[Clinical and cytological characteristics of primary thyroid lymphoma: a fine needle aspiration study of 10 cases].

<b>Objective:</b> To evaluate the cytological features of primary thyroid lymphoma diagnosed by fine needle aspiration (FNA), using histology as the gold standard. <b>Methods:</b> FNA samples from the 10 cases of primary thyroid lymphoma diagnosed between October 2016 and November 2024 at the Department of Pathology, Beijing Hospital were analyzed. Clinical data were retrospectively reviewed. The smears, cell block slides, immunochemical and histological findings were examined. <b>Results:</b> Among the 10 patients, there were 2 males and 8 females. The median age was 72 (range, 70 to 76) years. Six patients showed rapid enlargement of new or existing tumors within a short period of time (1-2 months). The median diameter of the lesions was 5.0 (range, 4.5 to 5.3) cm. Thyroid FNA samples showed diffuse dysplastic lymphocytoid cells. Diffuse large B-cell lymphoma (DLBCL) was composed of highly consistent and dysplastic lymphocytoid cells with significant nuclear enlargement, obvious nucleoli, frequent mitoses and karyorrhexis, while indolent lymphomas were composed of mixed and mild atypical lymphocytoid cells with various cell sizes, small nucleoli, infrequent mitosis and karyorrhexis. A small number of thyroid follicular cells with eosinophilic alteration were also present. Immunochemistry on smear indicated a predominant B-cell population with Ki-67 index of 5% to 90%. All patients underwent core needle biopsy or resection except one case that only had cell block preparation. The final pathological diagnoses included 5 cases of DLBCL, 3 cases of extranodal marginal zone lymphoma of mucosa associated lymphoid tissue, 1 case of follicular lymphoma (grade 3b) with transformation of diffuse large B-cell lymphoma, and 1 case of chronic lymphocytic leukemia/small lymphocytic lymphoma with concurrent thyroid papillary carcinoma. Six patients had concurrent Hashimoto's thyroiditis. <b>Conclusions:</b> Primary thyroid lymphoma is rare, almost all of them are B-cell origin, and it often presents with rapid enlargement of neck masses. Aggressive lymphoma has prominent cell atypia and can be diagnosed by FNA. For indolent lymphoma, although the cell atypia is mild, there are still cytological characteristics of lymphoma. Therefore, FNA can be used to diagnose hematopoietic and lymphoid malignancies when combined with thyroid diffuse lesion on imaging and the clinical history of rapid growth in a short period of time. An accurate classification generally requires biopsy and molecular clonality examination.