Effect of angle correction factors of MatriXX on VMAT dose verification

Objective: To investigate the effect of the angle correction factors of ionization chamber array Matrixx on the dosimetric verification of volumetric modulated arc therapy plan. Methods: The Matrixx (IBA) was put in the middle of the MultiCube Phantom. Computed tomography scan was performed and the results were sent to the Raystation treatment planning system (TPS). All data calculated from TPS and actually measured using MatriXX were gained under the following conditions: 6 MV, 100 MU, 28 cm × 28 cm radiation field. And the directions of beam wee from 0° to 180° with an interval of 5°(except an interval of 1° from 85° to 95°). The angle correction factors (CF)(θ) cor of MatriXX were calculated by the calculated dose values from TPS and the measured values using MatriXX and then mirrored to the opposite side. CF (θ) cor values were compared with CF (θ) def values that were given by manufacturer. The comparison was made by paired t-test. Results: CF (θ) cor and CF (θ) def were different, and the difference was significant between 85° and 95°, with a maximum of 7.5%(P < 0.05). The CF (θ) cor correction method had a higher gamma pass rate than the CF (θ) def correction method, with a maximum of 17%(P < 0.05). Conclusions: The ionization chamber array MatriXX has an angular dependence and the angle correction should be performed. The angle correction factors have the individual characteristics and are different in every unit ionization chamber of the MatriXX. Therefore, it is necessary to do angle correction for every unit ionization chamber of the MatriXX and the possible errors caused by various factors should be considered in order to improve the accuracy rate and gamma pass rate of the plan verification. (authors)