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Case of the Month



Metastatic thyroid carcinoma

The patient is a 75 year old female with a lytic lesion in the left ileum. She reports history of abnormal urine protein, so plasma cell neoplasm is a possibility as well as metastatic carcinoma. CT/PET scan showed a central lower neck mass, which was favored to represent an enlarged thyroid. No other visceral lesions or lymphadenopathy is seen.

Core biopsy of a lytic lesion shows tumor in trabeculae and tightly packed follicles. Eosinophilic material consistent with colloid is present in each lumen. Nuclei are vesicular and fairly monomorphic. No definite nuclear features of papillary thyroid carcinoma are appreciated.

The tumor cells stain diffusely positive with the TTF-1 and PAX-8 immunohistochemical stains. The staining results, histologic features, and enlarged thyroid gland are consistent with metastatic thyroid carcinoma, and follicular carcinoma is favored.

Bone is the second most common site of metastasis of thyroid carcinomas after lung. Among the different types of thyroid carcinoma, follicular carcinoma is most likely to show bone metastasis. However, the overall case number of metastatic papillary carcinoma to bone may be higher since papillary carcinoma has a much higher incidence than follicular carcinoma.