Angular dose dependence of Matrixx TM and its calibration

Abstract

The MatrixxEvolution (IBA Dosimetry), is an ionchamber array used in the experimental verification of absolute dose for conventional IMRT and rotational therapies such as VMAT and TomotherapyTM. One of its many applications consists of cumulative plan verification against treatment planning systems (TPS) where dose from all treatment gantry angles is delivered to the stationary detector. Experimental calibration of the detector recommended by the manufacturer involves only an AP calibration field and does not address angular dependence of MatrixxEvolution. This angular dependence may introduce dose bias in cumulative plan verification if not corrected. In this study, the angular dependence of MatrixxEvolution has been characterized and a method for its calibration was developed. Relatively large discrepancies were found in dose response for anterior vs. posterior fields for four different MatrixxEvolution detectors (up to 11%). And relatively large variability of responses as a function of gantry angle, specifically in angle ranges of 91°-110° and 269°-260°. With our calibration method, the bias due to angular dependence is effectively removed in experimental verification of IMRT and VMAT plans.