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

[ 75 Se]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 75 Selenium. These patients had received lutetium-177 oxodotreotide ( 177 Lu-DOTATATE) therapy 98 and 151 days before their SeHCAT tests. Traces of 177 Lu may be retained longer than typically modeled, along with the contaminant 177m Lu. This work includes a retrospective audit to examine the prevalence of SeHCAT tests being affected by 177 Lu and phantom measurements to investigate the potential impact. Of 579 patients who received 177 Lu-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 177 Lu/ 177m Lu 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 177 Lu/ 177m Lu were present was determined. All SeHCAT measurement windows are affected by the 177 Lu/ 177m Lu, producing clinically significant errors. Patients requiring SeHCAT testing should be asked whether they have ever received 177 Lu-DOTATATE. Patient-specific background measurements may be useful for checking for significant levels of other radionuclides.