Granular cell tumor of thyroid: a case series with molecular characterization highlighting unique pitfalls.

Primary granular cell tumors (GCTs) of the thyroid are exceptionally rare. We report the clinicopathologic and molecular features of three cases and review the literature. Two patients (20-year-old, Case 1, and 26-year-old, Case 2, black American females) presented with painless masses with a preoperative fine-needle aspiration biopsy (FNAB) diagnosis of "Hürthle cell neoplasm," while one additional patient, 51-year-old white American female (Case 3), presented as an incidental finding within a background of chronic lymphocytic thyroiditis. On resection, morphologic, histochemical and immunohistochemical features were typical of GCT in all cases. Cases 1 and 2 had adequate material for molecular testing and demonstrated a clonal ATP6AP1 p.G381Vfs*15 frameshift mutation (Case 1) and a clonal ATP6AP2 p.L182Pfs*22 frameshift mutation along with a PIK3CA H1047R hotspot mutation (Case 2). All patients showed no evidence of GCT following resection (Cases 1, 3: 96-month follow-up; Case 2: 48-month follow-up). A literature review demonstrates similar clinicopathologic features and indolent course with only rare histologically or clinically aggressive outcomes. On FNAB, lesional cells are frequently miscategorized as Hürthle cells or oncocytes. In summary, GCT of the thyroid is rare but shows similar clinical, morphologic, immunophenotypic and genetic characteristics of GCT of other sites. This unusual site poses unique differential diagnostic pitfalls by mimicking other oncocytic head and neck lesions, particularly thyroid Hürthle cell neoplasms. We confirm that thyroid GCT also harbor V-ATPase component inactivating mutations that characterize these tumors, and that additional PI3K pathway alterations may not necessarily predict aggressive behavior.