Platform-specific comparison of IMRT and VMAT angular resolution for Ethos-based online adaptive radiotherapy in cervical cancer.

BACKGROUND: The Ethos™ platform enables online adaptive radiotherapy (oART) to mitigate inter-fractional anatomical variations. However, the lengthy "scan-to-beam" workflow introduces the risk of intra-fractional motion. This study aimed to investigate the dosimetric and efficiency differences between IMRT and VMAT with varying angular resolutions, providing data to guide the selection of planning strategies in Ethos-based oART. MATERIALS AND METHODS: Fifteen patients with cervical cancer treated on the Ethos platform were retrospectively analyzed. For each patient, five plans were generated: a standard 9-field IMRT plan and four VMAT plans optimized with angular resolutions of 2°, 3°, 4°, and 5° (VMAT2-VMAT5). Dosimetric metrics for Planning Target Volume (PTV) and Organs at Risk (OARs), along with plan generation efficiency (optimization and calculation time), delivery efficiency (beam-on and total delivery time), and deliverability (gamma analysis), were systematically evaluated. RESULTS: All plans met clinical requirements. IMRT demonstrated superior dose homogeneity (HI: 0.07 ± 0.01) compared to all VMAT groups (0.09-0.10, P< 0.001). For OARs, IMRT provided better sparing of the rectum, bladder, and bone marrow, and reduced the high-dose bowel metric D2cc; however, bowel V30 was lower in the VMAT groups, indicating metric-dependent bowel sparing. In terms of efficiency, IMRT required substantially shorter optimization time (122.8 ± 14.6 s) than VMAT (705.9-742.9 s, P< 0.001). Conversely, VMAT reduced the total delivery time by approximately 50% compared to IMRT (158.1 s vs. 305.3 s, P< 0.001). Among VMAT plans, finer resolutions (2°-3°) offered slight dosimetric advantages over coarser resolutions (4°-5°) but required longer calculation times. Gamma passing rates were >98% (3%/3mm) for all techniques, with IMRT showing marginally higher accuracy. CONCLUSION: On the Ethos platform, the IMRT technique offers a favorable balance for cervical cancer oART, combining superior target homogeneity, improved rectum, bladder, and bone marrow sparing, lower bowel D2cc, and rapid plan generation that minimizes the critical scan-to-beam interval. However, bowel V30 was lower with VMAT, suggesting that bowel sparing depends on the selected dose-volume metric.