We should be careful, activity in mCi or MBq doesn’t really measure what we want it to measure.

Objectives: Immediately before radiopharmaceutical administration, patient syringes are measured using a dose calibrator. The activity of the radioactive sample is recorded, but the intention is to measure the amount of radioactive material that will be administered to the patient. While the two quantities are closely related, they are not the same, creating scope for much confusion particularly when different isotopes are involved.

Methods: Activity is the rate of nuclear disintegrations and is the product of the number of radioactive atoms in a sample, multiplied by the probability per unit time that an atom will decay (λ, decay constant). The decay constant is inversely related to the radioactive half-life (λ = 0.693 / half life) and is accurately known for all isotopes of interest. The total number of radioactive atoms in a sample can be estimated by dividing the activity by the decay constant.

Results: A sample of ¹⁸F (λ = 0.006315 / min) with an activity of 10 mCi would be expected to contain 3.5 x 10¹² radioactive atoms. In contrast a sample of ¹³N (λ = 0.069558 / min) with a higher activity of 20 mCi contains only 0.6 x 10¹² radioactive atoms and a sample of ¹²⁴I (λ = 0.000115 / min) with an activity of just 1 mCi contains 19.3 x 10¹² radioactive atoms. In this example, the highest activity is associated with the smallest amount of radioactive atoms and the smallest activity is associated with the highest amount of radioactive atoms. Activity is therefore quite misleading if the intention is to convey the amount of radioactive material that will be given to a patient. Of course, 2 mCi of a particular isotope contains twice as much radioactive material as 1 mCi of the same isotope. The confusion arises when different isotopes are considered, as is often the case. Quantifying radiopharmaceutical administrations in terms of the total number of radioactive atoms could be achieved with a small modification to the output of existing dose calibrators and may be more intuitive for lay audiences. Indeed Nuclear Medicine professionals may also prefer to measure the amount of radioactive material using a quantity that actually measures the amount of radioactive material.

Conclusions: The current practice of using activity as a way of quantifying the amount of radioactive material administered to a patient is potentially misleading. Greater awareness of the relationship between activity and the number of radioactive atoms may help avoid confusion.