[Impact of Aperture Shape Controller on Volumetric Modulated Arc Therapy Treatment Planning for Nasopharyngeal Cancer]

Purpose: The aperture shape controller (ASC) decreases the complexity of the multi-leaf collimator (MLC) aperture for volumetric modulated arc therapy (VMAT). The purpose of this study was to evaluate the effect of ASC settings on the VMAT plan quality.

Method: First, VMAT plans were created (ASC=off) for three test patterns of The American Association of Physicists in Medicine (AAPM) Task Group 119 (TG-119) and 20 cases of nasopharyngeal cancer. Next, for these VMAT plans, only the ASC settings were changed from very low (complexity reduction: low) to very high (complexity reduction: high) in five steps, and VMAT plans were created respectively. To evaluate the created VMAT plans per each ASC settings, we analyzed the modulation complexity score (MCS_V) and dosimetric parameters for the planning target volume (PTV) and organ at risk (OAR).

Result: In three test patterns, there were no major dosimetric differences between the VMAT plans. In nasopharyngeal cancer, the mean MCS_V were 0.413, 0.325, 0.320, 0.307, 0.303, and 0.272 for very high, high, moderate, low, very low, off settings, respectively. Therefore, the most complex MLC aperture was off, followed by very low, low, moderate, high, and very high. In terms of dosimetric parameters, the VMAT plans created using the very high setting showed an increase of D_2% in the PTV and worse OAR sparing than that using other ASC settings. On the other hand, the dosimetric results for the very low to moderate setting obtained similar results to those for the off setting, respectively.

Conclusion: The ASC was able to decrease the complexity of the MLC aperture according to the setting level. From very low to moderate settings, a plan equivalent to the off setting could be created in terms of dose parameters.