The low energy (30-50 kVp) beams from an intra-operative X-ray source are modelled using a basic analytical model considering just primary beam attenuation and absorption. Spatial dosimetry at such low energies is difficult due to the rapid changes in dose-rate from the radiation source. The purpose of the model was to determine the variation with distance in water of coefficients required for beam dosimetry and to validate beam measurements performed in water of high-gradient dose distributions. The model predicts a change in mean mass-energy absorption coefficient of up to 3 % over the range of clinically-relevant distances in water. Distance-dose distributions (variation in dose with distance in water) for the X-ray source were calculated with the model and found to be in agreement with measurement (at clinically-relevant distances), to within a spatial distance comparable to the dimensions and positional accuracy of the ionization chamber used, and comparable to the expected dosimetric anisotropy of the radiation source. Measured and calculated distance-doses begin to diverge at relatively large distances from the radiation source, which is where dose-rates are so low that detector signal levels are comparable with noise.