Evaluation of 99mTc-succimer dosing in pediatric patients

Balancing image quality with radiation dose is a goal with every diagnostic procedure requiring radiation. Our institution compared the dosing of (99m)Tc-labeled succimer, commonly referred to as dimercaptosuccinic acid ((99m)Tc-DMSA), to pediatric patients using 2 methods of calculation, body surface area (BSA, the method we used from 2009 to 2010) and body weight (BW, the method we used in 2011).

A retrospective study was conducted in a 230-bed inpatient, tertiary-care academic pediatric hospital to obtain objective data on patients under the age of 17 y who received a renal nuclear medicine procedure with (99m)Tc-DMSA using a 300,000-count parallel image and four 150,000-count pinhole images. Data collection included patient age, sex, height, weight, calculated activity, assayed activity, administered activity, residual syringe activity, imaging time, and notable patient or equipment factors affecting the procedure.

Calculated activities based on BSA were higher than calculated activities based on BW. (99m)Tc-DMSA adsorption to the plastic syringes was significant, with a range of 3%-82%. Because of the adsorption, an average of 23.7 MBq (SD, ±31 MBq) was added to the patients' calculated dose when the order was placed. Therefore, assayed activities were significantly higher than calculated activities in both groups. Administered activity correlations to BSA and BW calculations were 0.75 and 0.83, respectively. Administered activities from BSA and BW groups were outside the American College of Radiology (ACR)-recommended guidelines 59% and 45% of the time, respectively. Overall, children less than 2 y old were above the ACR recommendations 80% of the time. There was a poor correlation between administered activity and total imaging time (r = 0.23). Average imaging time overall for 5 planar views was 14.8 min (±7.1 min). Patients receiving less than the ACR-recommended administered activities (<1.85 MBq/kg) had an average increase in imaging time of 4.5 min (±3.4 min).

The activity administered to patients was significantly affected by the amount of (99m)Tc-DMSA activity adsorbed to the syringe. Syringe residual should be considered when standardizing (99m)Tc-DMSA imaging protocols and calculating patient dose. Although (99m)Tc-DMSA adsorption was variable, the administered activities correlated with calculated activities. In all but one of our patients, the total imaging time was far less than recommended by the ACR and European Association of Nuclear Medicine guidelines. The study indicates that using the BW calculation of 3.7 MBq/kg resulted in a range of administered activity of 1.85-2.59 MBq/kg. (99m)Tc-DMSA dosing of 3.7 MBq/kg for pinhole imaging should be appropriate for most studies.