3332 - Enhancing Precision in Bladder Cancer Adaptive Radiotherapy: The Benefit of Plan of the Day Treatment for Improved Target Coverage and Reducing Toxicity

Purpose/Objective(s): This study demonstrates the effectiveness of cone-beam computed tomography (CBCT) in adaptive radiotherapy (ART) for muscle-invasive bladder cancer (MIBC), a highly prevalent and challenging cancer to treat due to the bladder’s variable anatomy. By using daily CBCT for image-guided radiotherapy (IGRT), through a plan-of-the-day approach, clinicians could better account for the bladder’s motion and shape fluctuations, reducing the chances of geographical misses—a common challenge in bladder radiotherapy that can compromise treatment accuracy.

Materials/Methods: Eighteen patients with localized MIBC (T2-T4aN0M0) received 55 Gy in 20 fractions with chemotherapy (cisplatin or 5-fluorouracil/mitomycin). CT simulation under an empty bladder protocol defined two planning target volumes (PTVs): a small PTV (CTV + 5 mm) and a large PTV (CTV + 15 mm), with corresponding radiotherapy plans. Daily CBCT guided plan selection based on bladder volume, and offline evaluations assessed geographical misses. The primary endpoint was target coverage and bowel volume spared using ART, while secondary endpoints included bladder movement patterns, acute toxicities, and treatment response.

Results: Between September 2023 and February 2024, 18 patients (median age: 63) were treated; 55% had T2 tumors, 61% received neoadjuvant chemotherapy, and 50% had concurrent radiosensitizing therapy. Geographical misses occurred in 26 of 360 fractions (7.2%), primarily in the superior direction (17 cases; mean displacement: 0.49 cm). The mean V50 for small bowel exposure was 16.18 cc with the small PTV plan and 36.68 cc with the large PTV. Of 360 fractions, 251 (71%) used the small PTV, preserving a mean bowel volume of 324.03 cc per patient (SD: 280.34). Predominant bladder movement was superior, anterior, and posterior. Acute grade 2 genitourinary toxicity occurred in five patients, and two experienced grade 2 gastrointestinal toxicity. No grade 3+ radiation-related toxicities were observed. Clinically, 11 patients achieved a complete response, and seven had a partial response.

Conclusion: CBCT-guided ART enables precise, individualized radiotherapy for MIBC, significantly improving treatment accuracy while minimizing radiation exposure to surrounding healthy tissues. The plan-of-the-day approach effectively mitigates the impact of bladder volume variations, enhancing therapeutic outcomes. These findings underscore the potential of ART as a viable alternative to more invasive interventions such as radical cystectomy, emphasizing its role in optimizing bladder cancer management.