Author: Denis Bergeron, Larry A. DeWerd, Sean Jollota, Peyton Alexandra Lalain, Likhitha Polepalli 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, National Institute of Standards and Technology 🌍
Purpose: 90Y is a challenging radionuclide to measure because it is a pure beta-emitter and is typically quantified using a radionuclide calibrator. These devices, such as the Capintec model used in this study, rely significantly on Bremsstrahlung radiation, and are inefficient for pure beta-emission measurement. This study aimed to evaluate whether an air-communicating well-type ionization chamber could serve as an alternative for measuring the signal from 90Y solutions.
Methods: A Capintec 55tR radionuclide calibrator was used to establish a baseline for activity determinations. Multiple HDR1000+ well chambers were tested, including aluminum- and custom C552-walled designed to minimize beta attenuation. A 90Y solution, calibrated at NIST, with an initial activity of 166 MBq was measured in syringe and 10R vial geometries. Volume effects were assessed by varying syringe volumes, and additional measurements were taken with aluminum shielding to fully attenuate beta particles. Calibration coefficients [MBq/pA] were determined for each chamber.
Results: All chambers detected signals from the 90Y source with acceptable signal-to-noise ratios. We measured very similar signals in both the aluminum- and C552-walled chambers. With additional aluminum in the standard HDR1000+, the signal measured dropped to nearly zero. Assuming a minimum detectable signal of 1 pA, the minimum detectable activity (MDA) in the aluminum-walled chamber was 2.78 MBq for the 10R vial and 0.63 MBq for the syringe. The MDA in the C552-walled chamber was 2.95 MBq for the 10R vial and 0.63 MBq for the syringe.
Conclusion: The chamber wall material does not significantly impact the detected signal, and ionization currents primarily result from beta particles rather than Bremsstrahlung radiation. The strong SNR for 90Y measurements suggests that well chambers could be viable tools for a wide range of difficult-to-measure radionuclides. The low MDA also indicates viability, as clinical 90Y solution has typical activity on the order of GBq.