Correlation Between Average Segment Width and Gamma Passing Rate as a Function of MLC Position Error in Volumetric...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• Multi-leaf collimator (MLC) position errors can introduce dose uncertainties in volumetric modulated arc therapy (VMAT).
• Accurate dose delivery is critical, especially for complex VMAT plans involving irregular fields, such as those used in prostate cancer treatment.

Purpose of the Study:

• To investigate the correlation between average segment width (ASW) and the gamma passing rate in VMAT.
• To establish a relationship that can predict dose uncertainty based on MLC position errors.

Main Methods:

• 21 VMAT plans for prostate cancer were analyzed.
• Average segment width (ASW) was calculated using custom VBA code.
• Gamma passing rates were evaluated using ArcCHECK with induced MLC position errors (-1 mm to +1 mm).
• Linear regression analysis was performed to determine the correlation.

Main Results:

• A strong linear correlation (R²=0.88) was found between ASW and changes in gamma passing rate due to MLC position errors.
• A 10 mm increase in ASW corresponded to an approximate 11.9% improvement in gamma passing rate for a 1 mm MLC error.

Conclusions:

• ASW is a significant predictor of dose uncertainty resulting from MLC position errors in VMAT.
• These findings can serve as guidelines to optimize VMAT planning and reduce dose uncertainties.