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Published on: July 29, 2013
Improved patient-specific quality assurance for VMAT on film measurement with 2.5D gamma analysis
Hideharu Miura1, Masanori Miyazawa2, Shuichi Ozawa1
1Hiroshima High-Precision Radiotherapy Cancer Center, Hiroshima, Japan; Department of Radiation Oncology, Institute of Biomedical & Health Sciences, Hiroshima University, Hiroshima, Japan.
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.
