Dosimetric Benefits of Hybrid CT-Guided Adaptive Radiotherapy for Pelvic Malignancies
Abstract
Purpose
To quantify organ-at-risk (OAR) sparing achievable using a hybrid approach combining conventional image-guided radiotherapy (IGRT) with a CT-guided online adaptive radiotherapy (CT-ART) cone-down boost for pelvic malignancies.
Methods
Four patients with pelvic malignancies were evaluated. Three cases with cone-beam CT (CBCT) imaging were retrospectively simulated using the Ethos 2.0 emulator, and one patient was treated prospectively using the same hybrid workflow on a clinical Ethos system. The hybrid treatment consisted of conventional IGRT for initial fractions, followed by daily CT-ART boost fractions. For each adaptive fraction, daily anatomy was recontoured on CBCT, and AI-generated auto-segmented contours were reviewed and edited by the attending physician. Two plans were generated per fraction: a scheduled plan (original plan recalculated on daily anatomy) and an adapted plan reoptimized on daily anatomy to maintain target coverage while reducing OAR dose. Standardized Ethos planning directive templates ensured consistent target objectives and OAR prioritization. Dose accumulation and dosimetric comparisons were performed in MIM software. Endpoints included clinically relevant dose-volume metrics for bowel, bladder, rectum, testes, femoral heads, and spinal structures.
Results
Across 19 evaluable OAR metrics, 73% showed dose reduction with adaptive boost delivery. The largest improvements were seen in large bowel V54Gy (mean 29% reduction, up to 55%), small bowel V45Gy (19.4% reduction), and bladder V54Gy (11.8% reduction). Testes maximum dose decreased by an average of 4.9% (up to 13%). Additional reductions were observed in small bowel D0.1cc (2.2%), rectum V45Gy (1.8%), and femoral head doses. Target coverage was maintained in all adapted plans while improving OAR sparing relative to established dose-toxicity thresholds.
Conclusion
Hybrid sequential CT-ART provides meaningful OAR sparing during pelvic boost phases while preserving target coverage. This resource-efficient strategy extends adaptive benefits beyond ultra-hypofractionated regimens while minimizing clinical resource burden and supports broader clinical implementation.