Varian Halcyon dosimetric comparison for multiarc VMAT prostate and head-and-neck cancers

The Varian Halcyon linear accelerator is designed for faster treatment time and fewer treatment errors than conventional linear accelerators. Its increased gantry speed (4 rpm), dose rate (800 MU/min), and leaf speed (5 cm/s) reduce treatment time. The 2 multileaf collimator banks of 1 cm leaf width could cause planning difficulties, but utilizing many arcs could address this challenge. While 3 or more arcs are often avoided due to the increased treatment time, the Halcyon can deliver 5 arcs in under 3 minutes. For prostate and head-and-neck, we hypothesized plan quality would increase with increasing arcs without compromising treatment time. Fifteen prostate and fifteen head-and-neck previously treated patients were replanned to 78 and 66 Gy, respectively. A total of 150 volumetric modulated arc therapy (VMAT) plans, ranging from 1 to 5 arcs, were created to compare organ-at-risk (OAR) sparing, target coverage, homogeneity, and hotspot volume. Plans were normalized to the prescribed dose of 98% and 95% of the primary target volume for prostate and head-and-neck, respectively. All prostate 4-arc and 5-arc plans met MD Anderson Cancer Center (MDACC) constraints, while 86.7%, 20%, and 6.7% of the 1-arc, 2-arc, and 3-arc plans did not meet constraints, respectively. As the number of arcs increased, all OAR maximum doses decreased. All head-and-neck 5-arc plans met MDACC constraints, while 73.3%, 26.7%, 13.3%, and 6.7% of the 1-arc, 2-arc, 3-arc, and 4-arc plans did not meet constraints, respectively. The contralateral parotid mean dose and the spinal cord maximum dose decreased with increasing arcs. Homogeneity and conformity indices decreased when increasing from 1 to 5 arcs for both prostate and head-and-neck. Overall, homogeneity and conformity indices and OAR doses improved when using three or more arcs. Therefore, using at least 3 arcs for VMAT plans on the Halcyon will increase plan quality without compromising treatment time.