Purpose: The assessment of uncertainty in personalized, image-based, radiopharmaceutical therapy (RPT) dosimetry is complicated and is commonly ignored in clinical routines because it is not as developed as it is for external beam radiotherapy. As such, this work investigates the uncertainties associated with ¹³¹I SPECT/CT based Jaszczak phantom studies and provides the groundwork for the estimation of dosimetric uncertainties for patient-specific RPT treatments.
Methods: ¹³¹I SPECT/CT cylindrical Jaszczak phantom studies were conducted. The first phantom study (5.64L with 105 kBq/ml ¹³¹I) established a calibration factor for the SPECT camera and the second phantom study included hot-sphere inserts (257 kBq/ml ¹³¹I with 10:1 insert to background ratio) with volumes of 0.5-16ml to establish recovery coefficient (RC) partial volume corrections. GE Xeleris 4.0 was used to reconstruct images with CT-based attenuation, scatter, and resolution corrections enabled. Segmentations were manually performed on the CT image. The uncertainties in the volume, RC, mean counts, total sensitivity, and cumulative uncertainty were then calculated.
Results: In general, the uncertainty decreased as the size of the structure increased. The largest uncertainties were the mean counts and RC which are dependent on the volume determination and the RC fitting function. The range of uncertainties for volume (17.3% to 54.7%), RC (22.32% to 95.71%), mean counts (13.2% to 106.4%), total sensitivity (7.47%), and cumulative activity (25.6% to 113%) were calculated.
Conclusion: The uncertainties for ¹³¹I SPECT/CT RPT can be larger than 100% for small structures and thus require the utmost consideration in the administration of RPT treatments. Patient-specific RPT dosimetry becomes much more valuable for clinical translation if the associated uncertainties are also measured and reported for the physician’s reference.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by NIH SPORE CA196513-01. BB and JG are co-founders of Voximetry, Inc., a nuclear medicine dosimetry company in Madison, WI.