Abstract
The combination of electron filter material, thickness and position which would minimise the surface dose for a 10 MV X-ray beam from a varian Clinac 018 was investigated. It was found that a lead filter of 0.17 g cm-2 thickness, placed below the lower collimator at a distance of 50 cm from the X-ray target, offers the greatest reduction in build-up region dose. Tin and copper filters with thicknesses of 0.19 and 0.23 g cm-2 respectively are slightly less effective in reducing the dose in the build-up region. The maximum reduction in surface dose achieved by added filtration is only 7% for a field size of 25 cm*25 cm. Smaller improvement is achieved with smaller field sizes. In cases where blocking trays are used, the filters should be placed under the trays toward the patient's skin.
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