A three‐dimensional photon beam calculation is described which models the primary, first‐scatter, and multiple‐scatter dose components from first principles. Three key features of the model are (1) a multiple‐scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions. The results of calculations for cobalt‐60 in a homogeneous phantom are used to verify the accuracy of the model.