When should Mo-99 contamination in Tc-99 eluate be measured according to the NRC?

The correct answer is that measurement of Mo-99 contamination in Tc-99 eluate should be performed only after the first elution. This practice is established to ensure that any potential contamination from molybdenum-99, which is the parent isotope of technetium-99m, is monitored closely right at the onset of the radioisotope's use.

By measuring after the first elution, practitioners can determine if there are residual levels of Mo-99 that could pose a safety concern. This is important because Mo-99 can have significant radiological implications if present in excessive amounts, given its half-life and decay characteristics.

Subsequent elutions tend to show consistent levels of contamination, allowing facilities to establish a baseline after the initial measurement, thus minimizing the need for constant measurement after each individual elution. Other suggested frequencies, such as daily or monthly, do not align with the NRC's requirement for initial contamination checks that are crucial for patient safety and compliance with regulatory standards.