Diagnostic efficacy of dynamic radiothallium uptake in thyroid nodules determined by computer-assisted scintigraphy. Reevaluation of a radioisotopic procedure.

Radiothallium (201Tl) has been proposed as a tracer in nuclear medicine for the diagnosis of cold thyroid nodules. Its potassium-mimetic characteristics, like those of 131Cs, can provide information on its turnover in the thyroid nodules; this serves as a basis for the distinction between benign and malignant thyroid tumours. The author presents a study on the diagnostic efficacy of 201Tl as a tracer for thyroid nodules with the aim of evaluating the dynamic uptake and the biodistribution of 201Tl after intravenous injection of 555-740 kBq/kg. A quantitative analysis was made by continuous scanning of 201Tl distribution during the first 20-30 min following thallium injection, with further surveys from 40 to 60 min or more when fixation in a nodule was demonstrated. The ratio between 201Tl uptakes in the nodule and in healthy tissue (density index) was calculated and a multiparametric analysis of the corresponding activity/time curves was performed. This study included 176 patients. Concordance between clinico-histological diagnosis and radio-isotopic findings was demonstrated in 102 out of 106 colloidocystic goitres, in 42 out of 48 benign nodules including thyroiditis, and in 21 out of 22 malignant tumours; a significant statistical difference in the density index between solid and cystic nodules, but no difference between benign and malignant nodules; and a different 201Tl dynamic behaviour in nodules of different nature, with a clear statistical difference in the thallium release between malignant nodular and healthy tissue (nodular disappearance index). This method can improve the diagnosis value of scintigraphy for thyroid nodules enhancing the sensitivity of the radioisotopic applications in comparison with other non-invasive techniques.