bladder volume changes could support a pPS margin to optimize treatment delivery.

Material/Methods: Eight patients affected by MIBC were enrolled. They underwent trimodal therapy with a RT protocol of 55 Gy in 20 fractions. The preparation consisted of a comfortable filling of the bladder. The oART workflow includes the execution of a first CBCT on which the contours provided by the AI software are reviewed and the reoptimization of the treatment plan is performed. A second CBCT is performed before starting treatment. The average process time is 20 minutes. At our center, PTV is obtained with 3mm isotropic expansion to account for possible intrafraction patient motion, while CTV includes the bladder plus 5mm isotropic margin to account for intrafraction bladder filling. In this study we retrospectively evaluated the effectiveness of reducing the CTV margin to a pPS-CTV expansion. The new PTV formula, incorporating the pPS margin is the following: PTV=[bladder+pPS-CTV margin] + 3mm geometric margin. Bladder volume was off-line re- contoured across 320 CBCTs. For each fraction, the mean displacement(∆) between the bladder contours in the two CBCTs was calculated. The pPS- CTV was computed as the maximum ∆ among the first five sessions. For each patient, the final 15 sessions were used to evaluate the capability of the pPS-CTV to completely account for the intra-fraction bladder filling. Results: The pPS-CTV margin from the first sessions ranges from 0.11 to 0.31 cm, indicating a high inter-patient variability. Such margins effectively accounted for the intra-fraction bladder filling for the remaining fractions, with only 4 out of 160 fractions exceeding the pPS-CTV by more than 0.05 cm (marked with a dotted red line in Fig 1).