"Measurements of surface dose using build-up curves" obtained with an extrapolation chamber.

Surface dose was measured in an extrapolation chamber and was found to be less than previously reported. The slope of the build-up curve was less steep with the Clinac 4 than with 60-Co and much steeper with the Clinac 35 at 25 MV than with the betatron at the same energy. The gradient of the depth-dose curve in the build-up region is probably the best indicator of skin damage.