Room: AAPM ePoster Library
Purpose: Targeted radionuclide therapy (TRT) dosimetry is typically based on converting the measured activity of TRT agents to absorbed dose. This work aims to directly measure absorbed dose to water from commonly used TRT radionuclides and validate these measurements with Monte Carlo simulations.
Methods: The absorbed dose to water measured using unlaminated GafChromic EBT3 film (a variant of EBT3 with a 14 µm active layer and a single 125 µm polyester base) from liquid solutions of ?°Y and ¹³¹I injected into custom acrylic phantoms equipped with 7 µm polyimide windows were compared to Monte Carlo statistical modeling from egs_chamber. The film was calibrated using a 250 kVp NIST traceable x-ray beam. Activities and exposure times were adjusted to deliver approximately 350 cGy to the surface film based on energy deposition from EGSnrc 2018. Five stacks consisting of layers of 4, 1 mm slabs of virtual water and 5 pieces of film were used. Setup of the stacks varied slightly depending on radionuclide with virtual water slabs placed between each piece of film for high-energy ?°Y betas or beneath the film stack for low-energy ¹³¹I betas.
Results: Experiments with ?°Y agree with EGSnrc within uncertainty for film in the top 3 mm of the stack. Standard deviations of film were on the order of 10% and increased with depth due to lower signal to noise ratio (SNR). Data from ¹³¹I showed lower than calculated dose to the film of approximately 20-30 cGy through all depths, except for at the surface.
Conclusion: Agreement between film measurements and Monte Carlo simulation was best for ?°Y where all dose points were within calculated uncertainty. Due to the variety of radionuclides used in TRT, further investigation of low-energy, beta-emitting radionuclides, such as ¹³¹I and ¹77Lu, in solution is required for more thorough Monte Carlo benchmarking.
Funding Support, Disclosures, and Conflict of Interest: BB and JG are co-founders of Voximetry, Inc., a nuclear medicine dosimetry company in Madison, WI