Complexity metric as a complement to measurement based IMRT/VMAT patient-specific QA

Abstract

IMRT/VMAT treatment plans contain treatment fields with MLC openings of various size and shape. Clinical dose calculation algorithms show limitations in calculating the correct dose in small and irregular parts of a MLC opening which leads to differences between the planned and delivered dose distributions. The patient-specific IMRT QA is often designed to compare planned and measured dose distributions and is therefore heavily dependent on the measurement equipment and the evaluation method. The purpose of this study is to develop a complexity metric based on shape and size of MLC openings that correlates to the dose differences between planned and delivered 3D dose distributions. Different MLC openings are measured and evaluated and used to determine a penalty function to steer the complexity metric and make the complexity scores correlate to dose difference pass rates. Results of this initial study show that a correlation was found between complexity scores and dose difference pass rates for static fields with varied complexity. Preliminary results also show that the complexity metric can distinguish clinical IMRT fields with higher complexity.