Dosimetric advantages of dual arc increments for head and neck volumetric-modulated arc therapy in the Monaco treatment planning system.

Purpose: This study aimed to assess the dosimetric advantages of dual arc increments in head and neck volumetric-modulated arc therapy (VMAT) in the Monaco treatment planning system (TPS).

Methods: Three VMAT plans were created for each of the 10 patients by prescribing 70 Gy in 35 fractions with arc increment combinations of 30°/30°, 15°/15°, and 30°/15° in the Monaco TPS. The dose to 0.03 cm volume (D), conformity number, and homogeneity and gradient indices was compared for the planning target volume (PTV), and the D and mean dose (D) of the spinal cord, brain stem, parotid glands were compared. For plan complexity evaluation, the monitor unit and various related metrics were compared. Wilcoxon signed-rank tests were performed across plans for the evaluated indicators.

Results: For PTV, plans with 30°/15° showed comparable D and homogeneity and gradient indices to those of plans with 30°/30° and 15°/15° while exhibiting a better conformity number. The D for spinal cord and brain stem for plans with 30°/15° were 26.0% and 20.8% less than those with 30°/30° and 16.8% and 19.0% less than those with 15°/15°, respectively. The D for the left and right parotid glands under plans with 30°/15° were 17.4% and 13.2% less than those with 30°/30° and 14.0% and 9.8% less than those with 15°/15°, respectively. The total monitor unit in plans with 30°/15° was less than that in other plans but with no significance. The plans with 30°/15° showed higher modulation complexity and plan-averaged irregularity, while no significant differences observed in both plan-averaged area and modulation compared with other plans.

Conclusion: In head and neck VMAT, a dual arc increment of 30°/15° seems advisable because it can substantially reduce doses to normal tissues with comparable delivery efficiency while maintaining target dose coverage.