Poorly differentiated thyroid carcinomas with primordial cell component. A group of aggressive lesions sharing insular, trabecular, and solid patterns.

Poorly differentiated carcinomas of the thyroid share insular, trabecular, and solid histological patterns that are different from those of papillary, follicular, medullary, and anaplastic varieties. We have collected 63 cases of poorly differentiated thyroid carcinomas. Thirty-one tumors (Group A) corresponded to the so-called insular carcinomas, and 32 tumors (Group B) had predominant trabecular and solid or focally follicular patterns in the presence of a minor insular component. The cells characterizing these lesions were relatively small and globoid, with uniform nuclei and intracytoplasmic deposits of thyroglobulin. They were in every respect similar to primordial cells present in the early stages of fetal thyroid development. None of the tumors proved fatal within 6 months, and most responded to radioiodine therapy. Although no differences in survival between the two groups were found, a significantly (p < 0.01) higher percentage of recurrences or distant metastases was observed with Group A tumors. The term primordial cell carcinoma appears appropriate for this type of tumor, which displays characteristic histocytological features and production of thyroglobulin. Clinically, these tumors are aggressive but generally show a slow course and good response to radioiodine therapy.