Sensitivity of the IQM and MatriXX detectors in megavolt photon beams.

Abstract

This study focused on evaluating the sensitivity of integral quality monitoring (IQM®) system and MatriXX detectors. These two detectors are recommended for radiotherapy pre-treatment quality assurance (QA). IQM is a large wedged-shaped ionisation chamber mounted to the linear accelerator (linac) head in practice. MatriXX consists of an array of ionisation chambers also attached to the linac head. In this study, the dosimetric performance and sensitivity of MatriXX and IQM detectors were evaluated using the following characteristics: reproducibility, linearity, error detection capability and three-dimensional conformal radiotherapy (3D-CRT) plans of the head and neck, thorax and pelvic regions. This study indicates that the signal responses of the large ionisation chamber device (IQM) and the small pixel array of ionisation chambers device (MatriXX) are reproducible, linear and sensitive to MLC positional errors, backup jaw positional errors and dose errors. The local percentage differences for dose errors of 1%, 2%, and 3% were, respectively, within 0.35-8.23%, 0.78-16.21%, and 1.10-24.41% for the IQM device. While for the MatriXX detector, the ranges were between 0.24-3.19, 0.57-6.43 and 0.81-12.95, respectively. Since IQM is essentially a double wedge-shaped large ionisation chamber, its reproducibility and detection capability are competitive to that of MatriXX. In addition, the sensitivity of the two QA systems increases with an increase in escalation percentage, and the signal responses are patient plan specific. The two detectors response signals have good correlations and they are accurate for pre-treatment QA. Statistically, (P<0.05) there is a significant difference between the IQM and MatriXX response to dose errors.