Poorly differentiated insular thyroid carcinoma. A case report with identification of intact insulae with fine needle aspiration biopsy.

Background: Subsequent to the publication of a report in 1984 entitled "Poorly Differentiated ("Insular") Carcinoma: A Reinterpretation of Langhans "wuchernde Struma," poorly differentiated insular thyroid carcinoma (PDITC) has become recognized as a distinct thyroid neoplasm. It is classified morphologically and biologically as an intermediate entity between well-differentiated (papillary and follicular) and undifferentiated (anaplastic) thyroid carcinomas. Only a few publications have addressed the findings with fine needle aspiration biopsy (FNAB).

Case: A 67-year-old female presented for evaluation of a massively enlarged thyroid gland. Fine needle aspiration biopsy of the thyroid with a 22-gauge needle showed many large, multilayered, round to oval nests of tumor cells, 0.2-0.4 mm in diameter. Rosettelike configurations of 8-15 cells, 0.025-0.050 mm in diameter, were also observed. Nests of neoplastic cells in the histologic sections were virtually identical to those in the fine needle aspiration biopsy specimens. When the patient developed metastatic cervical adenopathy one year later, a microfollicular pattern was seen on both the FNAB and histologic sections.

Conclusion: When nests of tumor cells, 0.2-0.4 mm in diameter, are identified in a thyroid FNAB specimen, PDITC should be included in the differential diagnosis. A microfollicular pattern in a metastatic lymph node does not exclude the possibility that the primary tumor is a PDITC.