[Usefulness of a 201Tl dynamic study of thyroid tumors]

A thallium 201 dynamic study was performed on 39 patients with thyroid tumor by use of a digital gamma-camera, which was connected to the computer data system. They consisted of 12 patients with a benign lesion and 27 patients with a malignant lesion. Regions of interest (ROIs) were assigned for tumor, normal thyroid, and background. Time activity curves (TACs) were generated from these ROIs. The following six parameters were created from the TACs and compared between benign and malignant tumors: 1) Tmax, 2) T3/4max, 3) distribution index (DI) = peak count determined for a tumor divided by that for the normal thyroid, 4) mean transit time (MTT), 5) relative MTT (r-MTT) = MTT for a tumor divided by that for the normal thyroid, and 6) perfusion curve (PC) = TAC that is drawn on the basis of the quotients obtained by dividing A by B (A = counts in the individual frames determined for a tumor; B = those for the normal thyroid). Among the first five parameters, DI, T3/4max, and r-MTT were significantly different between the two sorts of lesions. To be more precise, DI was significantly lower (p less than 0.01) and T3/4max and r-MTT were higher (p less than 0.05) in the malignant group. PC was classified into three types: decreased type, parallel type, and increased type. In the benign group the PCs were decreased or parallel type in all cases except two. In the malignant group the PCs were parallel or increased type in all cases. In conclusion, the present observations suggest that thallium 201 accumulates in a malignant lesion at nearly the same rate as in the normal thyroid in an early phase, or otherwise its washout from the lesion is prolonged, and further that early-phase accumulation in a benign lesion is high and washout from the lesion is rapid compared with the normal thyroid. Functional images (peak count, MTT), which were obtained through computer analysis, facilitated visual diagnostic evaluation of thyroid tumor.