@article {Beijst103, author = {Casper Beijst and Jakob W. Kist and Mattijs Elschot and Max A. Viergever and Otto S. Hoekstra and Bart de Keizer and Hugo W.A.M. de Jong}, title = {Quantitative Comparison of 124I PET/CT and 131I SPECT/CT Detectability}, volume = {57}, number = {1}, pages = {103--108}, year = {2016}, doi = {10.2967/jnumed.115.162750}, publisher = {Society of Nuclear Medicine}, abstract = {Radioiodine therapy with 131I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic 124I PET/CT with a low activity (\~{}1\% of 131I activity) can be performed to determine whether uptake of 131I, and thereby the desired therapeutic effect, may be expected. However, false-negative 124I PET/CT results as compared with posttherapeutic 131I 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 124I PET/CT and 131I SPECT/CT and, hence, whether the administered 124I 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 124I activity was expressed as the percentage of 131I 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 124I activity was sufficient to achieve similar lesion detectability between 124I PET/CT and 131I 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{\textendash}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 124I activity of 74 MBq (\~{}1\% of 131I activity) is sufficient to achieve similar lesion detectability between 124I PSF TOF PET/CT and 131I SPECT/CT for small spheres (<=10 mm), since the reported DEPs are close to 1\%. False-negative 124I PET/CT results as compared with posttherapeutic 131I 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, 124I activities as high as 170 MBq may be warranted to obtain equal detectability.}, issn = {0161-5505}, URL = {https://jnm.snmjournals.org/content/57/1/103}, eprint = {https://jnm.snmjournals.org/content/57/1/103.full.pdf}, journal = {Journal of Nuclear Medicine} }