An analytical approximation for the scatter to primary dose ratio (SPR) onthe central axis was validated against Monte Carlo results and experimentalmeasurements for homogeneous and inhomogeneous phantoms. The analytical approximation only included first-order Compton scatter. The contributionto the total SPR from first-order Compton scatter, multiply scatteredphotons and electron scatter was investigated using Monte Carlo simulationfor homogeneous phantoms (up to 30 cm thick for 6 and 18 MV beams; sourceto detector distances from 150 to 230 cm) as well as for a neck, thorax andpelvis phantom. SPRs were measured on the central axis with an ionizationchamber for water phantoms (up to 20 cm thick at 4 MV, 30 cm for 6 MV and10 MV and 40 cm for 18 MV; source to detector distances of 185 and 200 cm)and for phantoms representing the neck, thorax and pelvis (for air gaps of50 cm and larger). The mean difference between the experimental andanalytical SPRs on the central axis for source to detector distances of170 cm or greater was within: -0.003 (neck); -0.012 (thorax); -0.028(pelvis, 10 MV) and 0.008 (pelvis, 18 MV) respectively.