LATERAL ELECTRON EQUILIBRIUM AND ELECTRON CONTAMINATION IN MEASUREMENTS OF HEAD-SCATTER FACTORS USING MINIPHANTOMS AND BRASS CAPS

The head-scatter factor (S-h) can be measured with a narrow miniphanto m or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the S-h values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (gamma(LE E)) required to achieve complete LEE for photon beams ranging from Co- 60 to 24 MV. The measurement shows that the error introduced to the S- h value due to lateral electron disequilibrium is negligible. The radi i of the miniphantoms or the sidewall thicknesses of the caps can be r educed below gamma(LEE) provided they are thick enough to prevent the effect of electron contamination.