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Introduction: Primary thyroid lymphomas comprise 5% of all thyroid malignancies and 2% of extranodal lymphomas. Chronic lymphocytic thyroiditis is a risk factor for thyroid lymphomas. The surgical specimen was formalin-fixed and paraffin embedded. The sections were stained with routinary H&E. Immunohistochemistry was also performed.

Case Report: A 76 year-old man admitted to outpatient clinics with a neck mass. He has neither abnormal laboratory tests nor hematologic disease in his profile. Macroscopically, thyroid had 3 nodules and the dominant one was solid and circumscribed with 5x5x4cm dimensions, and a creamy nodule. Sections prepared from this nodule, demonstrated atypical lymphoid cells infiltration limited within the nodule. Immunohistochemically, CD45(+), CD20(+), Bcl-6(+), C-myc 90% (+), ALK-1 (-), MUM-1 (-), HBME-1(-), PANCK (-), CK19 (-), TTF-1 (-), calcitonin (-) in neoplastic cells so these histologic and immunohistochemical findings indicated diffuse large B-cell lymphoma (Germinal center phenotype).

Conclusion: Large cell lymphomas may demonstrate epitheloid appearance and can be misdiagnosed as a carcinoma. Primary thyroid carcinoma was initially considered in this case. Awareness of primary thyroid lymphomas especially diffuse large B-cell lymphoma is important. The diagnosis is easy when the patients have systemic Non-Hodgkin lymphoma diagnosis, but primary thyroid lymphomas also should be kept in mind even in patients without a lymphoma history.

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