P32

Brachytherapy of superficial skin tumors using beta-emitting sources is a method that has been investigated by some researchers in both simulation and experimental studies with promising results. In the current study, the effect of geometrical parameters of some relevant radionuclides including Y-90, Re-188, P-32, and Ho-166 on the depth dose distribution in skin tissue has been investigated through Monte Carlo simulation. MCNPX Monte Carlo code was employed to model the above-mentioned patch sources in cylindrical format and then the effect of patch geometrical parameters including the source-to-skin distance (SSD), patch thickness, and patch diameter on depth dose distribution was assessed through modeling and calculation of the dose inside a cubic phantom mimicking the skin tissue. The obtained results demonstrated that increasing the SSD, patch thickness, and patch diameter (with the same activity) will reduce the depth dose distribution. Changing the SSD has a more significant effect on the dose gradient within the depth than other geometrical parameters. It was also observed that the effect of patch diameter on the skin-delivered dose gets less sensible as the patch size goes beyond the range of beta radiation inside tissue. Finally, it can be concluded that the patch source geometrical parameters can affect the depth dose distribution inside the skin tissue. This fact may be of concern regarding the delivery of a high radiation dose in a single treatment session. Therefore, variations of patch source geometrical parameters should be considered during the skin dose calculation plan.

Vertebroplasty is a minimally invasive outpatient procedure to stabilize compression fractures in the spine. This procedure involves injecting bone cement into the vertebrae that have been cracked or broken, typically due to osteoporosis. The cement hardens inside the bones, providing stability to the fractures and supporting the spine. Additionally, radioactive bone cement and brachytherapy sources have been utilized to suppress tumor growth in the vertebral body.

We present a novel brachytherapy technique for treating vertebral body metastases using a liquid form of radioactive sources, Phosphorus-32 and Lutetium-177, separately mixed with bone cement and injected into vertebral body bone prostheses. We also investigated the dose distribution of the radioactive bone cement by theoretically calculating it using GEANT4 Monte Carlo and measuring it using TLD dosimeters for Phosphorus-32 and Lutetium-177 loaded in vertebral bodies.

CT-scanned images of each vertebral body (L2 and L3) were imported into GEANT4 for simulation purposes. Two simulations were performed to evaluate the possibility of using PLA prostheses in ex vivo measurements, using bone and PLA material as a bone substitute for brachytherapy of Lutetium-177 and Phosphorus-32. The simulations calculated the dose distribution, dose rates, and deposited dose to the spinal cord and aorta. Next, 3D-printed bone prostheses were drilled and separately filled with bone cement, including PMMA-P32 and PMMA-Lu177, in liquid form using the Vertebroplasty technique. The dose to regions of interest was measured using Thermoluminescence dosimeters.

When comparing the simulated and measured results of dose rates, it was observed that P32 delivers higher doses to normal organs such as the spinal cord and aorta. At the same time, Lu177 has better sparing in these regions of interest. Therefore, while P32 and Lu177 are suitable for radioactive bone cement treatment, Lu177 delivers relatively lower doses to vital organs such as the spinal cord and aorta. Additionally, Lu177 has characteristics such as a shorter range and lower energies of beta particles in tissue and the presence of gamma rays that make it a better choice for the same treatments. It also provides the possibility of SPECT imaging.