Lymphoma is a disease in which lymphocytes (i.e. blood cells that fight infection) turn into cancer cells. In thyroid lymphoma, the lymphocytes of the thyroid turn into cancer cells. Thyroid lymphoma is a very rare disease that accounts for 1 to 2% of all thyroid cancers and 1 to 2% of all lymphomas outside the lymph nodes. Most patients are older with an average age in the late 60's. If properly diagnosed and treated, the prognosis is very good.