Application of the scaling factor method to estimation of beta dose distributions for dissimilar media separated by a planar interface

Energy response of an imaging plate exposed to standard beta sources

Imaging plates (IPs) are a reusable media, which when exposed to ionizing radiation, store a latent image that can be read out with a red laser as photostimulated luminescence (PSL). They are widely used as a substitute for X-ray films for diagnostic studies. In diagnostic radiology this technology is known as computed radiography. In this work, the energy response of a commercial IP to beta-particle reference radiation fields used for calibrations at the National Institute of Standards and Technology was investigated. The absorbed dose in the active storage phosphor layer was calculated following the scaling procedure for depth dose for high Z materials with reference to water. It was found that the beta particles from Pm-147 and Kr-85 gave 68% and 24% higher PSL responses than that induced by Sr-90, respectively, which was caused by the different PSL detection efficiencies. In addition, normalized response curves of the IPs as a function of depth in polystyrene were measured and compared with the data measured using extrapolation chamber techniques. The difference between both sets of data resulted from the continuous energy change as the beta particle travels across the material, which leads to a different PSL response.

Dose model for a beta-emitting stent in a realistic artery consisting of soft tissue and plaque

A model for the description of the near-field dose deposition from a 32p impregnated stent in an arterial system consisting of soft tissue and dense plaque is presented. The model is based on the scaling property of the dose-point-kernel (DPK) function which is extended to a heterogeneous medium consisting of a series of layers of different materials. It is shown that, for each point source originating from the stent surface, the DPK function for water can be scaled consistently along the path through the different layers of material to predict the dose at a given point in the heterogeneous medium. Radiochromic film dosimetry on actual 32p stents is used to test the new model. The experimental setup consists of a water-equivalent phantom in which a stent is deployed and on which a thin layer of polytetrafluoroethylene (PTFE) is deposited to simulate the presence of plaque. Layers of radiochromic films stacked over the phantom are used to measure the dose at distances varying from approximately 0.1 mm to approximately 3 mm from the stent surface with and without PTFE. It is shown that the proposed new DPK model for a heterogeneous medium agrees very well with the experimental data and that it compares favorably to the usual homogeneous DPK model. These results indicate that the new model can be used with confidence to predict the dose in a realistic artery in the presence of plaque.