Replacing Re-Simulation CT Using Extended–Field-of-View Hypersight CBCT In Gynecologic Cancer for Offline Adaptive Radiotherapy
Abstract
Purpose
Gynecologic (GYN) cancer patients frequently experience significant weight loss and nodal volume changes during conventional pelvic radiotherapy, often necessitating re-simulation and potential treatment adaptation. A TrueBeam linear accelerator equipped with HyperSight cone-beam CT (HS-CBCT), featuring a 70-cm extended field of view and faster gantry rotation, provides improved Hounsfield unit accuracy (within ±20 HU from CT) and enhanced image quality, thereby enabling direct dose calculation. This study evaluates the feasibility of utilizing a standard linac with HS-CBCT to perform offline adaptive radiotherapy without re-simulation.
Methods
A dedicated CBCT imaging protocol for overweight GYN patients was developed, incorporating extended-FOV acquisition and metal artifact reduction to achieve clinically acceptable image quality. External contour accuracy and HU fidelity were evaluated using a pelvis phantom. A retrospective evaluation was performed for five GYN patients. A single HS-CBCT image obtained more than 3 weeks from treatment start, using clinical rigid registrations, was imported into the treatment planning system. The original clinical plan was transferred to the HS-CBCT images and recalculated to determine delivered dose. An offline adaptive plan was also generated using RayStation’s fine-tuning module instead of full re-optimization. Plan quality for the initial, delivered, and adaptive plans was retrospectively evaluated in five randomly selected patients.
Results
Across all patients, both delivered and adaptive plans demonstrated small deviations (<1.6%) in rigidly copied target coverage (V100) relative to the initial plan. However, delivered plans demonstrated substantial increases in bladder V90 up to 16% and rectum V90 up to 11%, due to on-treatment anatomical changes. In contrast, adaptive plans provided reduced bladder and rectum dose deviations, restoring organ-at-risk doses closer to baseline.
Conclusion
HS-CBCT–based offline adaptive radiotherapy effectively mitigates dosimetric deviations caused by anatomical changes without compromising target coverage. This approach provides a practical and clinically effective alternative to re-simulation for GYN radiotherapy delivered on C-arm Linacs.