Three-dimensional dosimetry around small distributed high-Z materials

Abstract

Patients are increasingly undergoing radiotherapy procedures, in which small metals are implanted in the body for target localization for IGRT or targeted therapies. Previous, interface dosimetry studies focused high-Z materials irradiated by low energy beams where the dose enhancement is large. In the majority of the cases, they used one or two dimensional detectors. Therapeutic beams, however, are mostly 6 MV and higher with significantly less dose enhancement. Over the last decade, significant improvements in polymer gel dosimetry have been made allowing for improved 3D dose measurements. The purpose of this study was to better understand the dose around distributed high-Z materials irradiated by high energy photon beams and investigate the feasibility of 3D dose measurements. A Monte Carlo code was used to determine the effect of various foil configurations. The dosimetric effect of foil thickness, separation, energy and other factors were investigated. Software tools were also developed to process the data. These results were used to help identify suitable experimental setups. The dose around two foils was compared to the dose resulting from adding the dose of two single foils. The dose around a single foil was also compared to the dose around a fiduciary marker. Later on, we looked at how distributing the thickness of the high-Z foil over a wider area affected dose and how that compared to a to the dose around a single foil. Finally, we looked at the effect of pair production and how it affected the distribution of dose in select configurations. Several polymer gel dosimeter (PGD) were evaluated and two were selected for further study. Various formulations were investigated and procedures developed to meet the needs of the project. Materials compatibility studies were performed to ensure that there were no reactions between the PGD and inserted materials within the time frame of the studies. PGDs were manufactured and thin lead foils with the configuration determined earlier were inserted into the polymer gel. The PGDs was irradiated with 18 MV photons and the dose was quantified using MRI with a multiple spin echo technique for the measurement of the spin-spin relaxation rate (R2). The measured dose data were compared to theoretical data obtained from Monte Carlo experiments. The dose profiles around the foils from the PGD were in agreement with dose values from simulation. This project demonstrated that it is feasible to use polymer gel dosimetry to measure the fine dosimetric structures around a small metallic object. We also determined that material, foil thickness, separation and photon energy had the largest effect on the dose in-between a two foil configuration. When the foils were close, we found that the dose around the two foils was larger but not significantly different from the combined dose of two single foils with the same separation. We also found that the dose upstream and downstream of a distributed foil is less that the upstream and downstream dose around a single foil of equivalent thickness.