Electron depth absorbed doses for small phantom depths. Comparison between different accelerators.

Depth dose distributions at small phantom depths in polystyrene were measured with a liquid ionization chamber at six different accelerators. The absorbed dose at 0.5 mm depth relative to the absorbed dose maximum in the central beam varied between 0.77 and 0.92 for 10 MeV electrons. Generally the surface absorbed dose increased with increasing energy or decreasing field size. Results from measurements off axis indicate small differences from the data available for the central beam.