Correction of the angular dependence of MatriXX Evolution detectors and its impact in the IMRT and VMAT treatment validation

Abstract

The purpose of the study was to create detector element-specific angular correction factors for each detector of the MatriXX planar ion chamber array and compare them to vendor-default angular correction factors.  Additionally, the impact of both factors on gamma index was quantified using two corrections.

The correction factor of each element is determined irradiating the detector at different incidences by the ratio of the calculated expected dose to the MatriXX measured dose as a gantry angle function. To evaluate its impact, sixty-five pre-irradiated patient-specific dose validations were re-analyzed using the gamma index with: 3%/3 mm, 2%/2 mm, 1%/1 mm criteria.

The factors for 6 MV were found to differ (7%) from the default ones for specific angles –particularly for 85° to 95°. For 10 MV, differences (1.0%) existed when correction factors were created using various ROI’s. Two corrections were proposed, absolute differences for 3%/3 mm, 2%/2 mm, and 1%/1 mm were up to 1.5%, 4.2% & 4.1% ( p < 0.01), respectively.

Large differences in the default and specific factors were noted for 6 MV and lead to improvement of the absolute gamma index value of up to 4.2%. In general, gamma index value increases for patient specific dose validations when using device specific factors.