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  • Title: Contaminant electrons in the build-up region of a 4 MV photon beam.
    Author: Rustgi SN, Gromadzki ZC, Ling CC, Yorke ED.
    Journal: Phys Med Biol; 1983 Jun; 28(6):659-65. PubMed ID: 6410419.
    Abstract:
    The per cent ionisation 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 ionisation curves were measured at 86 cm SSD with a parallel-plate ionisation chamber embedded in a polystyrene phantom. The surface ionisation 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 ionisation with field size.
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