Integrated Boost vs. Sequential Scheme in Volumetric Modulated Arc Therapy (VMAT) for Trimodal Bladder Cancer Therapy: A Comparative Study

Background Trimodal therapy (TMT), combining transurethral resection of the bladder tumor, radiotherapy, and concurrent chemotherapy, is an established bladder-preserving approach for muscle-invasive bladder cancer (MIBC). Advances in radiotherapy, such as volumetric modulated arc therapy (VMAT), have enabled the use of different dose delivery schemes, namely simultaneous integrated boost (SIB) and sequential boost (SEQ) techniques. This study aimed to compare dosimetric outcomes, organ-at-risk sparing, and treatment tolerance between these two VMAT schemes. Methods Fourteen patients with MIBC treated with TMT between December 2016 and December 2021 were retrospectively analyzed. Seven patients received SIB-VMAT, and seven received SEQ-VMAT. Treatment plans were generated using Eclipse^® software (Varian Medical Systems, Palo Alto, CA, United States), and dose-volume parameters were evaluated for the rectum, small bowel, and femoral heads. Tumor response and toxicities were assessed according to CTCAE v6.0 criteria. Statistical analyses were performed using IBM SPSS Statistics for Windows, Version 25.0 (Released 2017; IBM Corp., Armonk, NY, USA). Results Target coverage and conformity were comparable between SIB and SEQ plans (V98%: 95.7% vs. 96.8%; V2%: 102.2% vs. 101.9%). Rectal and femoral head doses were similar across both schemes, whereas the small bowel received higher V15Gy and V45Gy in the SIB group (863.6 mL and 130.6 mL vs. 517.3 mL and 71.6 mL, respectively). Acute toxicities were predominantly urinary (Grade 1-2 cystitis) and occurred in 21.4% of patients. The complete response rate was slightly higher in the SIB group (18%) compared with the SEQ group (17%), although the difference was not statistically significant. Conclusions Both SIB and SEQ-VMAT schemes provide effective and well-tolerated treatment options for bladder preservation in MIBC. SIB offers a shorter overall treatment duration and marginally improved target conformity, while SEQ may better spare the small bowel at lower dose levels. The choice between these approaches can be individualized according to patient anatomy and institutional expertise.