Exhibit Hall | Forum 7
Purpose: Treating synchronous oligometastatic (≤5) lesions via a SIMT-SBRT plan on fast rotating coplanar Halcyon Linac can improve delivery efficiency, patient comfort and compliance, and patient throughput. However, aligning multiple extracranial tumors on a single pre-treatment conebeam-CT on Halcyon can be challenging. We simulated site-specific potential target(s) coverage loss due to small, but clinically observable rotational set up errors.
Methods: After confirming weekly reproducible spatial accuracy of <1.0 mm for off-axis targets up to 7cm from isocenter on Halcyon, previously treated site-specific SBRT patients (lung/spine) with 2 extracranial planning targets (PTVs) were selected for dosimetric evaluation. Previously treated 50Gy(lung) and 35Gy(spine) in 5-fraction SIMT-SBRT on TrueBeam 6MV-FFF with non-coplanar geometry and Acuros dose-deposition were retrospectively reoptimized for 6MV-FFF SIMT on Halcyon using similar arc-geometry, dose-engine and planning objectives. Rotational errors of ±0.5⁰ to ±3.0⁰ on Halcyon were simulated about all 3-axes and recalculated dose in Eclipse. Dosimetric impact of rotational errors was evaluated for target(s) and organs-at-risk(OAR).
Results: Average distance-to-isocenter was 3.3±1.2cm(spine) and 6.2±1.9cm(lung) SBRT plans. Change in Paddick’s conformity index ratio of spine plans were 0.98±0.04, 1.02±0.04, and 0.96±0.07 for 3° pitch, roll, and yaw-rotation. For lung SBRT plans, it was 0.92±0.04, 0.87±0.10, and 0.84±0.13. Maximum drop of PTV(D95) in spine and lung SBRT plans was 9Gy and 12Gy, respectively. Spine and bilateral-lung SBRT cases demonstrated robustness against roll and pitch errors, respectively. However, for spine and lung SBRT plans, partial cord received up to 10.9Gy higher, and heart received up to 7.6Gy higher than planned dose, respectively.
Conclusion: The severe dosimetric impact including significant loss of target conformity and unacceptable OAR dose increases was observed due to clinically observable rotational set up errors–warranting caution when performing SIMT-SBRT treatment on Halcyon. Multivariant data analysis is ongoing to fully characterize the Halcyon for site-specific extracranial oligometastatic SIMT-SBRT treatment in future.
Funding Support, Disclosures, and Conflict of Interest: This project was partially supported by Varian Medical Systems (Palo Alto, CA)
Not Applicable / None Entered.