Follicular thyroid carcinoma with diffuse mucin deposition: cytologic and histologic differential diagnosis and review of the literature

Diffuse mucin deposition in a follicular thyroid carcinoma is an exceedingly rare morphologic variation, with only a limited number of cases that have been documented in the literature. We report a case of encapsulated angioinvasive follicular thyroid carcinoma with diffuse intrafollicular and stromal mucin deposition in a woman in her late 70s, who presented with a 9-year history of anterior neck swelling. Fine-needle aspiration cytology performed one year prior to surgery was interpreted as benign, showing only a small number of follicular cell clusters with mild nuclear atypia in a fibrino-myxoid background without nuclear features of papillary thyroid carcinoma. Computed tomography demonstrated a well-circumscribed 10-cm right thyroid mass with heterogeneous enhancement, suggesting degenerative change. A right thyroid lobectomy revealed an 11 × 7.7 × 7.7 cm solid, gray-white mucinous nodule. Histologically, the tumor was encapsulated and exhibited focal intracapsular vascular invasion, consistent with encapsulated angioinvasive follicular thyroid carcinoma. Diffuse mucin deposition was present within both follicles and the stroma. The mucinous material was positive for Alcian blue and negative for periodic acid-Schiff stain. Immunohistochemically, tumor cells were positive for thyroglobulin, TTF-1, PAX8, and CD56, and negative for CEA, chromogranin A, and synaptophysin. PTEN expression was retained, and BRAF V600E and NRAS Q61R were not detected by immunohistochemistry. This case highlights an exceptionally rare thyroid neoplasm and underscores the importance of morphologic differential diagnosis in both cytologic and histologic evaluations when encountering mucin-rich thyroid lesions.