ESTRO 2024 - Abstract Book

S4921

Physics - Quality assurance and auditing

ESTRO 2024

2839

Digital Poster

Adaptive radiotherapy for bladder cancer: A retrospective audit of two bladder filling protocols

Diana Nohemi Briceno-Guel 1 , Nicola Laverick 1 , Nicholas MacLeod 2 , Ronan M Valentine 1

1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom. 2 Beatson West of Scotland Cancer Centre, Clinical Oncology, Glasgow, United Kingdom

Purpose/Objective:

Bladder cancer is the 6 th most common occurring cancer worldwide. There are over 550,000 new cases of bladder cancer and more than 200,000 deaths each year 1,2 . External beam radiotherapy (EBRT) is used to deliver radiation to the tumour site, but the bladder’s constant movement poses challenges in accurately targeting the treatment area. To address this, image guided radiotherapy (IGRT) and adaptive radiotherapy (ART) techniques alongside volumetric modulated arc therapy (VMAT) have been employed to improve the accuracy of treatment dose delivery. The purpose of this work was to investigate differences observed in PTV coverage and OAR sparing between two bladder filling protocols; empty bladder and full bladder.

Material/Methods:

This study involved 40 patients who underwent adaptive radiotherapy for Muscle Invasive Bladder Cancer (MIBC) between 2021 and 2023. The patients were divided into two groups: 20 were treated with a full bladder filling protocol [FP] and the other 20 with an empty bladder protocol [EP]. Adaptive bladder VMAT patients have two scans acquired 30 minutes apart. For the empty bladder protocol, scans are acquired at 0 minutes after voiding of the bladder followed by a second scan at 30 minutes, while the filling bladder protocol demands that the patient void their bladders then drinks 350ml of water 30 minutes prior to their first scan with a second scan acquired at 60 minutes. Small, medium and large plans are in turn generated using three different PTVs, i.e., PTV_Sm, PTV_Med and PTV_Lar, respectively, which are created from bladder volumes outlined on planning CTs (pCTs). Adaptive plan of the day (PoD) radiotherapy is delivered with a prescription of 55Gy in 20fractions. The bladders of these patients were outlined on pre cone beam CTs (CBCTs) taken at specific fractions (1, 2, 3, 6, 11, 16 and 20) throughout treatment. The bladder volumes from both the pCTs and CBCTs were recorded to compare the differences between the two protocols. Differences in bladder volumes between the first and last fractions were compared for both protocols. Also, the number of small, medium and large plans selected for each patent and for each protocol during their treatment course was documented. Finally, PTV and OAR dose statistics were recorded by adapting each patients’ dose constraint form depending on the number and type of plan received.

Results:

When comparing between the protocols, we found a significant difference in mean bladder volume (cc) on the first pCTs (131.6±19.3 [EP] vs 231.4±29.9 [FP]; p<0.05) and second pCTs (171.9±21.6 [EP] vs 315±35.5 [FP]; p<0.05). Mean bladder volumes recorded during treatment from CBCTs were significantly different between both protocols (117.3±14.8 [EP] vs 191.2±23.3 [FP]; p<0.05). While we observed no significant difference in bladder volumes between