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

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 99mTc 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 99mTc. 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 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.&#xD.