Surface and build-up region dose analysis for clinical radiotherapy photon beams.

The standard clinical approach of dose measurement using a Farmer type fixed plane parallel cylindrical ionization chamber produces erroneous results in the build-up region. We studied dose distribution in this region using a Monte Carlo simulation technique and compared our results with data measured using extrapolation, parallel plate, and cylindrical farmer type ionization chambers for 6 and 10 MV photon beams from two different accelerators. The extrapolation chamber data agreed favorably with the Monte Carlo results, suggesting that dose at the skin surface and a few mm beneath is significantly lower than conventionally accepted values.