Author: David Chighvinadze, Chieh-Wen Liu, Sugandima Weragoda, Ping Xia 👨🔬
Affiliation: Cleveland Clinic Sandusky, Cleveland Clinic 🌍
Purpose: To explore the feasibility of using coplanar beams with collimator angle optimization for brain SRS plans with Halcyon system.
Methods: This study used a dataset of 26 patients with 2-3 brain metastases. The conformity index (CI) and R50 (volume receiving 50% of the prescription dose/PTV) for plans from 5 of these patients were reported here. The individual target volumes (12 targets total) ranged from 0.75 to 9.71cc and were prescribed to 12-24 Gy in single fraction. For each patient, 3, 5, and 8 full arcs plans were created using Halcyon system. For each arc, an optimized collimator angle was determined by an in-house developed program. Subsequently, because of the limit of total 12 arcs in the TPS, each full-arc of 3 arc plans was divided into 4 sub-arcs with collimator angle optimization.
Results: For targets >4cc, 8-arc plans were more conformal than 5-arc plans (CI = 1.30±0.10 vs. 1.33±0.16), and 3arc-sub plans were more conformal than 3-arc plans (CI = 1.25±0.20 vs. 1.28±0.12). The average R50 for 5-arc plans was lower than that of 8-arc plans (5.34±0.81 vs. 5.39±0.90); 3arc-sub plans were better than 3-arc plans (4.7±0.7 vs. 5.2± 0.6). For targets < 4cc, 5-arc plans were more conformal than 8-arc plans (CI = 1.69±0.35 vs. 1.75±0.41), and 3arc plans were more conformal than 3arc-sub plans (CI = 1.65±0.25 vs. 1.80±0.51). The R50 for 5-arc plans was lower than that of 8-arc plans (9.61±1.93 vs. 10.01±2.31), while 3arc plans had lower R50 than 3arc-sub plans (8.75±1.56 vs. 9.19± 2.11).
Conclusion: For large brain tumors, increasing number of arcs and introducing sub-arcs with collimator optimization can enhance the plan conformity and dose gradient. Our data will be further validated using the remaining 21 patients.