Figure 1. Differences between depth dose curves measured with an ionization chamber and an energy-compensated photon diode for a 10 MV beam. Results are shown for 10 cm x 10 cm (bold line) 20 cm x 20 cm (feint line) and 40 cm x 40 cm (broken line) field sizes. The differences are percentages of maximum dose, and a positive difference indicates that the ionization chamber records a higher dose than the diode. Large differences are seen in the build-up region, but thereafter the differences are comparable with the magnitude of the noise in the respective measurements.