Purpose: As targeted radionuclide therapy (TRT) is increasingly popularized in radiotherapy, its lack of traceable dosimetry compared to other forms of radiotherapy has gained interest. This work introduces a new insert capable of housing dosimeters and radionuclide solutions for use in a custom SPECT/CT phantom.
Methods: Modified phantom inserts, capable of housing active and passive detectors, and liquid radionuclide solutions, were designed and constructed for a previously introduced cylindrical, acrylic SPECT/CT phantom. The new inserts contain a thin polyimide window which separates a sealable source cavity from an insertable detector. Detectors used in this investigation include a Standard Imaging A20 parallel plate ion chamber, an IBA EFD diode, unlaminated EBT3 film, and 150 micron-thick TLD-100 chips. Custom probes were designed to house the TLDs and film. All detectors were calibrated for absorbed-dose-to-water using a 250 kVp NIST traceable x-ray beam or a NIST traceable Co-60 beam. A pilot experiment was conducted using approximately 322 mCi of Tc-99m with future experiments planned for Lu-177, I-131, and Y-90. Automation of the electrometer allowed for data collection over the extended acquisition time. Measurement data was appropriately corrected for applicable background subtraction, pressure and temperature fluctuations, radiation quality, recombination, polarity, and electrometer corrections, and subsequently compared to Monte Carlo simulations using the egs_chamber user-code in EGSnrc 2019.
Results: Ion chamber measurements agreed with predicted Monte Carlo simulated doses within k=1 uncertainty values but a systematic offset was observed between the two. EFD measurements started with a similar offset and agreement but drifted over the course of the experiment.
Conclusion: Our results show that measurements can be obtained within agreement of Monte Carlo simulations. An investigation into the long-term drift of the EFD must be performed. Future work will incorporate beta-emitting radionuclides to assess the feasibility of measuring dose from these commonly used TRT agents.
Absolute Dosimetry, Monte Carlo, SPECT
IM/TH- Radiopharmaceutical Therapy: Anthropomorphic dosimetry phantoms