Metastatic thyroid carcinoma without identifiable primary tumor within the thyroid gland: a retrospective study of a rare phenomenon.

Metastatic papillary thyroid carcinoma (PTC) without an identifiable primary tumor despite extensive microscopic examination of the thyroid gland is a rare but true phenomenon.We retrieved 7 of such cases and described in detail the clinical and pathologic features of these tumors. BRAF V600E immunohistochemistry and Sequenom molecular profile were conducted in selected cases. All patients harbored metastatic disease in the central (n=3), lateral (n=3), or both neck compartments (n=1). The histotype of the metastatic disease was PTC (n=5), poorly differentiated thyroid carcinoma in association with a PTC columnar variant (n=1), and anaplastic thyroid carcinoma in association with a PTC tall cell variant (n=1). Fibrosis was present in the thyroid of 5 patients. All patients with PTC were alive without evidence of recurrence. The 76-year-old patient with poorly differentiated thyroid carcinoma did not recur and died of unknown causes. Finally, the patient with anaplastic thyroid carcinoma was alive with distant metastasis at last follow-up. The median follow-up for this cohort was 2.2years (range, 0.8-17). BRAF V600E was detected in 4 of 6 cases by immunohistochemistry. In conclusion, metastatic nodal disease without identifiable thyroid primary is a rare but real phenomenon of unknown mechanisms. Although most tumors are low grade and well differentiated, aggressive behavior due to poorly differentiated or anaplastic carcinoma can happen. Most cases are BRAF-positive thyroid tumors. A papillary carcinoma phenotype is found in all reported cases.