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Electron dosimetry of angular fields

Royal Marsden NHS Foundation Trust, Fulham Road, London SW3 6JJ, UK

Shaping electron fields through the use of lead cut-outs may result in there being acute angles in part of the field. Using both experimental techniques and EGSnrc Monte Carlo simulations an investigation was carried out to determine the dosimetric consequences of this. Measurements were made to investigate how the field dose was related to the angle between adjacent sides in the cut-outs. The study involved two electron energies (9 MeV and 12 MeV) and source–skin distances (SSDs) in the range 1000–1100 mm. For angles less than about 120° the dose received in the angular region decreased significantly, the effect being more pronounced at 12 MeV than at 9 MeV, and at longer SSDs. The planar shapes of the Monte Carlo dose distributions agreed with those experimentally determined to within ±1.5 mm at 9 MeV and ±1.0 mm at 12 MeV, demonstrating the validity of using such calculations for this purpose. Graphs are presented which may help in the prospective assessment of the dose reductions likely to be incurred.