Electron contamination due to Lucite in a 45-MV photon beam

The presence of the Lucite blocking tray in a 45-MV betatron beam leads to a considerable loss in the skin-sparing properties of this unit. Measurements conducted with a parallel-plate chamber show that the surface dose is approximately doubled (up to 58% of the maximum absorbed dose) and the depth of maximum dose is reduced by as much as 1.5 cm by the Lucite. These results suggest that separate dosimetry data should be kept for blocked and unblocked fields. The penetration of the electron contamination to the open field depth of maximum dose can lead to erroneous results if attenuation measurements are made at that depth. Studies of the effect of the lead sheet and leaded acrylic on the surface dose showed that under normal circumstances the presence of high-Z materials increases the surface dose, hence these materials cannot in general be used as electron filters on very high energy x-ray units. A Luciteless blocking tray with universal triangular corner blocks has been designed for use in special cases requiring a reduced surface dose.