Effect of scatter material on diode detector performance for in vivo dosimetry

The accuracy of Scanditronix EDE, EDP10 and EDP20 diodes for entrance dosimetry of complex fields has been assessed using static and dynamic multileaf collimator test fields on phantoms. Specifically, surrounding scatter material size and composition have been investigated. The EDP10 and EDP20 diodes incorporate steel build-up caps. The effect of varying the diameter of the wax scatter discs on the dosimeter response showed a systematic detector under-response for the smaller discs with errors up to 4.1% relative to full scatter conditions. In static fields, all diodes over-respond at a field size of 1 x 1 cm(2). Diodes with non-water-equivalent build-up material exhibit over-response of up to 10.8%. In dynamic fields, diodes over-respond when there is an increased contribution from phantom scatter and under-respond in shielded regions due to low dose rate and beam hardening. For high dose regions, all diodes over-respond with the greatest over-response of 3.8% observed with a 6 mm sliding window field. The EDE diode with a 6 cm scatter disc correlated best with the reference dosimeter. A diode design with minimal non-water-equivalent components and the addition of a 6 cm diameter water-equivalent disc for scatter material is recommended for the in vivo dosimetry of 6 MV complex fields.