Primary thyroid lymphoma is an uncommon malignancy; it accounts for <2% of all extranodal lymphomas and <5% of all thyroid malignancies.
Case report: A 52-year-old male patient who was known to have a nodule in his thyroid gland for many years presented with instantaneous enlargement of his existing nodule and with appearance of ipsilateral cervical lymphadenopathy. On thyroid ultrasonography, a giant nodule of 38.4×31.6×45.5 mm size, and containing macro and microcalcifical areas was detected in the left lobe. In the ipsilateral anterior cervical region, a lymphadenopathy with ultrasonographically malign appearance, in size of 23.4×15.6×32 mm was observed. The patient was euthyroid in laboratory findings and his thyroglobulin and calcitonin values were in normal ranges. As cytomorphological appearance was consistent with anaplastic carcinoma in ultrasonography-guided aspiration biopsies of thyroid nodule and lymphadenopathy, immunohistochemical staining was performed for differential diagnosis of lymphoma. Diffuse large B-cell lymphoma cells showing CD20 involvement were seen in cytological specimens. The histological specimens of the patient who underwent total thyroidectomy and cervical neck dissection were examined. While normal thyrocytes were detected in thyroid gland, atypical lymphoid cells showing diffuse infiltration in thyroid tissue were observed in the nodule, which were stained diffuse, strongly positive for CD20 and BcL and negative for CD45 RO. In the lymph node, diffusely infiltrating atypical lymphoid cells with similar immunohistochemical staining properties were observed. According to Revised EuropeanAmerican Classification of Lymphoid Neoplasms, the diagnosis was diffuse large B-cell lymphoma. The patient was accepted as stage I-EB. Rituximab, Doxorubucin, cyclophosphamide, vinciristine, and prednisone chemotherapy was started. We introduce the case in order to emphasize that infiltration of diffuse large B-cell lymphoma should be recalled with instantaneous enlargement of nodules known to be benign previously, and when cytological specimen is uncertain, immunohistochemical staining should be performed for differential diagnosis.