Primary thyroid lymphoma can be defined as a lymphoma that arises from the thyroid gland. This definition excludes those that invade the thyroid gland as a consequence of either metastasis or direct extension. Primary thyroid lymphomas are practically always non-Hodgkin lymphomas (NHLs). Primary thyroid Hodgkin lymphoma is extremely rare.
NHLs can be divided into aggressive and indolent cell types. Aggressive NHLs comprise a large number of cell types, the most common of which is large-cell lymphoma. They most frequently arise from lymph nodes, but an extranodal site is the primary source in approximately 30% of cases, and the thyroid gland is among the most common of these extranodal sites.
Thyroid NHL represents approximately 1.2 to 1.7% of all NHLs.  It is highly curable, without the need for extensive surgery. Accordingly, early recognition and correct treatment of this condition is vital. The most common cell type is diffuse large-cell lymphoma, either associated or unassociated with mucosa-associated lymphoid tissue (MALT) lymphoma (MALToma).The best treatment results for primary thyroid large-cell lymphoma are with combined-modality therapy; for primary thyroid MALToma, radiation therapy alone is probably adequate. [2, 3] Primary T-cell lymphoma of thyroid is extremely rare, accounting for less than 2% of all primary thyroid lymphomas, and can present both diagnostic and therapeutic challenges. 
Thyroid lymphomas are very frequently associated with Hashimoto thyroiditis (HT). Conversely, there is a markedly increased incidence of primary thyroid lymphomas in patients with HT.  Such lesions are an aggressive or high-grade variant, whereas a low-grade MALT is an indolent type.
Like other low-grade MALTomas, such as those presenting in the parotid in association with Sjögren syndrome, those arising in the thyroid also occur in association with an autoimmune disorder (in this case, HT). The hypothesis is that chronic antigenic stimulation secondary to the autoimmune disorder leads to chronic proliferation of lymphoid tissue, which eventually undergoes a mutation that leads to the development of lymphoma.
Epidemiology and Prognosis
Primary thyroid lymphoma is rare, constituting only 1-2% of all extranodal lymphomas and approximately 2-8% of all thyroid malignancies. [6, 5, 7, 8] As with other non-Hodgkin lymphomas, the median age of presentation in patients with thyroid lymphoma is usually close to 60 years. [8, 7] Most cases occur in women.
The prognosis for patients with thyroid large-cell lymphoma usually is favorable because they typically present with localized disease, which is amenable to treatment with chemotherapy and radiation. The cure rate is typically high (see Treatment). A large population-based study that evaluated prognostic factors found that older age, advanced stage, histologic subtype, and lack of radiation/surgical treatment were adverse factors for survival. 
A rare case of pulmonary metastasis has been reported in a 65-year-old woman diagnosed with a primary thyroid MALToma who underwent total thyroidectomy, followed by chemotherapy. After 5 years of follow-up, she was diagnosed with metastatic thyroid MALT lymphoma. 
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