Brain metastasis from thyroid adenomatous nodules or an encapsulated thyroid follicular tumor without capsular and vascular invasion: a case report

Because benign-appearing thyroid nodules with metastasis are very rare, the author reports herein four thyroid nodules (one follicular adenoma and three adenomatous nodules) with brain metastasis. A 75-year-old Japanese woman was admitted to our hospital because of thyroid mass. Imaging modalities revealed four distinct nodules in the thyroid, and tumorectomies of all nodules were performed under the clinical diagnosis of benign thyroid nodules. Grossly, one of them was a completely encapsulated tumor (35 mm in diameter). Other three nodules were non-encapsulated nodules (10 mm, 8 mm, and 7 mm in diameters). Multiple sections were obtained from the largest nodule. One section was obtained from each of the smaller three nodules. Microscopically, the largest tumor was an encapsulated follicular adenoma. The tumor consisted of normofollicles and microfollicles surrounded by a fibrous capsule. Neither capsular invasion nor vascular permeation was recognized. The parenchyma lacked nuclear atypia, mitotic figures, degenerative changes, papillary structures, nuclear inclusions, nuclear ground-glass features, and nuclear grooves. Thus, the largest tumor was diagnosed as follicular thyroid adenoma. The remaining three small nodules were typical adenomatous nodules composed of normofollicles and macrofollicles without nuclear atypia, mitotic figures, degenerative changes, papillary structures, nuclear inclusions, nuclear ground-glass features, and nuclear grooves. Therefore, a diagnosis of adenomatous nodules (goiters) was made. However, six years later, the patient showed a brain metastasis of thyroid tumor composed of macrofollicles without cellular and nuclear atypia. A diagnosis of metastatic follicular thyroid carcinoma was made. The present case suggests that benign thyroid nodules can metastasize.