# Implementing log file‐based patient‐specific QA for VMAT plans: A comparative study of MobiusFX and measurement‐based approaches

## TL;DR

This study compares a log file-based QA tool (MobiusFX) with traditional measurement methods for VMAT plans, finding similar results and suggesting MobiusFX can be a reliable alternative.

## Contribution

The study evaluates MobiusFX as a feasible alternative to measurement-based QA for VMAT plans and establishes action and tolerance limits for clinical use.

## Key findings

- MobiusFX showed similar gamma passing rates to measurement-based methods at 3%/3 mm and 3%/2 mm criteria.
- RMS errors for MobiusFX were minimal, with maximum MLC error at 0.06 mm.
- No correlation was found between gamma passing rate and plan complexity metric (MU/Gy).

## Abstract

The current interest in utilizing log file‐based patient‐specific quality assurance (PSQA) is growing as an alternative to measurement‐based approaches for volumetric modulated arc therapy (VMAT) plans. However, implementing log file‐based PSQA requires further experience and understanding since there are no guidelines.

This study aimed to evaluate the feasibility of implementing commercial log file‐based PSQA (MobiusFX) for pretreatment verification of VMAT plans by comparing its outcomes with measurement‐based methods.

The study was conducted by selecting 40 VMAT plans and these plans were performed for pretreatment verification for both MobiusFX and measurement‐based methods, including portal dosimetry, ArcCHECK, and Octavius4D with gamma analysis at criteria 3%/3 mm, 3%/2 mm, and 2%/2 mm. The agreement of gamma passing rate between MobiusFX and measurement‐based methods was assessed through scatter plots and percentage agreement score. Additionally, the root mean square (RMS) errors obtained from MobiusFX were investigated in each machine parameter, and examined the correlation between the gamma passing rate and plan complexity metric (MU/Gy) by Spearman's rank correlation coefficient. The statistical control process concept from TG 218 was also applied to establish the action limit (AL) and tolerance limit (TL) for MobiusFX.

The scatter plots of MobiusFX illustrated a trend similar to the measurement‐based methods, with high percentage agreement scores observed at 3%/3 mm and 3%/2 mm criteria. RMS errors were zero for all parameters except multileaf collimator (MLC), and the maximum RMS was 0.06 mm. No correlation was found between the gamma passing rate and the complexity metric in this study. The most appropriate TL and AL for the gamma passing rate with MobiusFX were determined to be 95.38% and 94.75%, respectively, at the 3%/2 mm criterion.

MobiusFX showed interchangeable gamma passing rates with measurement‐based methods at the 3%/3 mm and 3%/2 mm criteria, supporting its use as an alternative for VMAT pretreatment verification. Machine‐ and system‐specific factors should be considered for reliable clinical implementation.