Po‐Poster ‐ 12: Evaluation of MOSFETS for Gamma Knife® helmet factor measurements

The Gamma Knife ® is calibrated annually using the 18 mm collimator helmet; the dose rates for the other helmets are determined by measuring the radiation output relative to the 18 mm helmet. These relative measurements yield a helmet factor for each collimator helmet, which is incorporated into the treatment planning software, and is validated during annual quality assurance testing. Traditionally, helmet factors have been measured using 1×1×1 mm 3 thermoluminescent dosimeters (TLDs). Determining helmet factors using this technique is a challenging process since the TLDs are small and fragile, and their preparation, calibration and readout is very time consuming. They also have a large uncertainty of 2 to 5%. The goal of this research is to investigate the use of metal oxide semiconductor field effect transistor dosimeters (MOSFETs) for Gamma Knife® helmet factor measurements. The MOSFET linearity and reproducibility was first characterized using a60 Co teletherapy unit; the linearity and reproducibility were both found to be within 2%. The helmet factors were then measured using TLDs, and again using MOSFETs. The TLD results gave helmet factors within 2.5% of the manufacturer's values; the MOSFETs provided helmet factors within 1.3% of the manufacturer's values, in a fraction of the time it took to generate the TLD results. This investigation confirms that MOSFETs can be used to measure Gamma Knife® helmet factors more quickly and accurately than TLDs.