SU-E-T-227: Least Restrictive Assignment of Dose-Distance Differences (LRAD) in IMRT On a Conventional Linac

Purpose: To develop a method to compare dose distributions based on the least restrictive of dose and distance differences, and to evaluate the method for IMRT QA on a conventional linac. Methods: Lrad is defined as the least restrictive of the dose difference at each point in the distributions being compared (%D), with dose normalized by the same factor, and the distance to agreement (DTA in millimeters), limited to directions parallel and perpendicular to MLC motion. If %D<%Do, Lrad=%D. Otherwise, if L=%D/DTA>Lo, Lrad=DTA, else Lrad=%D. Lrad fails at any point when Lrad>DTAo or Lrad>%Do. We compared LRAD (Lo=1%/mm, DTAo=2mm, %Do=2%) with Gamma index tool (2%/2mm pass/fail criteria) for IMRT QA with 2D detector arrays. Dose distribution measured for 6 MV and 18 MV x-rays for various field sizes and shapes, including 6 IMRT plans, were compared to measurements made with output, beam flatness, beam symmetry, MLC leaf positions, and detector array translation and rotation adjusted at up to twice published acceptable tolerance. Results: LRAD was more effective than Gamma in detecting a 2% drift in output (dose per monitor unit), 1–2 mm drifts in MLC leaf position for 1–8 leaves in the field, and beam flatness and symmetry drifts that resulted in a 3% change in the off-axis ratio. LRAD and Gamma were equally effective for larger output changes and for detector translations and rotations. For all ∼400 measurements performed, LRAD detected 78% of the error and Gamma detected 52% of the errors. By using a color scale to display dose differences and grey scale for spatial errors, the reason for failure was self evident, whether it be beam or mechanical changes. Conclusion: LRAD proved to be advantageous in its ability to detect and distinguish between realistic changes in beam and mechanical settings in comparison to the Gamma Index