Collimator angle selection for prostate and lymph node volumes on the Varian halcyon machine

Volumetric modulated arc therapy (VMAT), in conjunction with the split-jaw technique, has been widely used on TrueBeam linear accelerators to enhance dose conformity and mitigate the physical limitations of multileaf collimators (MLCs) when treating large, complex volumes. A new linear accelerator from Varian called the Halcyon does not have jaws and poses challenges in treating such volumes due to its unique MLC configuration. The goal of this study was to determine collimator angles effective in sparing organs at risk (OAR) and improving plan conformity for prostate and pelvic lymph node volumes exceeding 20 cm in length. Two VMAT plans (Plan A and B) were created. Plan A utilized departmental-standard collimator angles of 5°, 355°, 85°, and 90°, whereas Plan B was based on the work of Cozzi et al. and used collimator angles 20°, 45°, 340° and 90°. It was hypothesized that Plan A would reduce the volume of rectum and bladder receiving 40 Gy (V40) and volume of small bowel receiving 15 Gy (V15) by > 5% and achieve a conformity index closer to 1.0. Fourteen patients were retrospectively analyzed, and the 2plans were compared. The results showed no statistically significant differences in OAR sparing or conformity between the plans. Although researchers offered valuable insights into collimator angle selection for complex volumes on the Halcyon, future studies with larger sample sizes or other collimator angles may help refine treatment planning strategies.