Mucosa-associated Lymphoid Tissue Lymphoma of the Thymus: A Stuty of 2 Cases

  Objective  To study the clinicopathologic features and diagnosis of mucosa-associated lymphoid tissue (MALT) lymphoma of the thymus.

  Methods  The clinical, pathologic, immunohistochemical and polymerase chain reaction (PCR) findings of two patients (one 57-year-old Chinese Han man and one 53-year-old Chinese Han woman) with thymic MALT lymphoma were retrospectively analyzed.

  Results  Pre-operative computed tomography (CT) and magnetic resonance imaging (MRI) images and gross examination showed well-demarcated masses with multilocular cysts. Microscopy showed that the centrocyte-like lymphocytes formed broad, pale-staining bands and sheets, with frequent presence of epithelium-lined cysts and consistent plasma cell differentiation around small vessels. The dilated Hassall's corpuscle with eosinophilic material was observed. The lymphoepithelial lesions and reactive lymphoid follicle hyperplasia were common. Many small nodules, full of the lymphoma cells, located in the adipose tissue. The tumor cells were immunoreactive for CD20, CD79α, and Bcl-2. Immunostaining for AE1/AE3 highlighted the Hassall's corpuscles and epithelium lining the cysts. PCR for immunoglobulin heavy chain variable region (IgHV) rearrangements showed distinct bands indicating monoclonality in both two cases. No recurrence or metastasis was noted 27 and 6 months after surgery.

  Conclusions  Thymus MALT lymphoma is rare, and may represent a distinct subgroup of MALT lymphoma characterized by apparent predilection for Asians. It has a strong association with autoimmune disease, frequent presence of cysts, consistent plasma cell differentiation, and consistent lack of API2-MALT1 gene fusion. Surgery is the treatment of choice, usually resulting in good outcome.