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SeHCAT retention measurements may be compromised by traces of 177Lu/177mLu more than 90 days after 177Lu-DOTATATE was administered.

Tamar WillsonRichard Meades
Published in: Nuclear medicine communications (2024)
[75Se]tauroselcholic acid (SeHCAT) retention measurement provides a noninvasive test for bile acid diarrhea (BAD); however, it is sensitive to the presence of other radionuclides. Two SeHCAT patients at the Royal Free Hospital (RFH) had significant discrepancies between the lower photopeak (111-159 keV) and central photopeak (242-296 keV) windows, indicating contamination with a radionuclide other than 75Selenium. These patients had received lutetium-177 oxodotreotide (177Lu-DOTATATE) therapy 98 and 151 days before their SeHCAT tests. Traces of 177Lu may be retained longer than typically modeled, along with the contaminant 177mLu. This work includes a retrospective audit to examine the prevalence of SeHCAT tests being affected by 177Lu and phantom measurements to investigate the potential impact. Of 579 patients who received 177Lu-DOTATATE therapy at our center, 11 subsequently attended for a SeHCAT test. The two previously identified patients may have had compromised SeHCAT results; however, the other patients had longer intervals between their therapy and test, and their tests are believed to be valid. Spectra were acquired from a phantom containing either a SeHCAT capsule or a mixture of 177Lu/177mLu representative of a patient >90 days after their treatment. The SeHCAT spectrum was scaled to produce simulated day-7 spectra, and the SeHCAT retention that would have been calculated if 177Lu/177mLu were present was determined. All SeHCAT measurement windows are affected by the 177Lu/177mLu, producing clinically significant errors. Patients requiring SeHCAT testing should be asked whether they have ever received 177Lu-DOTATATE. Patient-specific background measurements may be useful for checking for significant levels of other radionuclides.
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