Prog Med Phys.  2024 Sep;35(3):65-72. 10.14316/pmp.2024.35.3.65.

Impact of Smaller Gantry Arc Increments on Volumetric Modulated Arc Radiation Therapy in the Monaco Treatment Planning System

• Kang S
• Ryu H
• Oh DH
• Yoo L
• Chun M

• KMID: 2559977
• DOI: http://doi.org/10.14316/pmp.2024.35.3.65

Abstract

Purpose
This study aims to evaluate the impact of smaller gantry arc increment (GAI) values on the plan quality and deliverability of volumetric modulated arc therapy (VMAT) for head and neck (HN) and prostate cancer cases using the Monaco treatment planning system. The study investigates whether a smaller GAI can enhance organ at risk (OAR) sparing without compromising target coverage or significantly increasing plan complexity.
Methods
VMAT plans were created for 20 patients (10 HN and 10 prostate cancer) using GAI values of 15° and 30°. Dose-volumetric parameters, such as conformity number, homogeneity and gradient indices, were assessed alongside plan complexity metrics like the modulation complexity score for VMAT (MCS v ) and monitor unit (MU). Statistical significance was determined using the Wilcoxon signed-rank test.
Results
For HN cases, a 15° increment significantly reduced the D_0.03cc for the spinal cord and the D_mean for both parotid glands compared to a 30° increment, improving OAR sparing. However, no significant differences were observed in the OAR doses for prostate cases. The 15° increment resulted in higher plan complexity, reflected by a lower MCS v , but the MU difference was not significant.
Conclusions
Smaller GAI values, such as 15°, can significantly reduce OAR doses in HN VMAT plans, offering potential clinical benefits despite increased plan complexity. However, no substantial advantages were observed in prostate cases. These findings suggest that smaller GAI values may be particularly beneficial for cases requiring high modulation.

Keyword

Volumetric modulated arc radiation therapy; Treatment planning; Prostate radiotherapy; Head and neck radiotherapy; Gantry arc increment

Figure

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  Fig. 1 Sample dose distributions for representative cases in head and neck cancer with GAIs of (a) 30° and (b) 15°, and in prostate cancer with GAIs of (c) 30° and (d) 15°. GAI, gantry arc increment; PTV, planning target volume.

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  Fig. 2 Sample dose-volume histograms for representative cases in (a) head and neck and (b) prostate cancer. The dotted and solid lines represent plans using 30° and 15° gantry arc increments, respectively. PTV, planning target volume.

Reference

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