Mucosa‐associated lymphoid tissue (MALT) Iymphoma
Authors: Mori Naoywhi , Yatabs Yasushi , Asai Junpei 
Published: December 2008

Isaacson et al. defined MALT lymphoma as a neoplasm that mimics MALT lymphocytes, which are normally present in the small intestine. However, there are various problems with this definition of MALT lymphoma. First, the incidence of MALT lymphoma is not high in sites where MALT lymphocytes are normally present. Lymphoepithelial lesions are most common in the small intestine and the tonsil, but MALT lymphoma is rare in these sites. In contrast, MALT lymphoma is frequent in the tissue in which MALT lymphocytes are not normally present, such as the stomach, the breasts, the lungs, and orbital tissue. In these tissues or organs, chronic infections, such as chronic gastritis, lymphocytic mastopathy, Hashimoto's thyroiditis, and myoepithelial sialoadenitis (or Sjögren's syndrome), can be a precursor condition of lymphoma. These findings suggest that in pathological conditions MALT lymphoma arises, not from MALT tissue normally present, but from the lymphoid tissue. Also evidence is lacking that lymphoepithelial lesions imitate the normal function of MALT lymphocytes. Arber et al. Found no difference in the frequency of lymphoepithelial lesions between primary and secondary breast lymphomas, suggesting that lymphoepithelial lesions may merely reflect the aggressiveness of the tumor. We have not observed proliferating monotypic plasma cells in the dome epithelium merging with the underlying CCL cells.

Isaacson et al. reported that MALT lymphoma is characterized by a good prognosis. However, extranodal lymphomas generally have a good prognosis and it remains to be determined if MALT lymphoma has a better prognosis than other extranodal lymphomas. There are reports indicating that the stage is a more important prognostic indicator than the grade in thyroid and stomach lymphomas. Hyjek et al. identified the presence or absence of capsular invasion as the most important prognostic indicator of thyroid lymphoma. We suggest that the relative prognoses of extranodal lymphomas should be determined by comparing various types of lymphomas at the same stage. Monocytoid B cell lymphoma in the salivary gland does not have a good prognosis.

If MALT lymphoma and monocytoid B cell lymphoma are the same disease, then the prognosis of MALT lymphoma would be expected not to be very good. Despite the unresolved issues concerning Isaacson's definition of MALT lymphoma, Isaacson et al.'s characterization of MALT lymphoma should be highly evaluated in that it has disproved the theory that all B cell lymphomas are of FCC origin and suggested the existence of B cell lymphomas associated with an interfollicular pattern of spread.

In addition to the stomach, the thyroid, the lung, and orbital tissue, MALT lymphoma can occur in the kidney, the breast, and the urinary bladder. However, it is doubtful that immune mechanism of MALT exists in these tissues under normal conditions. Tissues adjacent to the epithelium are exposed to massive antigenic stimulation when the epithelium is disrupted. MALT lymphocytes and monocytoid B cells may be B lymphocytes responding to such massive antigenic exposure. Further studies are needed to clarify the nature and histogenesis of MALT lymphoma.