Author: Nebi Demez, Shyam Pokharel, Suresh Rana, Lauren A. Rigsby, Tino Romaguera, Nishan Shrestha 👨🔬
Affiliation: Lynn Cancer Institute, Boca Raton Regional Hospital, Baptist Health South Florida 🌍
Purpose: Treatment plans on the ViewRay MRIdian are often planned with the target placed off isocenter due to the proximity of the bore to the patient, allowing for limited shifts. This is especially true for distal targets. However, in some cases patient positioning can be fine-tuned at the time of simulation to allow placement of the isocenter closer to the target or within the target itself. This study evaluated whether re-planning stereotactic body radiation therapy (SBRT) lung treatments on-isocenter could improve plan quality and reduce treatment times compared to off-isocenter setups on the ViewRay MRIdian by leveraging the peaked nature of the 6FFF center of field.
Methods: Four SBRT lung patients previously treated off-isocenter, with isocenter to target distances ranging from 2 cm to 6 cm, were retrospectively replanned with the isocenter placed at target center. Beam angles, dose constraints, and PTV normalization were maintained. Predicted treatment time, total number of segments, R50, conformity index (CI), and Paddick index were compared between the off-isocenter and on-isocenter plans.
Results: Off‐isocenter plans averaged 9.95 minutes of treatment time (9.56–10.58) with 51.75 segments (44–63). Mean R50 was 7.13 (5.00–8.24), CI was 1.31 (1.16–1.53), and Paddick index was 0.72 (0.64–0.79). On‐isocenter re‐plans showed reduced treatment time (mean 8.71 minutes, range 7.45–9.66) and fewer segments (mean 42, range 35–48). Mean R50 was 6.95 (4.73–8.23), CI was 1.27 (1.15–1.51), and Paddick index was 0.73 (0.65–0.80). Thus, on-isocenter plans either maintained or slightly improved dosimetric parameters relative to off-isocenter plans.
Conclusion: Results indicate that reducing target-to-isocenter distance, when feasible, may enhance both the efficiency and the quality of SBRT lung treatments on the ViewRay MRIdian by lowering treatment times and improving dosimetric metrics.