The radiation dose outside the radiotherapy treatment field can be of clinical concern and, therefore, a method of accurately predicting the peripheral doses received by tissues would be beneficial. This paper describes a semiempirical method developed for calculating the peripheral dose received at points outside the collimated field edge for incident photon beams with energies below neutron production thresholds (less than 10 MV). The dependence of the peripheral dose upon depth, distance, field shape and size, azimuthal angle about the central axis, external contour variations, and tissue heterogeneities are accounted for by this calculation. Predictions by this algorithm are compared with measurements and it is shown that the method is capable of reproducing the measured peripheral dose values usually to within the statistical uncertainties of the data.