Early and late lesion-to-non-lesion ratio of thallium-201-chloride uptake in the evaluation of 'cold' thyroid nodules

Forty-nine consecutive patients with 'cold' thyroid nodules were studied using early and late visual and semiquantitative measurements of ²⁰¹Tl uptake in the nodule to differentiate benign from malignant nodules. The visual method compared ²⁰¹Tl uptake in the nodule to the normal thyroid tissue. The semiquantitative method used a lesion-to-non-lesion (L/N) ratio of the same areas. Both measurements were carried out early (15 min) and late (3 hr) following ²⁰¹Tl injection. The reproducibility of the method for the early and late measurements was tested for intraobserver and interobserver variability as well as for repeatability coefficients. The visual method resulted in 43% sensitivity and 79% specificity for the detection of malignant nodules. The L/N method showed that an early threshold of 1.55 chosen by receiver characteristic analysis had a sensitivity of 57% and a specificity of 86%, while the late ratio of 0.99 had a sensitivity of 100% and a specificity of 62%. It is concluded that a L/N ²⁰¹Tl uptake method performed 3 hr following ²⁰¹Tl injection is superior to a visual scoring method as well as to the early L/N ²⁰¹Tl uptake in detecting malignant thyroid nodules.