ESTRO 2024 - Abstract Book

S2459

Clinical - Urology

ESTRO 2024

1802

Digital Poster

Effect of interfractional volume and motion displacement of trigone and bladder on prostate cancer

Davide Giovanni Bosetti 1 , Maria Antonietta Piliero 2 , Francesco Mose' Castronovo 1 , Klaudia Krzekotowska 2 , Gianfranco Angelo Pesce 1 , Stefano Leva 1 , Domenico Santoro 1 , Margherita Corsi 2 , Matteo Coppotelli 2 , Stefano Presilla 2 , Thomas Zilli 1,3,4 1 Ente Ospedaliero Cantonale (EOC) - Oncology Institute of Southern Switzerland (IOSI), Radiation Oncology Clinic, Bellinzona, Switzerland. 2 Ente Ospedaliero Cantonale (EOC) - Imaging Institute of Southern Switzerland (IIMSI), Medical Physics Divisionn Oncology Clinic, Bellinzona, Switzerland. 3 Università Della Svizzera Italiana (USI), Facoltà Di Scienze Biomediche, Lugano, Switzerland. 4 University of Geneva, Radiation Oncology Clinic, Geneva, Switzerland

Purpose/Objective:

Fractionated radiotherapy for prostate cancer (Pca) is associated with daily variations in bladder and trigone positions due to physiological changes and different treatment preparations. Optimization strategies aiming to reduce the dose delivered to these structures can potentially reduce radiation-induced toxicities. Aim of the present study was to quantify the inter-fractional bladder and trigone displacements and investigate the dosimetric impact on doses delivered to the trigone in PCa patients treated with moderate hypofractionation.

Material/Methods:

Structure and plan datasets from 10 PCa patients treated between 2022 and 2023 with a 20-fraction moderate hypofractionation schedule according to the DELINEATE trial were retrospectively analyzed. All patients were simulated and planned using a MRI-only workflow and were treated using a VMAT technique, using daily CBCT imaging guidance and robotic couch-based repositioning. Bladder and trigone were contoured on the planning MRI and in all 20 daily CBCTs. Interfractional variations in volume and position displacements were analyzed and a planning organ at risk volume (PRV) margin for trigone was calculated using the Van Herk formula. The dosimetric impact on delivered doses to the trigone was calculated for the entire treatment course and compared to the initial treatment plan evaluating different dose metrics (Dmax and Dmean). Among the 10 analyzed patients, the average of the mean trigone volume was 3.5 cc. The average variation during the 20 fractions was 1.1 cc with only one case showing a variation >2 cc. The mean bladder volume showed a large variability among patients, ranging from 168 cc to 266 cc, with an average difference of -42 cc compared to the planning MRI. Over the course of 20 fractions, the average variation in bladder volume was 79 cc. A statistically significant correlation was observed between the antero-posterior (AP) displacements of the trigone and the bladder volume (approximately 2mm every 100cc of bladder volume difference, p=0.0002), while this correlation was not observed in the latero-lateral (LL) and superior-inferior (SI) axes. The estimated PRV margins for the trigone were: 4.5mm, 16.7 mm, and 6.7 mm for the LL, AP and SI directions. Results:

The mean Dmax and Dmean doses to the trigone throughout the 20 fractions were 61.6 Gy and 51.5 Gy on average, with a difference of -0.2 Gy (1.3 Gy SD) and -0.5 Gy (5.1 Gy SD) when compared to the planning MRI estimates.