Evaluation of P-type semiconductor diodes for in-vivo dosimetry in a 6MV x-ray beam

The importance of dose verification in radiotherapy has long been recognised. Semiconductor diodes used for in-vivo dosimetry are well suited for this purpose. It was the aim of the present study to investigate the performance of a commercially available set of diodes (Rainbow diodes by Nuclear Associates) which had been in use for two years in our department. The reading of the diodes proved to be reproducible within +/- 0.5% and recalibration was necessary approximately every month. While no marked influence of field size or beam modifiers was found, SSD proved to be a significant factor influencing diode response. Using various radiation qualities in air and phantom experiments, it could be shown that dose rate was not the contributing factor to the SSD dependence of the diode reading, as has been reported by some authors. The results reinforce the need to thoroughly assess diode response prior to clinical use. Using a correction for SSD this work has resulted in the implementation of an accurate and reliable system for diode entrance dose measurements in our 6MV x-ray beams, allowing an action level of +/- 4% to be set in patient measurements.