Replacing Re-Simulation CT Using Extended–Field-of-View Hypersight CBCT In Gynecologic Cancer for Offline Adaptive Radiotherapy
Poster
AAPM Poster Hypersight CBCT XL_EH V2.pdf Hosted by AAPM · 0.8 MBAbstract
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.