Diode verification of routine electron-beam treatments

A platinum doped n type commercial silicon electron diode/electrometer system was evaluated for use in our comprehensive external beam radiotherapy quality assurance program. Directional dependence of diode response was investigated by positioning the diode at the radiation isocenter and recording the response versus beam incident direction. Dose rate response was examined by testing for nonidealities in inverse square law behavior. Energy dependence of response was explored using 6-21 MeV beams. Dose response linearity was investigated from 50-800 cGy. Radiation perturbation was measured using localization film. Finally, the system was evaluated for initial treatment quality assurance in a number of clinical cases (N = 34). The diode response exhibited +/- 5% variation within +/- 20 degrees of the detector's marked "preferred" direction for the 6 MeV beam. Higher energy electron beams exhibited much less directional dependence. Dose rate response was found constant from 178-326 cGy/min (or 96-130 cm SSD). Energy dependence of response was constant within +/- 5.5% from 6-21 MeV. Dose response was linear (r2 = 0.999). Radiation field perturbation was significant at the depth of dose maximum (dmax). The maximum perturbation measured was -12.5% at dmax using the 6 MeV beam. Clinical trials of the system demonstrated that the diodes could be utilized for initial treatment quality assurance. The system proved effective for routine initial treatment electron beam in vivo dosimetry when the directional and energy dependent limitations of the system were respected.