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

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Abstract 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 cm3 volume (D0.03cc), conformity number, and homogeneity and gradient indices was compared for the planning target volume (PTV), and the D0.03cc and mean dose (Dmean) 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 D0.03cc and homogeneity and gradient indices to those of plans with 30°/30° and 15°/15° while exhibiting a better conformity number. The D0.03cc 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 Dmean 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.

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