Build-up region of megavoltage photon radiation sources.

Measurements of surfacedose and build‐up have been measured for photonradiationsources with maximum energy from 1.2 to 25 MeV. A variable volume ionization chamber was used and the results extrapolated to what would be obtained with a zero volume chamber. The results are found to depend systematically on the plate separation of a parallel plate ionization chamber and an empirical method was derived for correcting measurements made with a fixed volume chamber. The relationship of dose build‐up curves with skin reactions in radiation therapy patients is discussed.