An investigation of the IQM signal variation and error detection sensitivity for patient specific pre-treatment QA.

Abstract

To evaluate the Integral Quality Monitor (IQM) as a clinical dosimetry device for detecting photon beam delivery errors in clinically relevant conditions. The IQM's ability to detect delivery errors introduced into clinical VMAT plans for two different treatment sites was assessed. This included measuring 103 nasopharynx VMAT plans and 78 lung SBRT VMAT plans with introduced errors in gantry angle (1-5°) and in MLC-defined field size and field shift (1-5mm). The IQM sensitivity was compared to ArcCheck detector performance. Signal dependence on field position for on-axis and asymmetrically offset square field sizes from 1×1cm2 to 30×30cm2 was also investigated. The IQM detected almost all introduced clinically-significant MLC field size errors, but not some small gantry angle errors or most MLC field shift errors. The IQM sensitivity was comparable to the ArcCheck for lung SBRT, but worse for the nasopharynx plans. Differences between IQM calculated/predicted and measured signals were within±2% for all on-axis square fields, but up to 60% for the smallest asymmetrically offset fields at large offsets. The IQM performance was consistent and reproducible. It showed highest sensitivity to the field size errors for these plans, but did not detect some clinically-significant introduced gantry angle errors or most MLC field shift errors. The IQM calculation model is still being developed, which should improve small offset-field performance. Care is required in IQM use for plan verification or online monitoring, especially for small fields that are off-axis in the detector gradient direction.