Comment on: the interference of medical radionuclides with occupational in vivo gamma spectroscopy.

Dear Editors: THIS informative article (Kol et al. 2003) prompted me to share some challenges that radiopharmaceuticals presented to the bioassay program at a power reactor facility. But first, I noticed that some medical radionuclides were not mentioned in the article (e.g., F, Ga, In, and I). Perhaps they were mentioned in the references that were given. This particular bioassay program screened workers with a stand-up whole body counter (WBC) with multiple NaI(T1) detectors. If the stand-up WBC indicated a radionuclide was present, then the activity was quantified using a scanning bed (WBC) with a NaI(T1) detector. A worker reported that he had a red blood cell (RBC) mass performed with Cr. He neglected to mention that he also had a plasma volume performed with I. The RBC mass was performed using Cr labeled autologous RBCs. The plasma volume was performed using I labeled human serum albumin. An accurate total blood volume can only be determined by measuring both blood components. This is because the peripheral hematocrit to total body hematocrit ratio can vary by a factor of 0.7 to 1.3. The I was not detected because the low energy ( 28 kev) sum peak was hidden by the instrument’s electrical noise. The solution would be to increase the instrument’s gain in order to shorten the range of the multichannel analyzer. Sometimes knowing what combinations of radiopharmaceuticals are used is key to a valid bioassay. Sometimes knowing the peculiarities of the radionuclides involved is key to a valid bioassay. Another worker reported that he had a nuclear medicine study performed using T1. He was being followed with serial bioassays. A bioassay performed during the afternoon was less than LLD. However, a bioassay the next morning was greater than LLD. This was a puzzle. Did the T1 go away and then come back? The answer is closer to the yes side than the no side. Thallous chloride is a potassium analog so it behaves similar to potassium in vivo. It redistributes in the body relative to blood flow. Muscles or organs that are stressed require increased blood flow. The activity during the afternoon bioassay was concentrated in the large muscles of his legs, arms, back, and buttocks because he used his muscles during work. This presented a large spatial distribution of activity to the multiple detectors of the stand-up WBC. Also, activity in his back and buttocks was attenuated by his torso. Consequently, the bioassay result was less than LLD. The activity when he had his bioassay performed shortly after breakfast the next morning redistributed to his stomach and liver. This presented a smaller spatial distribution to the multiple detectors of the stand-up WBC. Also, the stomach and left lobe of the liver are anterior structures, so attenuation was less. Consequently, the bioassay result was greater than LLD. Sometimes, knowing the pharmacokinetics of a radiopharmaceutical is key to a valid bioassay. Not all of the challenges presented to a bioassay program by radiopharmaceuticals are as subtle as those presented by the authors of this article. But there is a potential solution to many of these challenges. The nuclear medicine community knows a great deal about the radiopharmaceuticals they use. In my opinion, it is unreasonable to expect the bioassay programs in other types of practices to have the same level of expertise. Therefore, I think it would be a valuable service if some entity took the responsibility to provide this information to the bioassay programs of all licensees.