Author: Fatemeh Akbari, Artemis Dalby, Nelson A. Miksys, Prarthana Pasricha, Rowan M. Thomson 👨🔬
Affiliation: Ackerman Cancer Center, Carleton University 🌍
Purpose: To investigate dose evaluations for GammaTile brachytherapy, a novel treatment for brain malignancies. The analysis focuses on the impact of tissue heterogeneities, including the presence of cerebrospinal fluid (CSF) on dose distributions with comparisons to TG-43-based calculations.
Methods: Using the EGSnrc egs_brachy Monte Carlo simulation toolkit, three GammaTiles, each containing four Cs-131 seeds (Model CS-1 Rev2, 3.5 U) embedded in a collagen matrix, were modeled to deliver 60 Gy to voxels in a plane at a depth of 0.5 cm in brain tissue. The simulation geometry involved GammaTiles positioned along the edge of a 6×6×6 cm³ CSF cavity within a brain phantom composed of concentric layers: brain parenchyma (0.8 cm), bone (0.7 cm), and skin (0.5 cm). Dose distributions from this inhomogeneous brain phantom were compared to simplified simulation scenarios: i) TG-43: all tissues modeled as water and no interseed attenuation; and ii) brain-CSF model: CSF cavity in brain parenchyma. In each simulation, dose was scored to the local medium in a voxel.
Results: Doses to voxels in the prescription plane were lower with tissue media present than for TG-43 doses, by up to 4% (brain-CSF model) and 27% (inhomogeneous-brain-CSF model). Reduced dose uniformity was also observed when modelling tissues compared to homogeneous water in TG43. Additionally, the dose to bone near the brain-bone interface was found to be 4-5 times higher than TG-43 based calculations, while the dose to skin near the bone-skin interface was 6-7 times lower. These differences were higher than those reported in literature for single GammaTile simulations.
Conclusion: TG-43 may overestimate dose coverage in brain tissue. TG-186 model-based dose calculations can provide more accurate guidance for GammaTile clinicians, especially when assessing post-implant coverage or when pairing GammaTile with external beam boosts.