Scattered photons as the cause for the observed dmax shift with field size in high-energy photon beams.

Measurements on the Sagittaire linear accelerator and Allis-Chalmers betatron at M. D. Anderson Hospital indicate that the observed dmax shift with field size is due to the presence of Compton-scattered photons in the therapy beam, and not electrons as proposed by others. Separating the primary from other radiation components indicates that the secondary fraction represents a percentage contribution to the overall beam that increases as the collimators are opened. This is consistent with what would be expected from Compton scattering and explains the effective softening of the beam as field size increases.