Dose responses of a MOSFET detector for in-vivo dosimetry of clinical electron beams

Metal Oxide-Silicon Field Effect Transistor or MOSFET radiation detectors have become excellent in-vivo dosimeters for radiation therapy, but the MOSFET dosimeter's responses to clinical electron beams have yet to be fully evaluated. The authors measured MOSFET dose responses to varying electron beam energies and angles on patients and anthropomorphic phantoms. MOSFET responses in mV per cGy are compared against electron skin doses determined using a thin parallel plate ionization chamber. Results indicate the MOSFET dosimeter is near energy independent to within 1%. The MOSFET responses are dependent on the electron source to patient skin distances (SSD) within +/-5%, indicative of some dependence on dose rate or accelerator pulse rate by the field effect transistor. These SSD responses compare favorably with the response typically observed with diode in vivo dosimeters, except at extremely short SSD (i.e., 95 cm) close to the mouth of the electron cones where both the chamber and the MOSFET or diode readings are difficult to interpret. MOSFET responses correlated within 2% of the doses in different electron field sizes including blocked or small cutout down to 4 cm. The authors concluded MOSFET is dosimeter suitable in electron beams and it necessitates only a nominal dose calibration factor for in-vivo dosimetry treatment field conditions, except when patient treatment distances are considerably greater (20 cm) than accelerator nominal isocenter.