A NEW RADIOTHERAPY SURFACE DOSE DETECTOR - THE MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately meas urable by use of a MOSFET detector system. The MOSFET (Metal Oxide Sem iconductor Field Effect Transistor) is approximately 200-mu m in diame ter and consists of a 0.5-mu m Al electrode on top of a 1-mu m SiO2 an d 300-mu m Si substrate. Results for % surface dose were within +/-2% compared to the Attix chamber and within +/-3% of TLD extrapolation re sults for normally incident beams. Detectors were compared using diffe rent energies, field size, and beam modifying devices such as block tr ays and wedges. Percentage surface dose for 10x10-cm and 40x40-cm fiel d size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate r etrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.) (C) 1996 American Association of Physicists in Medic ine.