Room: Exhibit Hall
Purpose: To develop a contrast-based, marker-free, automatic dose-space registration technique for patient specific IMRT QA verification using radiochromic film. Dose-space registrations of the reference and evaluation dose distributions of clinical IMRT QA plans are typically done either through the recognition of made-ahead marks on films, CT fiducial marks or through the subjective method involving the tedious process of manual rotation and translation.
Methods: The evaluation dose pixels were resampled to match the pixel spacing of film measurement through bicubic interpolation by automatically detecting the pixel spacing information from both the reference film measurement and the treatment planning calculation. The algorithm employs the dose pixels above 20% of the maximum dose from the evaluation dose map to perform the registration by minimizing the normalized dose value-based metric residuals. The algorithm was programmed in MATLAB code. As a test case, a clinical lung IMRT plan was measured by Gafchromicâ„¢ EBT 3 film inside a solid-water phantom on an Elekta VersaHDâ„¢ linear accelerator. The triple-channel dosimetry method was deployed to generate the reference dose plane from film. An unexposed film piece was used to correct the scan-to-scan artifact of an Epson 10000XL photo scanner. The evaluation dose plane was generated by the RayStation 6 treatment planning system. Gamma index analysis was used for the 2-D spatial dose comparison after the automatic registration.
Results: The passing rate (Î³< 1) for global and local gamma analysis is 100% and 94.8% respectively with 2mm/2% and 10% threshold dose. The mean global and local gamma index value is 0.11 and 0.40 respectively.
Conclusion: An efficient, automatic dose comparison registration technique for radiochromic film dosimetry has been developed and evaluated, showing robust performance. The method requires no made-ahead marks or other fiducial markers for either film or phantom use during the routine clinical IMRT/VMAT QA.