Follicular lymphoma of the thyroid and the role of core needle biopsy.

Summary: Primary thyroid lymphoma (PTL) is a rare malignancy, accounting for less than 5% of all thyroid neoplasms. The follicular subtype is even more rare, accounting for approximately 10% of all PTL cases. We report a case of a 64-year-old woman, who presented with a rapidly growing goitre with mass effect and B symptoms. She had a history of Hashimoto's thyroiditis and her thyroid ultrasound revealed diffuse goitre with a dominant nodule (56 × 63 × 60 mm) within the right thyroid lobe. Ultrasound-guided percutaneous fine-needle aspiration of the right thyroid nodule was classified as benign, according to Bethesda System, with lymphocytic thyroiditis. A CT scan of the neck showed diffuse enlargement of the thyroid gland extending towards the anterior mediastinum with tracheal deviation and lymphadenopathy within levels VII and right II-IV. The core needle biopsy of the right thyroid nodule revealed a follicular non-Hodgkin's B cell lymphoma with a Ki67 of 60%. According to the Ann Arbor staging system, she was at stage IIIE. She underwent chemotherapy with R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone) with remarkable clinical improvement and is currently in remission 2 years after the diagnosis. PTL is an extremely rare malignancy that usually arises in a lymphocytic thyroiditis background, presenting as a rapidly enlarging goitre, which can lead to compressive symptoms or airway comprise. Learning points: Primary thyroid lymphoma (PTL) is a rare malignancy, accounting for less than 5% of thyroid neoplasms. PTL should be suspected when a patient presents with a rapidly enlarging goitre, especially in the setting of Hashimoto's thyroiditis. Fine-needle aspiration has a limited capacity for PTL diagnosis due to similar cytomorphological features of lymphoma with thyroiditis. Therefore, in case of clinical suspicion and if fine needle aspiration fails to diagnose PTL, a tissue biopsy should be performed. Treatment is dependent on both the stage and histology of PTL. Chemotherapy and local radiotherapy remain the mainstay treatment for PTL.