Findings for Differentiated Thyroid Carcinoma by γ-Camera-Based and Uptake-Probe-Based Methods in Comparison with...

The objective of the study was to calculate neck uptake by γ-camera-based and uptake-probe-based methods and compare it with uptake by diagnostic whole-body and neck radioiodine scanning in thyroid carcinoma patients. Methods: Of the 46 patients, 14 were male (average age, 47.5 y) and 32 female (average age, 38 y). All had already undergone thyroidectomy followed by ¹³¹I treatment at least once and came to the institute after 6 mo for follow-up. As per the institutional protocol, they underwent scanning 72 h after administration of a low-dose capsule (111-148 MBq) of ¹³¹I, and the uptake was calculated by both probe- and camera-based methods using medium-energy parallel-hole collimators. Results: The neck was negative for uptake in 24 patients by both the probe-based method (with 0.1% as the cutoff) and the camera-based method, as well as by the scan. The neck was positive for uptake in 10 patients by both methods and by the scan. In 10 patients, the neck was positive for uptake by the probe-based method but negative by the camera-based method and the scan; in most of these cases, nontarget counts contributed to high scatter radiation. In 2 patients, the neck was negative for uptake by the probe-based method but positive by the camera-based method and the scan. Thus, the two methods were concordant in 34 of the 46 patients (74%) but discordant in 12 (26%). On the basis of these results, we propose that 0.03% be the cutoff for positivity for the camera-based method. In 85.7% of patients with negative scan results and systemic metastases near the neck (n = 7), an uptake cutoff of 0.1% resulted in positive results. All patients had true-negative results with a camera-based cutoff of 0.03%. In view of this discordance between the probe-based method and the scan, we further propose that 0.2%, rather than 0.1%, be the cutoff for positivity for the probe-based method in order to reduce false positivity. Conclusion: In thyroid carcinoma patients undergoing ¹³¹I low-dose scans, camera-based uptake is as reliable as, and can substitute for, probe-based uptake in evaluating functioning metastases near the neck or nonspecific high physiologic accumulation.