What is Thyroid Lymphoma and How Common is it?

Thyroid lymphoma is a rare type of cancer that affects the lymphatic system, accounting for 1 to 2% of all thyroid cancers and 1 to 2% of all lymphomas outside the lymph nodes. The majority of patients are older, with an average age of 60 years. If diagnosed and treated properly, the prognosis is very good. Lymphoma usually occurs within the lymph nodes, but in rare cases, it arises from lymphocytes present in the thyroid gland.

This is known as primary thyroid lymphoma, to distinguish it from lymphomatous involvement of the thyroid gland due to metastasis or direct extension. Primary lymphomas of the thyroid gland account for approximately 2-5% of all thyroid malignancies. Most thyroid lymphomas are non-Hodgkin B-cell tumors, with malignant lymphoma of the mucosa-associated lymphoid tissue (MALT) being the next most common histological type. Hodgkin lymphoma, Burkitt cell lymphoma, and T-cell lymphoma have also been reported.

The incidence and prevalence of thyroid lymphoma are higher in areas with higher rates of thyroiditis. Although primary thyroid lymphoma is a rare cause of both thyroid and extranodal lymphoma, knowledge of this disease is important for early diagnosis and treatment implementation. Traditionally, open surgical biopsy was considered necessary to differentiate thyroid lymphoma from thyroiditis and anaplastic carcinoma. In rare cases, Hashimoto's thyroiditis can progress to MALT lymphoma, and as a result, patients with Hashimoto's thyroiditis have a higher risk of developing PTL than the general population.

BAAF may be useful in diagnosing thyroid lymphoma, but it is considered less reliable with thyroid lymphoma than with other thyroid malignancies. Patients who receive radiation for thyroid lymphoma are at high risk of developing hypothyroidism and need to have their thyroid function blood tests measured regularly. The risk of primary thyroid lymphoma increases 70 times in patients with chronic lymphocytic thyroiditis compared to the general population. AAF may have a limited ability to differentiate between anaplastic thyroid carcinoma and thyroid lymphoma.