Purpose: Radiotherapy quality assurance (QA) requires quantitative methods to compare two different dose distributions to determine whether the differences are significant or within tolerances. The gamma index (GI), which is based on predetermined dosimetric criteria, is the most popular method for comparing 1, 2, or 3D dose distributions. This study proposed a gamma-defined bounded dose-volume histogram (bDVH) QA technique, which is based on a dose difference (DD) and a distance-to-agreement (DTA) tolerance to better reveal the dosimetric cause of plan failure/error.
Methods: An algorithm that translates gamma criteria to a bDVH was developed. Gamma criteria of 1%/1mm, 2%/2mm and 3%/3mm were evaluated in three clinically relevant SBRT treatment plans for NSCLC with a dose calculation grid of 2x2x2 mm. Log file-based dose reconstruction is performed for QA deliveries of the 3 patient treatment plans. A DVH is extracted for each delivery and compared to the bDVH based on the 3 gamma criteria. Delivery is within tolerance if 100% of the points on the delivered DVH fall within the bDVH. All deliveries were synthetically shifted by 2x the distance criterion or scaled by 2x the dose criterion and tested again.
Results: The bDVH test passed for all deliveries that met the gamma criteria from log file-based dose reconstruction at the 2%/2mm and 3%/3mm levels at 100% of points falling within the bDVH bounds. The bDVH test also failed at less than 100% when the dose was shifted or scaled for all 1%/1mm, 2%/2mm, and 3%/3mm tests.
Conclusion: A bounded DVH generated based on gamma criteria can now be used as a dosimetric test for patient-specific quality assurance. For the first time, a DVH can be dosimetrically evaluated to determine whether it is within machine tolerances or it presents a risk.
Funding Support, Disclosures, and Conflict of Interest: Research supported by RefleXion Medical.