Simplifying the dosimetry of carbon-11-labeled radiopharmaceuticals

A two time-point sacrifice method is proposed as an alternative to conventional multiple time-point sacrifice methods to determine the organ cumulated activity of 11C-labeled radiopharmaceuticals.

Rat biodistribution data for 10 11C-labeled radiopharmaceuticals were analyzed to determine organ cumulated activity. Data were obtained at four sacrifice intervals 2-5 min, 10-15 min, 30-45 min and 1-1.5 hr postinjection. The organ absorbed dose per unit administered radioactivity (mGy/MBq) was calculated using all four data points and combinations of limited data. The objective was to determine if a limited sampling technique would provide sufficient accuracy in estimating absorbed dose.

Residence times calculated using two time-points acquired during the first half-life of 11C were either equivalent or positively biased compared to using all sacrifice times. Overall, 87% of the residence times assessed were conservative compared to the multipoint method. For bladder organs, a consistent negative bias was observed with the reduced sacrifice method.

Analysis of animal biodistributions using a reduced sacrifice protocol provides results in good agreement with and generally conservative to results using all sacrifice intervals. Correction factors are required for the urinary bladder and gallbladder when using the simplified technique due to bias. The bladder was often the limiting organ in determining human administered activity.