Poorly differentiated ('insular') thyroid carcinoma. Report of two cases and review of the literature.

The clinical, histopathologic, immunohistochemical, and electron microscopic findings of two cases of poorly differentiated ("insular") thyroid carcinoma are reported and compared with the 25 cases previously described in the literature. These 27 cases occurred in eight men and 19 women, aged from 34 to 77 years. All but 2 presented with a thyroid mass. Eleven (41%) of the 27 patients experienced local recurrences following surgery, 17 (63%) had development of cervical lymph node metastases, 5 (19%) had development of mediastinal lymph node metastases, 11 (41%) had pulmonary metastases, and 9 (33%) had bone metastases, and 9 (33%) had bone metastases. At least 15 patients (56%) are known to have died of their disease, usually within 8 years of diagnosis. Pathologically, the tumors are distinctive and grow as solid islands (insulae) of small cells separated by artifactually created clefts. In some instances, small follicles are also noted within the insulae. All tumors were positive on immunostaining for thyroglobulin. In view of the propensity for local recurrences and lymphatic and hematogenous dissemination, a total thyroidectomy and neck dissection would seem advisable. Additionally, adjuvant external beam irradiation, systemic chemotherapy, and/or radioactive iodine therapy should also be considered.