WE‐C‐BRB‐06: Analyzing the Impact of Intra‐Fraction Motion: Correlation of IMRT QA Metrics with D95%

Abstract

Purpose: Motion‐encoded dose distributions, taking into account intra‐fraction motion during intensity modulated radiation therapy (IMRT), can be determined both computationally and experimentally, with a number of analysis techniques available to quantify the dosimetric changes. The analysis results can provide important information regarding the clinical impact of motion and the appropriateness of a given treatment plan. This study provides a comparison of different analysis techniques performed on computationally predicted motion‐encoded dose distributions. Methods: A total of 3,768 motion‐encoded dose distributions were calculated computationally for 38 step‐and‐shoot and 38 compensator IMRT plans across 9 lung patients. The motion‐encoded dose distributions were compared to static dose distributions using four analysis techniques: 2D and 3D gamma, histogram (looking at the fraction of voxels with a dose deviation exceeding 3% of the prescription dose) and dose volume histogram (DVH) analyses. Tests were also implemented to separate failing voxels caused by either over‐dosing or under‐dosing. In addition tests were performed both globally (not taking into account the planning regions of interest) and within restricted regions of interest, such as the target volume. Results: Global analysis techniques did not accurately predict clinically relevant dosimetric changes in the target D<sub>95%</sub>. Three‐dimensional gamma and histogram analyses, localized within the target and considering only voxels experiencing decreased doses due to motion, provided the strongest correlations with changes in D<sub>95%</sub> (r,2</sup>=0.68 and r,2</sup>=0.71 respectively). A global 2D gamma analysis correlated less well with changes in D<sub>95%</sub> (r,2</sup>=0.44), and was both less specific and sensitive. Conclusions: This study suggests that any clinical decisions based solely on 2D or global measurement techniques may not provide adequate dosimetric information about the effect of motion on IMRT delivery. Care needs to be taken in determining the appropriate analysis technique and in interpreting the results in order to understand the relevant dosimetric changes caused by motion.This work was supported in part by .decimal, Inc.