Primary thyroid lymphoma (PTL) is a rare thyroid gland cancer, with diffuse large B-cell lymphomas (DLBCL) being extremely rare in children and adolescents. Thus, optimal therapy is debatable. We describe a rare case of thyroid DLBCL in an adolescent girl with a history of Hashimoto thyroiditis (HT), the difficulty in diagnosis and the outcome of treatment. A 12-year-old girl with a nine-year history of HT was admitted with a right-sided painless progressive swelling of the neck. Physical examination and imaging including ultrasound (US), computed tomography (CT) and positron emission tomography/CT revealed an enlarged thyroid gland with right side lymphadenopathy and no metastasis. Two fine needle aspirations were done showing suspected lymphoblastic lesions for non-Hodgkin lymphoma without precise diagnosis. US guided core needle biopsy was finally performed confirming the diagnosis of DLBCL. She was treated according to LMB 96-group B protocol with no surgical removal of thyroid. The patient responded very well to treatment and 14 months later there is no evidence of relapse or metastases. PTL is an extremely rare cause of thyroid malignancy in children. However, it should be considered in the differential diagnosis of a thyroid mass in adolescents presenting with a rapidly enlarging neck mass and a history of HT. It is a treatable condition with a good prognosis, even in aggressive histological subtypes, with no need for thyroidectomy.
