Uncertainty In SPECT Sensitivity Factor for Lu-177 Radiopharmaceutical Dosimetry
Abstract
Purpose
To quantify uncertainty in SPECT sensitivity factor used for Lu-177 radiopharmaceutical dosimetry.
Methods
Uncertainty in SPECT sensitivity factor due to (1) different dose calibrators, (2) phantom preparation, (3) camera stability, and (4) method of regions of interest (ROI) contouring on the SPECT images for determining counts was evaluated. 50mCi of Lu-177 was split evenly between two Nalgene bottles. Both were inserted into a Graves phantom and scanned with a Siemens Intevo Bold SPECT/CT camera at 8 timepoints over the course of a week. Activity measurements were made on two Capintec CRC-55tW dose calibrators: one which had been calibrated for Lu-177 and another using the same dial settings to evaluate the influence of dose calibrator differences. Regions of Interest (ROI) to obtain total counts were drawn using 5 different methods, with the “standard” being total counts in the entire SPECT FOV. Sensitivity factors were calculated as (total counts)/(acquisition time x activity). Sensitivity factors were compared for the two phantom datasets, the date/time of scan, and ROI contouring method.
Results
The sensitivity factor calculated at the first timepoint by following the established protocol was considered the baseline for comparisons. For Phantom 1 and Phantom 2, sensitivity factors were 0.990-1.006 and 1.002-1.014 of baseline respectively, across all timepoints, when the standard ROI was used. Sensitivity factors from the second phantom were consistently higher (0.62-1.24%) than those determined using the first phantom. When alternative ROI methods were considered, ratios ranged from 0.926-1.006. The date/time of SPECT scans did not produce significant differences in sensitivity factors, suggesting stability across the week. Measurements from both dose calibrators consistently showed the non-calibrated dose calibrator read ~2% higher than the calibrated one.
Conclusion
Standardization is critical to minimizing uncertainty when calculating SPECT sensitivity factors for Lu-177. Any deviations from a standardized process will impact downstream dose calculations.