Primary thyroid lymphoma (PTL) is a rare malignancy, representing less than 2% of all thyroid cancers. It typically presents in the seventh life decade, affecting predominantly women (female :male ratio— 4:1). The major known risk factor for PTL is Hashimoto’s thyroiditis, which was identified in up to 90% of patients.
Thyroid lymphomas are almost always of the non- Hodgkin type and of B- cell lineage; the most frequent are diffuse large B- cell lymphomas (DLBCL) and mucosa- associated lymphoid tissue (MALT) lymphomas.
PTL typically presents as a rapidly expanding neck mass which is often hard, sometimes tender, fixed to the trachea, and often extending substernally. Because of invasion into the adjoining neck structures, patients can present with hoarseness, dyspnoea, and dysphagia. Lymph node enlargement can be identified in approximately 50% of cases. Systemic B- type symptoms can also be pre sent: fever, night sweats, and significant weight loss. Occasionally, MALT lymphomas present as a single, asymptomatic, slow- growing thyroid nodule.
Patients are usually euthyroid; hypothyroidism sometimes occurs because of coexisting Hashimoto’s thyroiditis/ severe thyroid infiltration by the lymphoma. Rarely, transient thyrotoxicosis occurs due to rapid destruction of thyroid follicles.
Ultrasound reveals an intensely hypoechoic, pseudo- cystic structure, sometimes nodular, with ‘broccoli- like borders’, other times diffuse, infiltrative. Enhancement of posterior echoes is always present and can help to distinguish it from severe thyroiditis. FNAB can suggest the diagnosis, but management is decided after large- bore or surgical biopsy with immunohistochemistry which is needed to evaluate the histological subtype.
18FDG- PET, CT, or MRI are performed for staging purposes, to define local extension, to plan treatment, and monitor response to treatment.
Management depends on histological subtype and tumour stage. MALT lymphomas usually have an indolent clinical course and can be treated by surgery alone, radiotherapy alone or a combination of them. The benefit of surgical resection in these patients is debatable, so it is not typically recommended, but MALT lymphomas are sometimes detected incidentally after thyroidectomy performed for other indications, and in this case no further treatment may be needed. DLBCL are generally aggressive and require combined modality treatment using the monoclonal antibody rituximab, combined chemotherapy (cyclophosphamide, doxorubicin, vincristine, prednisone) and radiotherapy. Radiotherapy provides local control and combined chemotherapy is used to decrease the incidence of distant recurrences. If critical airway obstruction occurs, surgical resection may be an option; however, tumours typically respond in a few hours to chemotherapy and glucocorticoids and airway permeability is restored.
The prognosis of PTL depends largely on the histological sub type, patient age, disease stage, and treatment. The reported 5- year disease- free survival is 95% for MALT lymphomas and 75% for DLBCL.
