PT - JOURNAL ARTICLE AU - Casper Beijst AU - Jakob W. Kist AU - Mattijs Elschot AU - Max A. Viergever AU - Otto S. Hoekstra AU - Bart de Keizer AU - Hugo W.A.M. de Jong TI - Quantitative Comparison of <sup>124</sup>I PET/CT and <sup>131</sup>I SPECT/CT Detectability AID - 10.2967/jnumed.115.162750 DP - 2016 Jan 01 TA - Journal of Nuclear Medicine PG - 103--108 VI - 57 IP - 1 4099 - http://jnm.snmjournals.org/content/57/1/103.short 4100 - http://jnm.snmjournals.org/content/57/1/103.full SO - J Nucl Med2016 Jan 01; 57 AB - 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–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 (&gt;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.