Primary thymic extranodal marginal zone B-cell lymphoma (MZBL) of mucosa-associated lymphoid tissue (MALT) is a lymphoma consisting predominantly of small B cells with a centrocyte-like or monocytoid appearance, which surround reactive follicles and infiltrate the thymic epithelium to produce lymphoepithelial lesions. This lymphoma is derived from post-germinal center marginal zone B cells.¹

Extranodal MZBL of MALT arising in the thymus is a rare disorder with an apparent predilection for Asians and a strong association with autoimmune diseases, among which the strongest link appears to be with Sjögren syndrome. Most patients are in the fifth and sixth decades. There is female predominance (M:F = 1:4). The prevalence of autoimmunity is much lower in males with thymic MALT lymphoma compared to females (33% vs. 87%).² It is characterized by the frequent presence of thymic cysts, consistent plasma cell differentiation, tumor cells expressing IgA, and a lack of API2-MALT1 gene fusion.^{2,3,4,5,6,7,8,9,10,11,12,13} There is no association with Epstein-Barr virus.⁴ The bulk of the disease is in the anterior mediastinum, but the regional lymph nodes and other extranodal sites (e.g., stomach, salivary gland, lung) may be involved concurrently.

Grossly, the tumor is often encapsulated and comprises solid grayishwhite fleshy tissue commonly interspersed with multiple variable-sized cysts. Invasion into the adjacent pericardium and pleura is sometimes found.