Low-dose measurement with a MOSFET in high-energy radiotherapy applications

The ability of a MOSFET dosimetry system to measure low therapeutic doses has been evaluated for accuracy for high-energy X-ray radiotherapy applications. The MOSFET system in high sensitivity mode produces a dose measurement reproducibility of within 10%, 4% and 2.5% for 2, 5 and 10cGy dose assessment, respectively. This is compared to 7%, 4% and 2% for an Attix parallel plate ionisation chamber and 20%, 7% and 3.5% for a Wellhofer IC4 small volume ionisation chamber. Results for our dose standard thimble ionisation chamber and low-noise farmer dosimeter were 2%, 0.5% and 0.25%, respectively for these measurements. The quoted accuracy of the MOSFET dosimetry system is partially due to the slight non-linear dose response (reduced response) with age of the detector but mainly due to the intrinsic variations in measured voltage differential per applied dose. Results have shown that the MOSFET dosimetry system provides an adequate measure of dose at low dose levels and is comparable in accuracy to the Attix parallel plate ionisation chambers for relative dose assessment at levels of 2-10cGy. The use of the MOSFET dosimeter at low doses can extend the life expectancy of the device and may provide useful information for areas where low dose assessment is required.