Purpose: GammaTile (GT Medical Technologies, Tempe AZ) is a permanent brain implant used to treat certain brain tumors. A GammaTile treatment program has been established at our institution. Post-op treatment planning is performed clinically with a TG-43 based treatment planning system (TPS). The TPS calculates dose to water and doesn't account for heterogeneities. The purpose of this work was to investigate differences in dose deposited due to heterogeneities, in particular to examine dose to the skull and skin following GammaTile implantation utilizing Monte Carlo simulation.
Methods: Each GammaTile contains four CS-1, Rev2 131Cs seeds (IsoRay Medical Inc., Richland, WA) embedded in DuraGen (Integra Lifesciences, Plainsboro, NJ) collagen tile measuring 2.0 x 2.0 x 0.4 cm³. Each seed has an air kerma strength of 3.5U at the date of implant. MCNP Monte Carlo simulation (MCNP5) was used to simulate the dose distribution around a single GammaTile implanted permanently. MC simulated doses were obtained both in water and in a heterogeneous medium, including brain, skull bone (thickness=0.7 cm), skin (thickness=0.5 cm), and air. For comparison, the total dose deposited was also calculated using BrachyVision (Varian Medical Systems, Palo Alto, CA).
Results: Initial simulated dose distributions in water were within 7% agreement with comparable dose distributions generated by BrachyVision. When bone was located 1 cm from a single GammaTile, the total dose simulated to the bone and skin were 28.3 Gy and 2.74 Gy, respectively. Doses for bone and skin at the same distance calculated with the treatment planning system were 10.4 Gy and 4.96 Gy, respectively. MC simulated values are 2-3 times the values calculated using TG-43.
Conclusion: Considerable dose to the skull bone is possible and even likely from GammaTile therapy. Initial simulations included a single GammaTile. Future simulations will include more representative of a clinical arrangement containing multiple GammaTile units.