SU‐FF‐T‐360: Correction for Angular Dependence of the MatriXXEvolution Ionization Chamber Array

Abstract

Purpose: The MatriXXEvolution ionchamber array (IBA Dosimetry) is an attractive tool for plan verification of both conventional IMRT and rotational therapies (RapidArc, VMAT, TomoTherapy, etc). However, the response of this array has a significant angular dependence. We have quantified this dependence, and developed a correction method suitable for plan verification. Method and Materials: A MatriXX phantom (MatriXXEvolution ionchamber array + solid water) and a reference phantom (A12 ionchamber + solid water) were irradiated from different gantry angles (0°–360°, steps of 10°, with finer steps of 3° for lateral angles). Reference measurements were taken using a phantom of the equivalent dimensions. The calibration factor was calculated as the ratio of the MatriXX dose to the A12 dose for a given gantry angle. The water equivalent thickness of the MatriXXEvolution reported by the manufacturer was also examined in the TPS and by measuring 6MV attenuation due to MatriXXEvolution vs. solid water for variable total thickness of the phantom. Three patient‐specific IMRT and two VMAT cumulative plan verifications were delivered to the MatriXX phantom. Dose measurements collected by MatriXXEvolution were then corrected with in‐house software. Measurements were then repeated to the reference phantom. Corrected MatriXX, uncorrected MatriXX and A12 measurements were compared to the treatment planning system. Results: The angular response of the MatriXX array was found to vary as much as 11%, depending on the irradiation angle. The most variation was found to occur around 91–110° and 260–269°, and also close to 180°. The corrected MatriXX doses were found to agree within 1.0% of the A12 ion chamber dose, vs. 2.5% for uncorrected. Conclusion: The MatriXXEvolution ionchamber array has an angular dependent response which is clinically significant, and must be corrected. We have developed and tested the use of an angular‐dependent correction factor which corrects for these response variations.