Quantitative Comparison of 124I PET/CT and 131I SPECT/CT Detectability

Radioiodine therapy with I-131 is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic I-124 PET/CT with a low activity (similar to 1% of I-131 activity) can be performed to determine whether uptake of I-131, and thereby the desired therapeutic effect, may be expected. However, false-negative I-124 PET/CT results as compared with posttherapeutic I-131 SPECT/CT have been reported by several groups. The purpose of this study was to investigate whether the reported discrepancies may be ascribed to a difference in lesion detectability between I-124 PET/CT and I-131 SPECT/CT and, hence, whether the administered I-124 activity is sufficient to achieve equal detectability. Methods: Phantom measurements were performed using the National Electrical Manufacturers Association 2007 image-quality phantom. As a measure of detectability, the contrast-to-noise ratio was calculated. The I-124 activity was expressed as the percentage of I-131 activity required to achieve the same contrast-to-noise ratio. This metric was defined as the detectability equivalence percentage (DEP). Results: Because lower DEPs were obtained for smaller spheres, a relatively low I-124 activity was sufficient to achieve similar lesion detectability between I-124 PET/CT and I-131 SPECT/CT. DEP was 1.5%, 1.9%, 1.9%, 4.4%, 9.0%, and 16.2% for spheres with diameters of 10, 13, 17, 18, 25, and 37 mm, respectively, for attenuation- and scatter-corrected SPECT versus point-spread function (PSF) model-based and time-of-flight (TOF) PET. For no-PSF no-TOF PET, DEP was 3.6%, 2.1%, 3.5%, 7.8%, 15.1%, and 23.3%, respectively. Conclusion: A relatively low I-124 activity of 74 MBq (similar to 1% of I-131 activity) is sufficient to achieve similar lesion detectability between I-124 PSF TOF PET/CT and I-131 SPECT/CT for small spheres (<= 10 mm), since the reported DEPs are close to 1%. False-negative I-124 PET/CT results as compared with posttherapeutic I-131 SPECT/CT may be ascribed to differences in detectability for large lesions (>10 mm) and for no-PSF no-TOF PET, since DEPs are greater than 1%. On the basis of DEPs of 3.5% for lesion diameters of up to 17 mm on no-PSF no-TOF PET, I-124 activities as high as 170 MBq may be warranted to obtain equal detectability.