*5.2.1. Marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT)*

The great majority of gastric malignant lymphoma in Western countries belongs to the mucosa-associated lymphoid tissue (MALT) subtype. About 70–80% of MALT-lymphomas are associated with a chronic helicobacter pylori (Hp) infection. The Hp-infection is one of the main drivers of this type of lymphoma; eradication of Hp is the first choice of treatment and induces a regression of the MALT-lymphoma in about 75% of cases. Hp-negative MALTlymphoma can be associated with some other infections (like Hepatitis C) or are related to immunosuppression (due to AIDS or post-transplant) or some autoimmune diseases. Prognosis is mainly related to stage (Ann Arbor staging). Gastric MALT lymphoma occurs frequently multifocal. It is noteworthy that some gastric MALT lymphoma can affect other MALT-bearing organs like gut, salivary glands and bronchial [2, 33].

Endoscopically, (MALT)-lymphoma imitates carcinoma (including mucosa-ulceration) and is usually located in distal parts of the stomach. Sometimes non-characteristic gastritis-like or nodular appearance dominates.

Histologically, MALT-lymphoma shows the characteristics of other marginal zone lymphomas like dense infiltrations of small to intermediate-sized more or less monomorphic lymphoid cells with clear cytoplasm. Some tumours show a striking plasmacytoid-like differentiation. Lympho-epithelial lesions (destruction of epithelial components of the mucosa) are highly characteristic for this type of lymphoma. Scattered blasts are typical [34, 35].

Immunohistochemically, MALT-lymphomas are positive for CD20 and half each for CD43. Negative for CD10, cyclin D1, CD5, CD23.

#### *5.2.1.1. Clinical significance*

Hp-eradication is the first choice of treatment (independent of Hp status at the surrounding mucosa). But tumours with nuclear BCL10 expression and positive translocation t (11;18) (q21;q21)) fail to response to Hp-eradication. This subtype is associated with a low risk of progression into an aggressive B-cell-lymphoma [36–38].

All other B- and T-cell-lymphomas and some other rare differential diagnosis can primary occur in the stomach, but are frequently an expression of a secondary infiltration (compare Sections 5.2.2–5.2.4) [39].
