SU-E-T-311: Dose Perturbation Due to Thin Layers of High-Z in HDR Ir-192 Source Dose Delivery

To evaluate the dose perturbation associated with thin layers of high-Z materials in HDR 192I source dose delivery through Monte Carlo simulation and experimental measurement.

Dose perturbation of high-Z thin materials in HDR 192Ir source dose delivery was experimentally and theoretically studied. MCNP5 Monte Carlo simulation code was employed to calculate the dose enhancement effects at upstream and downstream of high-Z materials. The cutoff energy for photon was 1 keV and for electron 5 keV. A parallel plate ion chamber with window thickness of 0.9 mm, was used to measure the dose rates at the downstream of the high-Z thin layer. The high-Z layer was positioned at 0.625 cm from the 192Ir source. The thickness of layer was from 0.1 to 1 mm. The results were normalized by the dose rate in a homogeneous water phantom at the location of the interface. The high-Z materials used were lead, tin and titanium. In addition, the dose enhancement effects of different high-Z materials at the shortest distance from the interface were also calculated and measured to examine the Z dependence.

The Monte Carlo simulation results agreed very well with the experimental data. At the surface of the 0.1 mm high-Z interface, Ti, Sn and Pb respectively increase dose rate by 1.2, 1.4 and 2.5 times the dose rate in homogeneous water medium. The dose rate enhancement depends on Z and thickness of the high-Z layer. The dose enhancement increases with Z. However, an increased thickness in the high- Z layer results in less dose enhancement due to attenuation.

HDR 192Ir source dose enhancement by a high-Z material thin layer is similar to that reported from a kilo-voltage X-ray beam. Attention should be paid for high-Z thin layers with HDR source clinically as it could cause significant dose enhancement.