Clinical and Dosimetric Evaluation of CBCT-Based Adaptive Radiotherapy for Prostate SBRT
Abstract
Purpose
Ethos Version 2 has several new capabilities, including high-fidelity (HF) adaptive planning, AI-based auto-segmentation, and dose calculation using HyperSight CBCT. This study evaluates these new features for online adaptive radiotherapy (ART) for prostate SBRT.
Methods
Twenty prostate SBRT patients were analyzed using four planning techniques: 9/12-field IMRT and 2/3-arc VMAT. AI-generated contours were compared against physician-approved contours. Optimization time and plan quality—including Monitor Units (MU), target Dmax, target D95%, conformity index (CI), and Organ at Risk (OAR) sparing—were compared between HF "On" and "Off" conditions.
Results
AI auto-segmentation required manual refinement in most cases for the rectum, prostate and penile bulb, with volume discrepancies exceeding 10% compared to physician contoured volumes. HF mode increased IMRT optimization time by ~1.4min while for VMAT the optimization time increased from ~4min (HF Off) to >8min (HF On). HF mode yielded a substantial reduction in total MU, particularly for VMAT 2,3-arc (17%) plans, resulting in faster delivery time. No significant difference in CI was observed between the two modes and Target Dmax remained stable (about 8–14% >prescription dose). While bladder metrics remained consistent, HF On mode increased patient-specific variations in rectal Dmax and D20%, suggesting careful OAR monitoring during HF optimization. However, HF On did improve dose fall off/peripheral dose.
Conclusion
Ethos 2.0 HF mode provides superior delivery efficiency through MU reduction, which is advantageous for SBRT. However, the substantial increase in optimization time for VMAT prolongs the on-table time. Currently, IMRT-ART workflows offer the most balanced compromise between HF dosimetric benefits and temporal efficiency for prostate SBRT. Furthermore, our results indicate that HF Off is more practical to achieve homogenous prostate target dose. While HF On improved peripheral dose, it does so at the cost of increased dose deposition immediately surrounding the target.