Dose response effects of diodes due to the high atomic number of silicon relative to water are investigated. While quality chances in the primary component of a megavoltage beam with depth are minimal. Compton scattered photons are shown to have a substantial effect on the quality leading to their enhanced absorption in silicon via the photoelectric effect. Monte Carlo methods were used to study and model this phenomenon. Measurements of dose rate, depth and field size dependence are examined for commercially available diode detectors and ionisation chambers.