Actinium-225 generates multiple radioactive ‘daughters’ as it decays, each with their own unique decay profiles that make measuring the sample’s radioactivity a challenge
Within and beyond Canada’s borders, research on actinium-225 (Ac-225) radiopharmaceuticals is advancing into clinical trials. And at this stage of research involving human participants, it is vital that doctors know exactly how much radioactivity they’re injecting into their patients for safe and accurate treatment.
However, precisely measuring the radiation dose within an Ac-225 sample is challenging because of the radioisotope’s complex decay chain, says Canadian Nuclear Laboratories’ (CNL) radiopharmaceuticals scientist Svetlana Selivanova. “As Ac-225 undergoes radioactive decay, it generates multiple radioactive ‘daughters’ with their own unique decay profiles. And all these daughters being present in the mixture can interfere with accurate measurement,” she explains.
This is one of several obstacles the team at Chalk River Laboratories (CRL) is working to tackle as part of their research into systemically addressing the current limitations surrounding targeted, radiopharmaceutical cancer treatment therapies — research that they’re carrying out for the federal government.
(Pictured is a vial of Ac-225)
Late last year, the research team emerged victorious from a series of experiments that validated their new, standardized approach to measuring the radioactivity emitted by Ac-225 samples, affirming that their technique aligns with international quality control standards.
And it wasn’t just ‘daughter interference’ that they had to overcome to get to that point.
Given the emerging nature of Ac-225 radiopharmaceuticals, manufacturers of commonly used radionuclide calibrators either haven’t set calibration and correction factors specifically for this radioisotope, or they’ve based them on theoretical calculations specific to their instrument.
Specific to a radionuclide and the container the sample is held in, calibration and correction factors are standardized numerical values or settings applied to a radionuclide calibrator’s response that ensure accurate radioactivity measurements.
Without them, an instrument could over or underestimate the actual radioactivity of a sample, which could then result in incorrect radiopharmaceutical dosing down the line — making it clear why developing standardized calibration and correction factors for Ac-225 is critical to furthering research on radiopharmaceuticals powered by this alpha-emitting radioisotope.
Working to fill this gap, researchers determined calibration and correction factors specific to Ac-225 through a series of experiments. With support from the National Research Council’s Metrology Laboratory, they used different radionuclide calibrators available in their lab and different sized sample containers to complete a series of radioactivity measurements on liquid Ac-225 samples eluted from their thorium-229 generator.
From this, they developed standardized calibration and correction factors specific to Ac-225, as well as best practices and recommendations they’ve gathered from their day-to-day experience. They will be sharing their results with the nuclear medicine community broadly to further facilitate adopting Ac-225 radiopharmaceuticals in clinical applications.
This research is funded by Atomic Energy of Canada Limited’s (AECL) Federal Nuclear Science & Technology (FNST) work plan program, which connects federal organizations, departments, and agencies to the nuclear science expertise and facilities we have at Chalk River Laboratories.
Under this program, our researchers carry out projects designed to support the Canadian government’s core responsibilities and priorities across the areas of health, safety and security, energy, and the environment.