Diffuse Large B-Cell Lymphoma of the Thyroid in a Patient With Hashimoto Thyroiditis: A Diagnostic Dilemma.

Primary thyroid lymphoma (PTL) is a rare malignancy, comprising 1%-5% of thyroid cancers and 2%-5% of extranodal lymphomas, with diffuse large B-cell lymphoma (DLBCL) being the most common subtype. PTL often coexists with Hashimoto thyroiditis, leading to diagnostic delays. We report a case of a man in his early 70s who presented with a progressively enlarging, painless anterior neck mass over 2.5 years. Initial fine needle aspiration cytology (FNAC), repeated twice over 18 months, suggested lymphocytic thyroiditis. Due to persistent growth and inconclusive FNAC results, the patient underwent a right hemithyroidectomy with central neck dissection. Histopathological examination and immunohistochemistry confirmed DLBCL, activated B cell (ABC) subtype. He was started on R-CHOP chemotherapy. FNAC, although a first-line diagnostic tool for thyroid nodules, has a low sensitivity to PTL, especially in the context of underlying thyroiditis. Advanced diagnostic techniques such as immunohistochemistry, flow cytometry, and core needle biopsy (CNB) significantly improve accuracy but are often inaccessible in low-resource settings. CNB offers a preserved tissue architecture and a higher diagnostic yield, reducing unnecessary surgeries. This case underscores the diagnostic limitations of FNAC in PTL and highlights the need for improved diagnostic approaches, including CNB and auxiliary tests, to ensure early and accurate diagnosis, especially in settings with limited resources.