Contaminant electrons in the build-up region of a 4 MV photon beam

The per cent ionization curves for the 4 MV photon beam from a Clinac 4 exhibit pronounced enhancement with field size in the build-up region. The source and nature of this increase is investigated by analysing the radiation components of the 4 MV photon beam with a 0.23 T magnetic field. The build-up ionization curves were measured at 86 cm SSD with a parallel-plate ionization chamber embedded in a polystyrene phantom. The surface ionization increases from 18% to 32% as the field size is increased from 4.3 cm x 4.3 cm to 9.7 cm x 32.3 cm. With the electromagnet placed beside the treatment head, the field size dependence reduces to 16% to 24%. This and similar results at 113.2 cm SSD, clearly demonstrate that electrons from the Clinac 4 treatment head are responsible for approximately half the observed increase in the surface ionization with field size. (author)