title: The use of lead-lined cones to improve the beam characteristics of a 50-kVp contact therapy x-ray beam.
abstract: Measurements were made to calibrate a Philips RT-50 contact therapy unit using an EG&G extrapolation chamber. The measured half-value layer of 0.27 mm in aluminum agrees closely with published data. Exposure rates in air for 4-cm source-to-surface distance (SSD) cones fall off faster than can be explained by the inverse square law alone. Lining the cone with lead results in exposure rates that follow the inverse square law at all distances from the cone end. Depth dose measurements made for 2- and 4-cm SSD cones produced the anomalous result of greater depth dose for the shorter SSD. This is due to a low-energy component arising from the end of the 4-cm SSD cone, producing an artificially high surface dose. Lining the cone with lead reduces the component and significantly increases the depth dose. The low-energy component is believed to be 6-8 keV characteristic x rays that arise from the stainless-steel cone. Lining the cone with lead either absorbs these x rays or prevents their production.
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation*; D002138:Calibration; D006801:Humans; D007854:Lead; D011879:Radiotherapy Dosage
chemical_list: D007854:Lead