124 I Positron Emission Tomography Versus 131 I Planar Imaging in the Identification of Residual Thyroid Tissue and/or Metastasis in Patients Who Have Well-Differentiated Thyroid Cancer

Abstract

Background and Objective:

¹²⁴I emits a positron and can be imaged with a positron emission tomography (PET) scanner. The objective of this study was to compare the ability of diagnostic ¹²⁴I PET images versus ¹³¹I planar whole-body imaging in detecting residual thyroid tissue and/or metastatic well-differentiated thyroid cancer (WDTC).

Methods:

Patients were recruited prospectively for this study who (i) had WDTC, (ii) were suspected of having metastatic WDTC, and (iii) were referred for ¹³¹I whole-body dosimetry. The prescribed activity was 1–2 mCi (37–74 MBq) and 1.7 mCi (62.9 MBq) for ¹³¹I and ¹²⁴I, respectively. For each image, one blinded reader (D.V.N.) categorized every focus of ¹³¹I and ¹²⁴I radioiodine uptake as 1 = definite physiological uptake/artifact, 2 = most likely physiological uptake/artifact, 3 = indeterminate, 4 = residual thyroid tissue/metastases in the neck/bed, 5 = most likely metastases, or 6 = definite metastases. Foci categorized as 4, 5, or 6 were considered positive. When available, foci categorized as 4, 5, or 6 were correlated with other diagnostic studies.

Results:

Of the 25 patients, 8 patients (32%) had more positive foci on ¹²⁴I images than on ¹³¹I, of which 3 patients to date have had metastases confirmed in one or more of the additional positive ¹²⁴I foci. ¹²⁴I demonstrated the same number of foci as on ¹³¹I in 16 patients (14 with no positive foci, and 2 with two positive and five positive foci each). One patient had one additional positive focus on ¹³¹I not seen on ¹²⁴I, which has not yet been confirmed as a metastasis. A total of 97 positive foci were identified on either ¹²⁴I or ¹³¹I. ¹²⁴I identified 49 positive foci not seen with ¹³¹I, and ¹³¹I identified one positive focus not seen with ¹²⁴I.

Conclusion:

Relative to ¹³¹I planar whole-body imaging, ¹²⁴I PET identified as many as 50% more foci of radioiodine uptake suggestive of additional residual thyroid tissue and/or metastases in as many as 32% more patients who had WDTC.