Purpose: Targeted radionuclide therapy (TRT) is a growing field within the realm of radiation therapy despite its shortcomings in dosimetry. This work aims to introduce a novel method for benchmarking TRT Monte Carlo simulations using a custom-made SPECT/CT phantom and active and passive dosimeters.
Methods: A cylindrical, acrylic SPECT/CT phantom of diameter 8” and length 8” 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. All detectors were calibrated using a 250 kVp NIST traceable x-ray beam in either Virtual Water or a water tank. Three experiments were conducted with exposure times ranging from 11 to 21 hours with starting activities of approximately 350 mCi. Automation of the electrometer allowed for data collection over the extended acquisition time. Measurement data was appropriately corrected for pressure and temperature fluctuations, or energy dependence, and subsequently compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2018.
Results: In general, both the diode and ion chamber measurements agreed with the predicted Monte Carlo simulated doses within k=1 calculated uncertainty values. TLD measurements generally produced an under response relative to the predicted doses.
Conclusion: Our results show that measurements can be obtained with generally good agreement of Monte Carlo simulations for the active dosimeters, and within at least 20% for the passive dosimeter investigated. Future work will incorporate beta emitting radionuclides and modified attachments to the current phantom to assess the feasibility of measuring dose from these commonly used TRT agents.