Lymphoid lesions of the thyroid: review in light of the revised European-American lymphoma classification and upcoming World Health Organization classification.

Thyroid lymphomas are rare diseases and almost always rise in the background of chronic lymphocytic thyroiditis (Hashimoto's thyroiditis). Large cell lymphoma is an aggressive disease and usually is not a significant diagnostic challenge from the pathological point of view. Small cell lymphoma, however, can sometimes be difficult to distinguish from chronic lymphocytic thyroiditis. In the past, most thyroid lymphomas were considered to be of follicle center cell origin. Today, after the introduction of the mucosa-associated lymphoid tissue (MALT) and the extranodal lymphoid tissue (ELT) concepts, most of the lymphomas in the extranodal sites are thought to originate from the marginal zone of the lymphoid follicles. The distinction between the different types of lymphomas has significant impact on the patient's prognosis, treatment, and follow-up. It is imperative that clinicians (endocrinologists and surgeons) and pathologists are aware of these types of lymphomas in order for the most appropriate diagnostic procedures to be selected, specific staging principles to be applied, and appropriate disease-specific treatment to be implemented. It is also important that terms such as "lesion of uncertain malignant potential" as defined by ancillary studies be understood.