Challenges in migrating 90Y calibrations to new ionization chambers: geometry, height, and impurity effects.
Denis E. Bergeron, Jeffrey T. Cessna, Brian E. Zimmerman
Objectives: Yttrium-90 microspheres are used for targeted radiotherapy in patients with liver cancer. Activity measurements of irradiated glass microspheres are traceable to a standard developed at the National Institute of Standards and Technology (NIST) in 1991. In 2014, it was deemed necessary to transfer the NIST standard from one aging ionization chamber (dose calibrator) to several others. The very small pair production branch in 90Y decay allows (potentially quantitative) imaging by positron emission tomography (PET). We have recently achieved a PET recovery coefficient of 0.93 ± 0.02 with a uniform phantom filled with 90YCl, where the main uncertainty contributions came from the uncertainty on the pair production branching ratio and from the activity assay. In this work, we show that quantitation of 90Y microsphere PET images will be limited in both accuracy and precision by the dose calibrator measurements. Methods: Two v-vials containing 90Y microspheres were measured on a dose calibrator for which geometry-specific (with and without hand-shield) dial settings were established as part of the 1991 standardization. The calibration curve method was then applied with the two vials four other chambers to determine new calibration factors. Results: The initial assays revealed a ≈ 4 % difference between the activities determined with and without the hand-shield. It appears that a change in the source geometry between 1991 and 2014 introduced a systematic bias that must now be treated as a source of uncertainty. Calibration factors were determined for three of the four chambers, with total combined uncertainties ≈ 4 % mostly due to the uncertainty on the initial assay. The fourth chamber suffered from a >8 % activity-dependent bias. Height and impurity effects were studied in detail, offering plausible explanations for the observed bias. Conclusions: NIST calibrations for 90Y glass microspheres have been transferred to several newer ionization chambers. The tran
, Cessna, J.
and Zimmerman, B.
Challenges in migrating 90Y calibrations to new ionization chambers: geometry, height, and impurity effects., Journal of Nuclear Medicine, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=925002
(Accessed September 27, 2021)