4 presents electron fluence correction factors calculated according to eq.
Systematic uncertainties are much harder to estimate.
The fluence correction factors are not unity at the surface for clear and white polystyrene materials. We have studied fluence correction factors with a beam incident on a very thin slab of polystyrene and fluence correction factors are found to be unity in this case. This value is close to unity and hence the calculated value of drops at depths in the bremsstrahlung tail as seen in the lower panel of fig. Figures 5 to 7 show calculated electron fluence correction factors as a function of depth for incident electron beams of 6 to 18 MeV from a Clinac 2100C accelerator and those determined according to the AAPM TG-25 protocol. Since the fluence correction factors are not listed below 6 MeV in the protocol, the curves for 6 MeV from the AAPM TG-25 protocol are plotted for reference only.