Divergent histology in the primary and metastatic lesions of thyroid carcinoma

The metastatic lesions of malignant diseases tend to present similar histological findings as the primary tumor. However, thyroid carcinoma is one of the malignancies which may be an exception to this rule. We have reviewed 61 consecutive autopsies of thyroid cancer patients at the Ito Hospital and Kawasaki Medical School Hospital during a period between 1969-1982. Fifty-five of 61 cases of thyroid carcinoma were found to have metastatic spreads beyond the thyroid. The uniform pathological findings were present in 5 of 9 papillary adenocarcinomas, 14 of 15 anaplastic carcinomas, and 1 of two squamous cell carcinoma and one epidermoid carcinoma. The remaining 36 patients were found to have different histological findings in metastatic lesions from the primary tumor. The histological types in coexisting metastases varied from one lesion to the other; the primary tumor was adenocarcinoma, but one of the metastases was anaplastic and the others showed adenocarcinoma or its combination. Those histological divergence in the primary tumor and metastases can be considered as the results of anaplastic or squamous cell transformation from adenocarcinoma. Such alteration of the histological findings in thyroid carcinoma calls for the reassessment of the therapeutic strategy.