Improved patient-specific quality assurance for VMAT on film measurement with 2.5D gamma analysis.

PURPOSE: This study proposed a "2.5D gamma analysis" method for patient-specific quality assurance (PSQA) in volumetric modulated arc therapy (VMAT) using film measurements.

METHODS

A custom-designed water phantom was used, consisting of a main body and an insert box, each with a wall thickness of 10 mm, and filled with water. The insert box contained a film holder designed to accommodate several radiochromic films. The holder consisted of four separate 2 mm-thick plates and a cover, allowing the positioning of one to five films. For consistency, three films were used per case, positioned at 0 and ± 2 mm. Quasi-volumetric data were generated from the measured Gafchromic film data at various positions using B-spline interpolation, producing additional data points at 0.5 mm intervals. Five spine SBRT cases were analyzed by comparing 2.5D and 2D gamma analyses using different criteria and dose thresholds.

RESULTS

The 2.5D gamma analysis showed higher gamma pass rates compared to the 2D analysis, with an average gamma pass rate of 98.6 % for the 2.5D analysis and 87.4 % for the 2D analysis, using 3 %/1 mm gamma criteria with a 5 % dose threshold. This represented an 11.2 % point increase in the gamma pass rate.

CONCLUSIONS

The 2.5D gamma analysis provided higher gamma pass rates than the 2D analysis for film measurements, providing increased confidence in the accuracy of the spine VMAT treatment plan.