Quantitative comparison of I-124 PET/CT and I-131 SPECT/CT detectability

Abstract

Radioiodine therapy with ¹³¹I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pre-therapy 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/CTs as compared with the post-therapy ¹³¹I SPECT/CTs 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 detectability of lesions on ¹²⁴I PET/CT and ¹³¹I SPECT/CT, and hence, to determine whether the administered activity of ¹²⁴I is sufficient to achieve equal detectability. Methods: Phantom measurements were performed using the NEMA-2007 image quality phantom. As a measure of detectability the contrast-to-noise ratio (CNR) was calculated. The ¹²⁴I activity concentration was expressed as a percentage of the ¹³¹I activity concentration required to achieve the same CNR. This metric was defined as the detectability equivalence percentage (DEP). Results: Lower DEPs were obtained for smaller spheres, so that a relatively low ¹²⁴I activity concentration is sufficient to achieve a similar detectability of lesions with ¹²⁴I PET/CT as with ¹³¹I SPECT/CT. The DEP was 1.5, 1.9, 1.9, 4.4, 9.0 and 16.2% for the spheres with a diameter of 10, 13, 17, 18, 25 and 37 mm respectively, for attenuation and scatter corrected SPECT versus point spread function (PSF) modeled and time-of-flight (TOF) PET. For no-PSF no-TOF PET, the DEP was 3.6, 2.1, 3.5, 7.8, 15.1 and 23.3%. Conclusion: A relatively low activity of 74 MBq ¹²⁴I (~1% of ¹³¹I activity) is sufficient to achieve similar detectability of lesions on ¹²⁴I PSF TOF PET/CT and ¹³¹I SPECT/CT for small spheres (≤10mm), since the reported DEPs are close to 1%. False negative ¹²⁴I PET/CTs as compared with the post-therapy ¹³¹I SPECT/CTs may be ascribed to differences in detectability for large lesions (>10mm) and for no-PSF no-TOF PET, since DEPs are larger than 1%. Based on DEPs of 3.5% for lesion diameters up to 17mm on no-PSF no-TOF PET, activities as high as 170 MBq of ¹²⁴I may be warranted to obtain equal detectability.