CD10-Positive Extranodal Marginal Zone Lymphoma of the Thyroid Arising in Hashimoto’s Thyroiditis: A Rare Diagnostic Challenge

Patients with Hashimoto's thyroiditis (HT) have an increased risk of developing primary thyroid lymphoma (PTL), with extranodal marginal zone lymphoma (ENMZL) representing a significant subset of these cases.We present the case of a 49-year-old woman with a history of HT, Sjögren's syndrome, and systemic lupus erythematosus who was incidentally found to have fluorodeoxyglucose (FDG)-avid thyroid nodules on PET-CT.Initial and repeat fine-needle aspiration (FNA) biopsies were either benign or indeterminate; however, flow cytometry revealed a small kappa-restricted B-cell population with dim CD10 expression.Given the persistent diagnostic uncertainty, the patient underwent total thyroidectomy, and histopathologic evaluation confirmed low-grade ENMZL with plasmacytic differentiation.Staging demonstrated localized stage IE disease.This case highlights important diagnostic challenges, including the potential for thyroid ENMZL to be missed on cytology despite ongoing clinical suspicion and the ability of rare aberrant CD10 expression in marginal zone lymphoma to mimic germinal-center-derived lymphomas.