Author: Cindy Pham, Anurag Sharma, John Starner, Ching-Ling Teng, Michael A. Trager, Gabriella Wernicke 👨🔬
Affiliation: Northwell, Northwell Health, Radiation Medicine, Northwell Health 🌍
Purpose: Precisely identifying orientation of implanted seeds on post-implant CTs is challenging. This research addresses the dosimetric uncertainties stemming from uncertainties in identifying seed orientation at time of implant and beyond.
Methods: Seven patients who had low dose rate brachytherapy implants with Cs-131seeds (0.8mm x 4mm) for recurrent GBM were retrospectively evaluated. Prescription doses were 60Gy (n=5; GammaTile) and 80Gy (n=2; stranded seeds) at 5mm depth. The post-op cavity was contoured on post-implant CT. Post-implant dosimetry was calculated in BrachyVision (V16.1) using both 1D and 2D Cs-131 seed models (Perspective Therapeutics Cs-1). Commissioning of and calculations with 1D and 2D seed models follow standard TG-43 formalisms. Dose to normal brain tissue was evaluated as maximum, minimum, and median doses at depths from the post-op cavity at 0-1mm, 5-6mm, and 10-11mm.
Results: Cavity volumes were on average 18.9cc (range: 8.4-44.5cc). Absolute differences in maximum dose from 0-1mm, 5-6mm and 10-11mm from the cavity between 1D and 2D calculations were 13.64+8.15%, 1.41+0.37% and 1.46+0.77%, respectively. Absolute differences in minimum dose from 0-1mm, 5-6mm and 10-11mm from the cavity were 1.51+0.62%, 1.15+0.76%, and 2.48+2.11%, respectively. Differences in median dose at all depths were within 2%.
Conclusion: Differences in maximum dose to the cavity surface (0-1mm from cavity) was the only region showing significant deviations between 1D and 2D Cs-131 seed model calculations. Based on our preliminary findings, a 2D seed model is likely unnecessary for post-operative intracranial Cs-131 post-implant dosimetry unless maximum dose to the cavity surface is clinically significant. Further investigation is required to determine if this result is invariant to cavity size, seed count, and other case-specific variations.