Lymphoma, though primarily a neoplasm of leukocytes, can involve extranodal, nonlymphoid tissue including the thyroid. When the thyroid is involved by widely disseminated lymphoma, the surgeon is rarely involved. This chapter, therefore, focuses on primary thyroid lymphoma (PTL) confined to the thyroid and its regional lymph nodes. Although PTL occurs rarely, familiarity with this disease will benefit the surgeon who may encounter it in the course of the workup of a thyroid mass and may be called upon to aid in its diagnosis and management.
EPIDEMIOLOGY AND RISK FACTORS
PTL accounts for 1% to 2% of all extranodal lymphomas and approximately 1% to 5% of thyroid malignancies.1,2 It is more common among women, with a 3–4:1 female:male incidence, and it affects individuals between the ages of 55 and 75 years with a peak incidence in the late sixties.3-10
The primary risk factor for the development of PTL is a chronic lymphocytic infiltrate. Accordingly, Hashimoto’s thyroiditis (chronic lymphocytic thyroiditis) is strongly associated with PTL, and 20% to 60% of patients diagnosed have preexisting Hashimoto’s thyroiditis.4,6 One group examined clonal relationships between PTL and surrounding Hashimoto’s thyroiditis. Using the polymerase chain reaction, they demonstrated substantial genetic homology between PTL cells and the cells from the surrounding thyroiditis, suggesting that cells in the PTL are genetic descendants of the surrounding thyroiditis cells. These results give evidence that PTL evolves from thyroiditis.11 However, only a small percentage of the vast numbers of patients with Hashimoto’s thyroiditis ever develop PTL.
MOLECULAR BIOLOGY AND PATHOLOGY
Lymphomas are characterized broadly as Hodgkin’s lymphoma or non-Hodgkin’s lymphoma (NHL); NHL comprises the vast majority (98%) of PTL.3 These lymphomas are further classified into subtypes based on the cell of origin (B cell, T cell, or natural killer cell) and the level of differentiation according to the 2008 WHO guidelines for classification of lymphoid tumors.12 Although PTL of T-cell origin does occur, almost all lymphomas of the thyroid have B-cell origins.13 Three subtypes of B-cell NHL comprise 90% of the cases of PTL: diffuse large B-cell lymphoma (DLBCL), mucosa-associated lymphoid tissue (MALT) lymphoma, and follicular lymphoma.3
Diffuse large B-cell lymphoma is the most common subtype of PTL, comprising approximately two-thirds of the cases. DLBCL tends to be clinically aggressive and is associated with a worse prognosis when compared to other PTL types. DLBCL can coexist with other types of thyroid lymphomas such as MALT in approximately 40% of cases, indicating that some DLBCLs may arise from MALT lymphomas. The pathogenesis of DLBCL likely lies in disorders in the process of somatic hypermutation in which the gene encoding the variable region of antibodies undergoes a high rate of mutation in order to adapt to new antigens.14
The second most common type of PTL is MALT ...