Exhibit Hall | Forum 6
Purpose: To estimate dose to a fetus from a pregnant patient receiving intracavitary low dose rate (LDR) brachytherapy in the brain from permanently implanted collagen tiles embedded with Cs-131 seeds (GammaTile, GT Medical Technologies, Tempe, AZ). Each GammaTile contains four 3.5U Cs-131 seeds (14U/tile), which emit ~30keV photons with a half-life of 9.7days.
Methods: This analysis assumes an implant-to-fetus distance of 30cm. The initial fetal dose rate per tile from primary internal radiation traveling through the body was calculated using the AAPM TG43 point-source formalism with S(k)=14U, Λ=1.056cGy/h-U, r₀=1cm, r=30cm, g(p)(30cm)=1.33x10⁻³, and φ(an)(30cm)=0.964. The range of initial fetal dose rates from primary leakage radiation transmitted through cranium was calculated using the typical (2mR/hr) and worst-case (6mR/hr) exposure rates at 1-meter representative of postoperative GammaTile implant survey measurements. The corresponding exposure rates at 30cm were calculated, assuming inverse-square fall-off, and converted to dose rate using an f-factor of 0.921cGy/R. Fetal dose contributions from primary internal and primary leakage radiation were calculated separately by integrating the exponential radioactive decay equation over time (assuming full source decay).
Results: Fetal dose from primary internal radiation was calculated to be 7.11x10⁻³cGy/implanted tile and dose from primary leakage radiation for the typical and worst-case implant scenarios range from 6.9–20.6cGy. This analysis demonstrates that >99% of fetal dose is attributed to primary leakage radiation.
Conclusion: A formalism for estimating fetal dose from permanent LDR brachytherapy using GammaTiles in the brain has been presented. This analysis demonstrates that primary leakage radiation is the major contributor of dose to the fetus and the calculated estimated dose value depends almost entirely on the postoperative exposure measurement. This estimate is conservative as it ignores attenuation from the patient's abdomen. These findings suggest external patient shielding could limit fetal dose and will be explored in future work.
Funding Support, Disclosures, and Conflict of Interest: Author is an employee and shareholder of GT Medical Technologies, Inc.