Medullary carcinoma of the thyroid: accuracy of diagnosis of fine-needle aspiration cytology.

BACKGROUND

A preoperative diagnosis of medullary carcinoma of the thyroid (MCT) allows for the investigation of associated multiple endocrine neoplasia/ pheochromocytoma, and definitive surgery without the need for frozen section. Criteria for cytodiagnosis are well known but variable patterns of presentation may cause diagnostic difficulty.

METHODS

This study examines the accuracy of cytodiagnosis and the value of ancillary tests in 17 patients seen between 1976 and 1997. Nine patients underwent thyroid gland aspirations, five patients underwent fine-needle aspiration (FNA) of the thyroid and cervical lymph nodes, and three patients underwent cervical lymph node aspiration alone. Electron microscopy (EM) of aspirated material was performed in nine cases and immunocytochemistry in two cases.

RESULTS

In all cases the diagnosis was suggested by FNA. In four cases, diagnosis and management were based on cytology alone. EM of FNA material was confirmatory in nine cases, two of which also showed positive calcitonin immunocytochemistry. In three cases the diagnosis was not proven until surgical resection, and in one case FNA confirmed lymph node metastasis in known MCT. Frozen section in five patients did not change the level of diagnostic confidence over the FNA diagnosis in any case. In four other thyroid tumors (one Hürthle cell follicular carcinoma, two anaplastic carcinomas, and one hyperplastic nodule) MCT was suspected in the FNA differential diagnosis but later excluded. In the Hürthle cell tumor immunoperoxidase staining was positive for calcitonin and in one anaplastic carcinoma, a neuroendocrine phenotype was suggested. In the latter case, additional EM excluded MCT.

CONCLUSIONS

Although correct diagnosis is made by cytology in the majority of instances, other tumors may show cytologic findings similar to MCT. EM of FNA material was found to be the most definitive method of proving or excluding MCT. Immunocytochemistry may be misleading for rarely performed tests.