Characteristics of silicon diodes as patient dosemeters in external radiation therapy

Silicon diodes connected to an integrating instrument that are used to measure the entrance dose on patients undergoing radiation therapy have been investigated with special emphasis on practical clinical aspects. The variation of the diode response for different photon qualities with different field sizes and different irradiation situations including oblique fields, wedges, blocking filters giving different electron contamination have been measured. The diode response for the different situations met in clinical practice when using various electron energies have also been examined. The results from measurements for patients treated with high energy are presented. The study has shown that if the mean value of all measured entrance doses with the diode on a patient differ more than +/- 3% from the presented absorbed dose for 60Co gamma radiation, a correction of the given dose should be made. The corresponding figure for high energy X-rays is +/- 5%.