Purpose: To evaluate a quasi-3D dosimetry system using a solid polyurethane-based dosimeter when conducting patient-specific quality assurances (QAs) in radiation therapy.
Methods: The patient-specific QA is the procedure to verify the coincidence between measurement and calculated dose distribution for intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VAMT) plans by using gamma analysis. A VAMT plan for a pancreas patient was analyzed with a criterion of both 2%/2 mm and 3%/3 mm, a threshold value of 10%, and a tolerance level of 90%. For the plan, the 2D gamma analysis was fulfilled with a MapCHECK2, and the 3D gamma analysis was performed using in-house solid polyurethane-based dosimeters and a quasi-3D phantom. The phantom was produced by drilling 25 holes of 9.4 mm in diameter and 56 mm in depth at even intervals, and it was made to be used by inserting a solid polyurethane-based dosimeter of the same size as the hole. A total of 9 solid polyurethane-based dosimeters were positioned according to the dose distribution of the plan. The 3D gamma analysis was performed on a region of interest of 5.5 mm in diameter and 35 mm in depth using a free platform for biomedical research, (3D) Slicer.
Results: For the 3D gamma analysis, the average passing rates with 2%/2 mm and 3%/3 mm criteria for relative dose distribution were 91.6% ± 1.4% and 99.4% ± 0.7%, respectively. For the 2D gamma analysis showed passing rates of 97.5%, 99.3% with 2%/2 mm and 3%/3 mm criteria, respectively.
Conclusion: The quasi-3D dosimetry system was evaluated by performing patient-specific QAs with the solid polyurethane-based dosimeters and quasi-3D phantom. As the 3D gamma index for all solid polyurethane-based dosimeters showed more than 90% for 2%/2 mm criteria, this result means there was a possibility that the quasi-3D dosimetry system was clinically acceptable.