Correlation of single image Tc-99m MIBI scan and ultrasonography with fine needle aspiration cytology (FNAC) to assess neoplasia in solitary 'cold' thyroid nodules

Various investigative approaches for the management of solitary thyroid nodules have been adopted, which involve Tc-99m pertechnetate scan, ultrasonography and fine needle aspiration cytology (FNAC). Incidence of neoplasm in a solitary thyroid nodule showing cold lesion on Tc-99m pertechnetate scan vary from 9 to 26%. In the current study we evaluated the utility of Tc-99m labeled Hexakis2- methoxyisobutyl isonitrile (Tc-99m MIBI) in differentiating neoplastic from non-neoplastic lesions among nodules which are cold on Tc-99m pertechnetate scan and solid or mixed on ultrasonography. Forty-nine patients, all having solitary cold nodules on Tc-99m Pertechnetate scans were included in the study. All underwent ultrasonography, Tc- 99m MIBI scan and FNAC. Ultrasound findings were categorized as solid, mixed and cystic. Anterior images of thyroid were acquired 15 minutes after intravenous injection of Tc-99m MIBI. The thyroid nodules were classified into 5 categories depending on the degree of MIBI uptake. They were; Category-1 showing intense uptake, Category -2 showing slightly higher uptake than the surrounding normal thyroid tissue, Category - 3 with uptake equal to the normal thyroid, Category -4 showing less uptake than the surrounding normal tissue and Category -5 showing no or negligible uptake of Tc-99m MIBI. Receiver Operating Characteristic (ROC) curve was generated using Tc-99m MIBI results. FNAC revealed 10 nodules with neoplastic lesions (8 follicular and two pleomorphic), 29 with colloid goiter and 5 with benign cystic lesions. Tc-99m MIBI images revealed Categories- 1,2,3,4 and 5 scintigraphic patterns in 2,12, 11, 10 and 14 patients respectively. One patient in Category 1 and 9 in Category 2 proved to be neoplastic in nature, while none of the patients in category 3 to 5 had evidence of neoplasm on FNAC. The ROC curve revealed excellent performance of Tc-99m MIBI scan in diagnosing neoplastic lesions in the solitary cold thyroid nodules. Considering Category-1 and 2 as true positive results sensitivity, specificity, positive predictive and negative predictive values of Tc-99m MIBI scanning were found to be 100, 90, 71 and 100% respectively. Ultrasonography also had a sensitivity of 100% considering solid and cystic lesions as test positive. But specificity of US was found to be only 13%. The sensitivity and specificity of the combined results of Tc-99m MIBI scintigraphy and ultrasonography were also not found to be significantly different from those of Tc-99m MIBI scan alone (100% and 91% respectively). Hence in patients with solitary cold thyroid nodules on Tc-99m Pertechnetate study, a single Tc-99m MIBI scan can be used reliably to assess neoplastic nature of the nodule with high degree of sensitivity and specificity. (author)