The Impact of Plan Complexity on Dose Delivery Deviations Resulting from MLC Positioning Errors in Volumetric Modulated Arc Therapy.

Abstract

Objectives: To assess the effect of plan complexity on dosimetric alterations induced by multi-leaf collimator (MLC) misplacements in volumetric modulated arc therapy (VMAT).

Methods: VMAT plans for 14 cervical and 10 lung cancer cases were reoptimized utilizing three distinct Aperture Shape Controller settings (none, very high, very low), resulting in three plan groups: ASC-none, ASC-vh, and ASC-vl. Four types of MLC position errors were simulated: total shifts (Type 1), open/closed (Type 2), right-side shifts (Type 3), and left-side shifts (Type 4). Plan complexity was assessed using the small aperture score (SAS). Dose deviations resulting from various MLC positioning errors and SAS values were calculated and compared among the three ASC groups.

Results: The variations in planning target volume (PTV) D95% for cervical cancer were approximately 0.6%, 3.7%, 1.9%, and 1.8% per millimeter for Types 1-4 errors, respectively. In the case of lung cancer, the changes were 2.3%, 9.3%, 5.3%, and 4.6% per millimeter. The ASC-vh and ASC-vl groups exhibited significantly reduced dose changes and SAS values in response to MLC errors, as compared to the ASC-none group (p < 0.05).

Conclusions: Highly complex plans exhibit greater dose sensitivity to MLC positional errors. The application of ASC proves effective in reducing plan complexity and mitigating the influence of MLC errors on dose deviation.

Advances in knowledge: By elucidating the relationship between dosimetric impacts from MLC errors and plan complexity, this study offers valuable guidance for the design of radiotherapy plans, helping to enhance the accuracy and effectiveness of VMAT treatments.