Question
Does the myQA SRS exhibit a dose rate dependence?
Answer
The myQA SRS detector exhibits a dose rate dependence, meaning its response varies with the applied dose rate. The measured signal can change by approximately 5% when the dose rate increases from 200 MU/min to 2400 MU/min.
To compensate for this known dependence, the myQA Patients Software provides a Dose Rate Calibration feature.
Dose Rate Calibration Procedure
For each beam energy, the same number of monitor units (MUs) is delivered at several different dose rates. The acquired data is normalized to the maximum dose rate. A linear fit is then calculated, yielding an offset and a slope. This linear relationship is subsequently used to determine correction factors for the various dose rates, which are Linac-dependent.
Dose Rate Correction During VMAT Deliveries
During VMAT irradiations, the instantaneous dose rate is not directly known, as it varies continuously. The only available information is the signal intensity within each individual frame (expressed in counts per millisecond, cnts/ms). This signal information is used to apply the correction factor online to each pixel in real time.