Technical Note: Impact of impurities on Yttrium-90 glass microsphere activity quantitation

Glass ⁹⁰ Y microspheres are produced with known radionuclide impurities. These impurities are not independently monitored. Clinical instruments, including ionization chamber dose calibrators and positron emmission tomography (PET) cameras, can be much more sensitive in detecting signals from these impurities than to signals from ⁹⁰ Y itself.

The "typical" levels of ⁹⁰ Y impurities have been studied to assess their impact on dosimetry during internal implantation, and for the management of waste. However, unaccounted-for decay spectra of impurities can also have an impact on dose calibrator and PET readings. Thus, even what might be considered negligibly small impurity fractions, can in principle cause substantial overestimates of the amount of ⁹⁰ Y activity present in a sample. To our knowledge, quantitative effects of radionuclide impurities in glass microspheres on activity measurements have not been documented in the field. As activity quantitation for dosimetry and its correlations with outcome becomes more prevalent, the effects of impurities on measurements may remain unaccounted for in dosimetry studies.

In this letter, we review theoretical and physical considerations that will result in asymmetric errors in quantitation from ⁹⁰ Y impurities and estimate their typical and potential impact on clinical utilization. Among the common impurities ⁸⁸ Y is of particular concern for its impact on ⁹⁰ Y dose measurements because of its decay characteristics, along with other isotopes ⁹¹ Y and ⁴⁶ Sc which can also impact measurements.

The typical level of ⁸⁸ Y impurities reported by the manufacturer should only cause small errors in dose calibrator and PET measurements made within the 12-day label-specified use-by period, up to 2.0% and 1.6%, respectively. However, the product specification max allowable impurity levels, specified by the manufacturer, leave open the potential for much greater bias from within the 12-day use-by period, potentially as high as 13.2% for dose calibrator measurements and 10.6% for PET from the ⁸⁸ Y impurities.

While typical levels of impurities appear to have acceptable impact on patient absorbed dose, it should be noted that they can have adverse effects on ⁹⁰ Y radioactivity measurements. Furthermore, there is currently minimal independent verification and/or monitoring of impurity levels within the field.