Room: Exhibit Hall | Forum 4
Purpose: To develop and test a channel weighted perturbation method with non-uniformity compensation for improved triple channel radiochromic film dosimetry.
Methods: The algorithm was tested for Gafchromicâ„¢ EBT3 film using two clinical QA plans, an IMRT and a VMAT plan (6 MV X-rays, Elekta Versaâ„¢HD). Film scans were done with the transmission mode of an Epson Expression 10000 XL photo scanner. Scan-to-scan variability artifact was corrected by an unexposed film strip for each scan. The scanner settings were 72 dpi, uncompressed TIFF with all color correction features muted. A 4 mm thick glass sheet was used to compress film to avoid the Callier effect. Global and local gamma index analyses were used for 2-D dose map comparisons. Gamma analysis was performed with film dose maps as references and TPS plans as evaluations. The dose difference and distance to agreement tolerance for gamma index calculations were 1%/1mm, 2%/2mm and 3%/3mm. The passing rates for Î³<1 and the mean gamma values were calculated. The new method was compared to the iterative method by Micke et al., the channel-independent algorithm by Mayer et al. and the single channel dosimetry methods. The algorithm was programmed in MATLAB R2018b.
Results: Under each tolerance for gamma index analysis, the new method, channel weighted perturbation method gave the lowest mean gamma values. The greatest improvement on gamma pass rate was observed for the IMRT plan with 1%/1mm global gamma analysis: 88% for this method, 80%, 78%, 70%, 55% and 43% for methods by Micke et al, Mayer et al, red, green and blue channel respectively.
Conclusion: The improved method showed robustness over the single channel dosimetry methods with the tested cases. The profound improvements compared to previous triple-channel efforts by Mayer et al. and Micke et al. were observed with tighter tolerance(1%/1mm) applied for gamma index calculations.