Primary Thyroid Lymphoma: Maintaining Diagnostic Scrutiny

Abstract

Background: Primary thyroid lymphoma (PTL) is a rare condition representing <5% of thyroid malignancies with diffuse large B-cell lymphoma (DLBCL) as the most common subtype (1). Although not commonly managed in the endocrine sphere, identification is paramount to prognosis and treatment. We present a case of PTL to highlight the importance of diagnostic scrutiny and clinical follow up. Clinical Case: A 69-year-old man with a history of Hashimoto’s thyroiditis presented with an enlarging neck mass. The patient noted development of a left-sided anterior neck mass 3 days prior to seeking care. He denied compressive symptoms and B symptoms (weight loss, fevers, and night sweats). Physical exam revealed a large, firm left-sided thyroid nodule. Serum studies were notable for TSH 39.1 mcIU/mL (0.30-4.7 mcIU/mL), FT4 0.9 ng/dL (0.80-1.70 ng/dL), and TPO Ab 127 IU/mL (<=20 IU/mL). Levothyroxine 100 mcg daily was started. Neck ultrasonography showed a 54 mm hypoechoic, solid left thyroid nodule without calcifications. No abnormal cervical lymph nodes were present. FNA revealed a mixed population of small lymphocytes with no monoclonal B cell population or T cell aberrancies on flow cytometry. In the setting of ongoing nodular growth, the patient underwent core needle biopsy which revealed DLBCL (Ki67 proliferation index >80%, EBV-EBER negative). For diagnostic confirmation and staging, a whole-body FDG-PET scan was performed with an intensely FDG-avid left thyroid mass and no metabolic evidence of additional lymphoproliferative disease. Bone marrow biopsy did not show lymphomatous involvement. The patient was diagnosed with primary thyroid lymphoma and started on chemotherapy with R-CHOP. Conclusion: PTL commonly presents as a rapidly enlarging, painless neck mass that may be accompanied by B symptoms. Hashimoto’s thyroiditis is a known risk factor. Our case in particular required more diligence in the setting of an FNA with mixed lymphoid cells and negative flow cytometry. Initial differential diagnosis included intrathyroidal lymph node, Hashimoto’s thyroiditis, thyroid adenoma/malignancy with a false FNA, and lymphoma. Notably, FNA biopsy is only associated with 71% sensitivity in diagnosing PTL, whereas core biopsy has a 93% sensitivity rate (2). Upon histopathologic disease diagnosis, collaboration with oncology is needed for further staging and initiation of chemotherapy +/- radiation.