Biokinetics Study for Tc-99m MAG3 in Pediatric Molecular Imaging

The overall objective of this investigation is to optimize pediatric patient absorbed dose by keeping it as low as possible while maintaining and even improving the diagnostic quality of nuclear medicine images. Current dosimetric estimations in children are based on either animal biokinetic or pharmacokinetic data from adults. This is due to paucity of data that exists specifically for children. This situation will be improved through the following specific aims:

• Collect imaging-based pharmacokinetic (PK) data from patient volunteers in different age groups scheduled for selected, routine nuclear medicine studies for Tc-99m MAG3, a radiopharmaceutical commonly used in pediatric nuclear medicine
• Pool and analyze the data for different age groups for each radiopharmaceuticals and
• Generate biokinetic models to be used in subsequent dosimetric models for the optimization of pediatric nuclear medicine procedures.

Pediatric absorbed dose estimates that are typically reported apply adult PK data with pediatric variations in body size and anatomy but not for differences in physiology between children and adults. Depending on the diagnostic agent, such differences can be of greater impact than anatomical differences. The investigators will acquire image data that will allow us to develop PK models for Tc-99m mercaptoacetyltriglycine (MAG3), a radiopharmaceutical routinely used in pediatric nuclear medicine for renal imaging. Patients undergoing standard of care imaging will be asked to consent to being imaged at one additional time point, either prior or subsequent to the time typical for clinical imaging. No patient will be asked to undergo more than one additional imaging time-point.

It is important to note that the patient volunteers will not receive any additional radiation exposure for inclusion in this study. They are only being ask to allow imaging at an additional time point.

Children ages 1-6 years old will be eligible to participate. Routine imaging is performed immediately as a dynamic acquisition with 80 frames over 20 min. (15 s per frame). Subjects in each age group will be additionally imaged 2-3 h post-administration.

The additional imaging will occur on the day of the clinically indicated procedure. Other than that, there is no timeline associated with this study.

Image data acquired from the subjects will be analyzed by the principle investigator and by colleagues at Johns Hopkins University and the University of Florida. Regions of interest will be defined around pertinent target organs and tissues and the count data recorded. The specific target organs will depend on the particular radiopharmaceutical. The data for each age range and time point will be pooled, normalized and fit to models describing the pharmacokinetics. The resultant models will be evaluated for age-based variations in the PK data and compared to existing, published models based on adult data to evaluated age based differences. Lastly, the impact that the more accurate PK has on dosimetric estimates of patients of different ages will be analyzed.

The number of subjects required at each time point will be determined using nonlinear mixed effects modeling software to model the data and adjust for covariates. The likelihood ratio test based on the objective function value (OFV) will be used to estimate PK parameters for varying doses and ages using a Bayesian approach. The proposed sample size plan with subjects imaged at different time points is predicated on the Monte Carlo Mapped Power (MCMP) method to achieve 80% power for detecting age and dose effects and robust coverage in estimating individual PK parameters. It is expected that there will be 5-10 subjects per age group depending on the statistical requirements as described above.