Dosimetric Validation of Ethos Online Adaptive Radiotherapy for Prostate Cancer: An End-to-End Phantom Study.

Abstract

Background/objectivesOnline adaptive radiotherapy (oART) re-optimizes treatment on the anatomy of the day, but automatically generated plans require dosimetric verification and dose-volume assessment. We performed an end-to-end validation of the Varian Ethos workflow for prostate cancer, including cross-adaptation between two phantom anatomies.MethodsA 3 Gy × 20-fraction reference plan was generated for a RUBY phantom. Scheduled, adapted, and cross-adapted plans were evaluated in two prostate, bladder, and rectum configurations. Absolute dose was measured with a Semiflex 3D ionization chamber at the clinical target volume (CTV), rectum, and bladder and compared with Ethos and independent Mobius calculations. Plan quality was assessed from cumulative dose-volume histograms using ICRU 83-oriented descriptors.ResultsFor scheduled, adapted, and cross-adapted Ethos deliveries, local differences were within 1.0% at the CTV and 3.6% at the rectum. Bladder percentages were larger, but the underlying absolute differences did not exceed 0.048 Gy. When a cross-scheduled plan was deliberately evaluated on the changed anatomy, dose discrepancies of 0.345 Gy per fraction in the rectum and 0.245 Gy in the bladder exceeded the predefined acceptance limits; adaptation reduced the corresponding SF_IS discrepancies to within 0.015 Gy. Adaptation preserved target coverage and reduced rectal dose in same-anatomy comparisons, with small bladder trade-offs. The representative combined standard uncertainty was 2.71%, with an expanded uncertainty of 5.42% (k = 2); these values were used for uncertainty interpretation and not as location-specific acceptance limits.ConclusionsThe Ethos workflow delivered accurate target dose and corrected clinically relevant plan-anatomy mismatch. Dose-volume benefits were anatomy- and objective-dependent rather than uniform.