The relationships between D,K and K_col are of fundamental importance in radiation dosimetry. These relationships are critically influenced by secondary electron transport, which makes Monte-Carlo (MC) simulation indispensable; we have used MC codes DOSRZnrc and FLURZnrc. Computations of the ratios D/K and D/K_col in three materials (water, aluminum and copper) for large field sizes with energies from 50 keV to 25 MeV (including 6–15 MV) are presented. Beyond the depth of maximum dose D/K is almost always less than or equal to unity and D/K_col greater than unity, and these ratios are virtually constant with increasing depth. The difference between K and K_col increases with energy and with the atomic number of the irradiated materials. D/K in ‘sub-equilibrium’ small megavoltage photon fields decreases rapidly with decreasing field size. A simple analytical expression for X, the distance ‘upstream’ from a given voxel to the mean origin of the secondary electrons depositing their energy in this voxel, is proposed: X_emp≈0.5R_csda(E_o), where E₀is the mean initial secondary electron energy. These X_emp agree well with ‘exact’ MC-derived values for photon energies from 5–25 MeV for water and aluminum. An analytical expression for D/K is also presented and evaluated for 50 keV–25 MeV photons in the three materials, showing close agreement with the MC-derived values.