Patient dose measurements in photon fields by means of silicon semiconductor detectors

Semiconductor detectors based on p-type silicon and designed for in vivo measurement of entrance dose at the reference point from photon radiation fields, are described. To estimate the absorbed dose at the reference point from measurements with a thin detector, field-size dependent correction factors must be applied to the reading, as the shape of the dose buildup curve varies with field size. To decrease or avoid field-size dependent correction factors, the detector can be covered with a buildup cap. The presence of such a detector will cause perturbation of the radiation field. Therefore, the design of a detector, irrespective of its type, intended for patient dosimetry involves a compromise between minimizing the radiation field perturbation and minimizing field-size dependent correction factors. Detectors with three different buildup caps were designed to cover the energy range from cobalt-60 to 16-MV x rays. The three different detector types were investigated with respect to their signal dependence on field size, field perturbation, and directional dependence. A summary of radiation damage effects on sensitivity, and of sensitivity variation with temperature is also presented.