Room: Exhibit Hall
Purpose: A commercial portable Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) system from Best Medical Canada was tested to evaluate the feasibility of performing in vivo skin dose measurements under bolus with MOSFETs during patient photon and electron irradiations.
Methods: Experiments were performed using dual-bias TN-502RD MOSFETs manufactured by Best Medical Canada. MOSFETs were connected to a portable dosimeter with bias supply set at standard sensitivity. Eight identical MOSFET dosimeters were investigated with three different photon energies and six different electron energies on three Varian Linear accelerators to characterize the energy dependence, stability, linearity, angular dependence and field size dependence of the detectors. Measurements were also performed to measure the accuracy of the MOSFET dose response with depth and the amount of attenuation induced by the MOSFET beneath the detector. The MOSFET measurements in phantom were compared to those of a thimble-type ionization chamber (PTW-31013) with sensitive volume of 0.3 cc and IBA CC13 with sensitive volume of 0.13 cc.
Results: Although the MOSFETs are nominally energy independent within 5%, a systematic difference in the calibration factor of ~3% was observed between photon and electron exposures. For this reason, separate calibration factors were used for photons and electrons. The calibration factors were then found to be within 2% (1SD) for both photons and electrons. The MOSFETs showed good reproducibility (Â±2%), angular independence (Â±3%), field size independence (Â±2.5%) and clinically minimal attenuation. The MOSFETs also showed good linearity over a range of dose from 0.01 Gy to 3 Gy for photons and up to 7 Gy for electrons.
Conclusion: This study shows that the portable MOSFET system can provide clinically acceptable accuracy and repeatability for in-vivo skin dose measurements if separate calibration factors are determined for photons and electrons