A Practical Method for Measuring Lu-177m Contaminants In Lu-177 Based Therapeutics
Abstract
Purpose
To develop a simple process for reliably detecting and quantifying the presence of the long-lived contaminant Lu-177m (t ½ = 160 d) in Lu-177-based therapeutics.
Methods
Twenty-four samples (14 Lutathera, 10 Pluvicto) were collected from cancelled patient doses. Lu-177m decays to Lu-177 (t ½ = 6.4 hr) and to Hf-177 (stable) by beta decay that includes several photopeaks. Samples were first counted on a HPGE to establish an absolute quantification standard for comparison to measurements in this work. Photopeak counts were collected on a 1-inch NaI well detector ranging from 80 keV to 430 keV and beta decay counts were collected on a liquid scintillation counter (LSC) from 0 to 500 keV. All detectors were calibrated with known standards and counting durations were adjusted to achieve <5% error in the integrated photopeaks and beta spectra. Data were reported as percent ratio of Lu-177m/Lu-177.
Results
The decay of Lu-177m yielded several closely spaced photopeaks at 108, 113, and 122 keV that were unresolvable with NaI and integrated together. For all Lutathera samples, the mean Lu-177m/Lu-177m ratio was 0.0062 +/- 0.0013% on the NaI well which agreed with HPGE at 0.0059 +/- 0.0017%. The Lu-177m/Lu-177 ratio from the LSC was higher at 0.0085 +/- 0.002% because of multiple beta decay transitions that could not be resolved. For all Pluvicto samples, the mean Lu-177m/Lu-177 ratio was two orders of magnitude lower than Lutathera on the NaI well with no observable photopeaks. Similarly for the LSC, Pluvicto measurements were three orders of magnitude lower than Lutathera.
Conclusion
This work demonstrates that NaI well and LSC systems common to institutions with theranostic programs can reliably detect and quantify Lu-177m. These capabilities are important for determining the correct waste stream of Lu-177 based therapeutics, depending on whether the long-lived contaminant is present or not.