A new radiotherapy surface dose detector: The MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOSFET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-mu m in diameter and consists of a 0.5-mu m Al electrode on top of a 1-mu m SiO2 and 300-mu m Si substrate. Results for % surface dose were within +/-2% compared to the Attix chamber and within +/-3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10x10-cm and 40x40-cm field 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 retrieval 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 Medicine.