Electron contamination in 6 0 Co gamma-ray beams

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Abstract
All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray and/or the air through which the beam passes. Skin sparing [in humans], the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron contamination. The present study of a 60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72-200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5 .times. 5, 20 .times. 20, and 35 .times. 25-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a clear Co .gamma.-ray beam of 1-cm diameter.