SU‐E‐T‐359: Comparison of Three Array Calibration Methods of MapCheck for Elekta Beam Modulator LINAC

Abstract

Purpose: To compare three array calibration methods of MapCheck for an Elekta Beam Modulator LINAC. Methods: Both Elekta Synergy and Elekta Beam Modulator LINACs are used in our clinic. While an Elekta Synergy LINAC is a conventional Linac with the maximu field size 40 × 40 cm2, an Elekta Beam Modulator LINAC only has a maximum field size of 21 × 16 cm2. Because the MapCheck requires a field size of 26 × 26 cm2 for array calibration according to its user manual, it is a challenge to calibrate MapCheck array for Beam Modulator. We have tested three methods for this Purpose: 1) use the array calibration file from Elekta Synergy; 2) calibrate the MapCheck using Beam Modulator at 135cm SSD; 3) calibrate the MapCheck with both Beam Modulator and Elekta Synergy, then combine the calibration files. For the third method, the MapCheck was first calibrated with Beam Modulator at both collimator angles 0 and 90 degrees at 110cm SSD. Then the MapCheck was calibrated with Elekta Synergy using the standard method at 100cm SSD. The three array calibration files were combined such that it best describes the response of MapCheck detectors to Beam Modulator and all the detectors are covered. After calibrating the MapCheck detector array with the three methods, four prostate and four H&N IMRT treatment plans were delivered. The measurements were compared with the calculations. The Gamma Index (Gamma Index < 1) pass rates with a criterion of 3% and 3mm from all the three calibration methods were compared. Results: The pass rate of Gamma Index ranges from 90.4% to 100%. The null hypothesis that all three methods come with the same QA pass rates can not be rejected (p>0.1). Conclusions: For the testing cases, no significant difference was found among the three array calibration methods in term of Gamma Index pass rate.