Validation of Monte Carlo simulated absorbed-dose-to-water inside a custom SPECT/CT phantom using active and passive dosimeters: a feasibility study using 99mTc

Objective.This project aims to provide a novel method for performing dosimetry measurements on TRT radionuclides using a custom-made SPECT/CT compatible phantom, common active and passive detectors, and Monte Carlo simulations. In this work we present a feasibility study using Tc-99m for a novel approach to obtaining reproducible measurements of absorbed-dose-to-water from radionuclide solutions using active and passive detectors in a custom phantom for the purpose of benchmarking Monte Carlo-based absorbed-dose-to-water estimates. Approach. A cylindrical, acrylic SPECT/CT compatible phantom capable of housing an IBA EFD diode, SNC600c Farmer type ion chamber, and TLD-100 microcubes was designed and built for the purpose of assessing internal absorbed-dose-to-water at various points within a solution of Tc-99m. The phantom is equipped with removable inserts that allow for numerous detector configurations and is designed to be used for verification of SPECT/CT-based absorbed-dose estimates with traceable detector measurements at multiple locations. Three experiments were conducted with exposure times ranging from 11 to 21 h with starting activities of approximately 10-16 GBq. Measurement data was compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2019. Main results. In general, the ionization chamber measurements agreed with the Monte Carlo simulations within k = 1 uncertainty values (+/- 4% and +/- 7%, respectively). Measurements from the TLDs yielded results within k = 1 agreement of the MC prediction (+/- 6% and +/- 5%, respectively). Agreement within k = 1 uncertainty (+/- 6% and +/- 7%, respectively) was obtained for the diode for one of three conducted experiments. Significance. While relatively large uncertainties remain, the agreement between measured and simulated absorbed-doses provides proof of principal that dosimetry of radionuclide solutions with active detectors may be performed using this type of phantom with potential modifications for beta-emitting radionuclides to be introduced in future work.