Gastric Lymphomas in

Pathology and Genetics of Tumours of the Digestive System - 2000

Definition
Primary gastric lymphomas are defined as lymphomas originating from the stomach and contiguous lymph nodes. Lymphomas at this site are considered primary if the main bulk of disease is located in the stomach. The majority of
gastric lymphomas are high-grade B-cell lymphomas, some of which have developed through progression from
low-grade lymphomas of mucosa associated lymphoid tissue (MALT). The lowgrade lesions are almost exclusively
B-cell MALT lymphomas.

Historical annotation
Classically, primary gastric lymphomas have been considered to be lymphomas that are confined to the stomach and the contiguous lymph nodes. While this excludes cases of secondary involvement of the stomach by nodaltype
lymphomas – which may occur in up to 25% of nodal lymphomas – this definition is excessively restrictive and excludes more disseminated, higher stage lymphomas arising within the stomach as well as those with bone marrow involvement. Today, stomach lymphomas are considered primary if the main bulk of disease is present in the stomach. Recognition of morphological features characteristic of primary extranodal lymphomas of mucosa- associated lymphoid tissue-type helps in defining these lesions as primary to the stomach irrespective of the degree of dissemination.

Epidemiology
Approximately 40% of all non-Hodgkin lymphomas arise at extranodal sites, with the gastrointestinal tract
as the commonest extranodal site, accounting for about 4-18% of all non-Hodgkin lymphomas in Western countries and up to 25% of cases in the Middle East. Within the gastrointestinal tract, the stomach is the most frequent site of involvement in Western countries while the small intestine is most frequently affected in Middle Eastern countries.
Lymphoma constitutes up to 10% of all gastric malignancies; its incidence appears to be increasing but this may, at least in part, be due to the recognition of the neoplastic nature of lesions previously termed ‘pseudolymphoma’ .
Gastric lymphoma has a worldwide distribution; somewhat higher incidences have been reported for some Western communities with a high prevalence of Helicobacter pylori infection. Primary Hodgkin disease is very rare in
the gastrointestinal tract.
Age and sex distribution
Incidence rates are similar in men and women. The age range is wide but the majority of patients are over 50 years at presentation.

Aetiology
Helicobacter pylori infection
Initial studies of low-grade MALT lymphoma suggested that the tumour was associated with H. pylori in 92-98% of cases; subsequent studies have suggested an association in 62-77%. H. pylori infection is seen less frequently
in high-grade lymphomas with a lowgrade component (52-71%) and in pure high-grade lymphomas (25-38%). The organism has been shown to be present in 90% of cases limited to the mucosa and submucosa, falling to 76% when deep submucosa is involved, and is present in only 48% of cases with extension beyond the submucosa. It has been shown that the infection by H. pylori precedes the development of lymphoma, both by sequential serological studies and by retrospective studies of archival gastric biopsy material. There is some controversy surrounding the role of the organism’s genetic features and the risk of lymphoma development. Studies of the association between MALT lymphoma and cagA bearing H. pylori strains have produced conflicting results, ranging from a lack of association between cagA and lymphoma to a strong association. One study claimed no association with low-grade lymphoma but a high frequency of cagA strains in high-grade lesions. Recently, a truncated form of an H. pylori associated protein, fldA, has been shown to be closely associated with gastric MALT lymphoma. All strains of H. pylori   associated with MALT lymphoma showed a nucleotide G insertion at position 481 of the fldA gene, compared
to 6/17 stains unassociated with lymphoma. This mutation causes a short truncation in the protein and antibodies
to this truncated protein could be detected in 70% of the patients studied with MALT lymphoma, compared to 17% of control patients .
Immunosuppression
Lymphomas may arise or involve the stomach in patients with both congenital and acquired immunodeficiencies. In
general, the incidence, clinical features and the histology of the lesions is indistinguishable from those that develop outside the stomach. Up to 23% of gastrointestinal tract non-Hodgkin lymphomas arising in HIV infected patients occur in the stomach and the vast majority of these are large B-cell or Burkitt/Burkittlike lymphomas,  although occasional low-grade MALT lymphomas are described.

Clinical features
Symptoms and signs
Patients with low-grade lymphomas often present with a long history of non-specific symptoms, including dyspepsia, nausea and vomiting. High-grade lesions may appear as a palpable mass in the epigastrium and can cause severe symptoms, including weight loss.
Imaging
Low-grade MALT lymphomas present as intragastric nodularity with preferential location in the antrum. A more precise assessment is obtained with spiral CT, particularly if this is used in conjunction with distension of the stomach
by water. This technique can identify up to 88% of cases, most of which have nodularity or enlarged rugal folds, and it can assess the submucosal extent of the tumour . High-grade lymphomas are usually larger and more frequently associated with the presence of a mass and with ulceration. In some cases, the radiological features may mimic diffuse adenocarcinoma. Endoscopic ultrasound is emerging as the investigation of choice in the assessment of the
extent of lymphoma infiltration through the gastric wall. Local lymph node involvement can also be assessed by
this technique.
Endoscopy
Some cases show enlarged gastric folds, gastritis, superficial erosions or ulceration  In these cases the surrounding normal appearing gastric mucosa may harbor lymphoma, and accurate mapping of the lesion requires multiple biopsies from all sites including areas appearing macroscopically normal. In a proportion of cases, endoscopic examination shows very minor changes such as hyperaemia and in a few cases random biopsies of apparently entirely normal mucosa may reveal lymphoma. High-grade lymphoma is usually associated with more florid lesions, ulcers and masses. It is often impossible to distinguish lymphoma from carcinoma endoscopically.

MALT lymphomas
Pathogenesis
The normal gastric mucosa contains scattered lymphocytes and plasma cells but is devoid of organised lymphoid tissue. The initial step in the development of primary gastric lymphoma is the acquisition of organised lymphoid tissue from within which the lymphoma can develop. In most cases, this is associated with infection by H. pylori, although it
has also been seen following infection by Helicobacter heilmannii  and in association with coeliac disease. This organised lymphoid tissue shows all the features of MALT, including the infiltration of the epithelium by B-lymphocytes reminiscent of the lymphoepithelium seen in Peyer patches. The cellular basis of the interaction between H. pylori and MALT lymphoma cells has been studied in detail. When unseparated cells isolated from lowgrade gastric MALT lymphomas are incubated in vitro with heat treated whole cell preparations from H. pylori, the tumour cells proliferate while those cultured in the absence of the organism or stimulating chemical mitogen rapidly die. The proliferative response appeared to be strain specific for individual tumours but varied between tumours from different patients. When T-cells were removed from the culture system the proliferative response was not seen and this could not be induced if the T-cells were replaced by supernatant from other cultures containing unseparated tumour derived cells. Together these studies show that the proliferation of the MALT lymphoma is driven by the  presence of the H. pylori but that this, rather than being a direct effect on the tumour cells, is due to a mechanism mediated via T-cells and that this help is contact dependant. Further studies have shown that the T-cells responsible for the proliferative drive are specifically those found within the tumour and their function cannot be replaced by T-cells derived from elsewhere (e.g. the spleen) in the same patient .
Histopathology
The organization of the lymphoma mimics that of normal MALT and the cellular morphology and immunophenotype is essentially that of the marginal zone B-cell. The neoplastic cells infiltrate between pre-existing lymphoid follicles, initially localized outside the follicular mantle zone in a marginal zone pattern. As the lesion progresses, the neoplastic cells erode, colonize and eventually overrun the lymphoid follicles resulting in a vague nodularity to an otherwise diffuse lymphomatous infiltrate. The morphology of the neoplastic cell can be variable even within a single case. Characteristically, the cell is of intermediate size with pale cytoplasm and an irregular nucleus. The resemblance of these cells to the centrocyte of the follicle centre has led to the term ‘centrocyte-like (CCL)’ cell being applied to the neoplastic component of MALT lymphomas. In some cases, the CCL cell may be more reminiscent of a mature small B lymphocyte while in other cases, the cell may have a monocytoid appearance with more abundant, pale cytoplasm and a well defined cell border. Plasma cell differentiation is typical and may be very prominent. Dutcher bodies may be identified. The CCL cells infiltrate and destroy adjacent gastric glands to form lymphoepithelial lesions. Lympho-epithelial lesions typical for MALT lymphoma are defined as infiltration of the glandular epithelium by clusters of neoplastic lymphoid cells with associated destruction of gland architecture and morphological changes within the epithelial cells, including increased eosinophilia.
Immunohistochemistry
The immunophenotype of the CCL cell is similar to that of the marginal zone B-cell. There is expression of pan-B-cell antigens such as CD20 and CD79a and the more mature B-cell markers CD21 and CD35. The cells do not express CD10. They are usually positive for bcl-2 protein and may express CD43 but do not express CD5 or CD23. They express surface and, to a lesser extent, cytoplasmic immunoglobulin (usually IgM or IgA, rarely IgG) and show light chain restriction. Immunostaining with anti-cytokeratin antibodies is useful in demonstrating lymphoepithelial lesions. Immunostaining with antibodies that highlight follicular dendritic cells (anti-CD21, anti-CD23 or anti-CD35) help to demonstrate underlying follicular dendritic cell networks in those cases in which the lymphoid follicles have been completely overrun by the lymphoma.
Differential diagnosis
The distinction between florid gastritis and low-grade MALT lymphoma may be difficult. In such cases it is essential to have sufficient biopsy material (up to eight biopsies from endoscopically suspicious areas) with good preservation of morphology and correct orientation of the biopsy specimen. For the distinction between reactive and neoplastic  infiltrates, histological evaluation remains the gold standard, but accessory studies may be helpful. In both reactive and neoplastic cases, lymphoid follicles are present and these may be associated with active inflammation, crypt abscesses and reactive epithelial changes. In gastritis, the infiltrate surrounding the lymphoid follicles in the lamina propria is plasma cell predominant while in MALT lymphoma the infiltrate contains a dominant population of lymphocytes with CCL cell morphology, infiltrating through the lamina propria and around glands. Prominent lymphoepithelial lesions, Dutcher bodies and moderate cytological atypia are associated only with lymphoma.
All of these features may not be present in biopsy material from a single case. In some cases it is justifiable to make the diagnosis of low-grade MALT lymphoma in the absence of one or more of these features if the overall histological appearances are those of lymphoma. Rare or questionable lymphoepithelial  lesions, dense lymphoid infiltration, mild cytological atypia and muscularis mucosae invasion are features more often associated with, but not limited to, lymphoma . In some cases it will not be possible to make a definite distinction between reactive infiltrates and lymphoma and in these cases a diagnosis of ‘atypical lymphoid infiltrate of uncertain nature’ is appropriate.
Effect of H. pylori eradication
The histological appearances of gastric biopsies from patients showing complete regression of lymphoma after H. pylori The lamina propria appears ‘empty’ with gland loss. Scattered lymphocytes and plasma cells are seen within the lamina propria and there are usually focal nodular collections of small lymphocytes. These collections frequently contain a mixture of B- and T-cells and may be based on follicular dendritic cell networks. In most cases, the appearances are insufficient for a diagnosis of residual lymphoma. The significance of these lymphoid nodules remains uncertain. In cases showing partial regression or no change following H. pylori eradication, the lamina propria contains an infiltrate morphologically indistinguishable from that seen at diagnosis, but in these treated cases lymphoepithelial lesions may be very scanty or absent. In some cases of partial regression and in cases with relapsed low-grade MALT lymphoma following H. pylori eradication, the lymphoma may be largely confined to the submucosal with only minimal involvement of the mucosa.
PCR based diagnosis
The role of genetic analyses in the diagnosis and follow up of low-grade MALT lymphoma remains controversial. Up to 10% of well characterized cases of MALT lymphoma identified as clonal through demonstration of rearrangement of the immunoglobulin heavy chain gene by Southern blot fail to show a clonal pattern when examined for immunoglobulin heavy chain gene rearrangement by PCR using fresh frozen tissue {418}. This false negative rate increases if paraffin embedded material is studied. Several studies have revealed by PCR the presence of clonal B-cell populations in biopsies from patients with uncomplicated chronic gastritis and no morphological evidence of lymphoma. In conjunction with histological assessment, PCR studies may be useful in monitoring regression of MALT lymphomas following conservative therapy. However, PCR detected clonal B-cell populations may still be detected in cases showing complete histological regression. Some, but no all of these will eventually show molecular regression but there may be a prolonged time lag between histological and molecular regression. In the absence of histological evidence of residual lymphoma, the clinical significance of a persistent clonal population remains uncertain.
Progression to high-grade lymphoma
The emergence of clusters of large transformed ‘blastic’ B-cells reflects transformation to high-grade lymphoma.
Eventually, these areas become confluent to form sheets of cells indistinguishable from the cells of a diffuse large B-cell lymphoma. As long as a low-grade component remains, these tumours may be termed high-grade MALT lymphomas but during further progression, all traces of the pre-existing low-grade lymphoma are lost, making it impossible to distinguish the lesion from a diffuse large B-cell lymphoma of unspecified type. In cases with both low- and high-grade components, genetic studies have confirmed the transformation of low-grade to high-grade lymphoma in the majority of cases while in other cases both components appear clonally unrelated, suggesting the development of a second primary lymphoma.
Molecular genetics of MALT lymphomas
Early studies confirmed the presence of immunoglobulin gene rearrangement in each case and suggested that there was no involvement of the bcl-1 or bcl-2 oncogenes. The translocation t(11;18)(q21;q21) has been identified in a significant number of low-grade MALT lymphomas and may be the sole genetic alteration in these cases. However,
this translocation appears to be less common in high-grade lesions. Trisomy 3 has been detected in up to 60% of cases in some studies using both metaphase and interphase techniques , but this finding has not been confirmed by other studies. The translocation t(1;14) (p22; q32) has also been described in a small proportion of cases and this is associated with increased survival of tumour cells in unstimulated cell culture. Cloning of the breakpoint involved in this translocation has led to the discovery of a novel gene, bcl-10, on chromosome 1 that may be significant in determining the behaviour of MALT lymphomas. Studies of the immunoglobulin gene of MALT lymphoma cells has shown the sequential accumulation of somatic mutations, consistent with an ongoing, antigen driven selection and proliferation. Study of the third complementary determining region of the immunoglobulin heavy chain gene shows
a pattern of changes associated with the generation of antibody diversity and increased antigen binding affinity.
Transformation of low-grade MALT lymphoma to a high-grade lesion has been associated with several genetic alterations. While the t(11;18) chromosomal translocation is not seen in high-grade MALT lymphoma and may be protective against transformation, alterations in the genes coding for p53, p16, c-myc and trisomy 12 have all been identified in high-grade lesions. Bcl-6 protein has also been described in high-grade lymphomas while being absent from lowgrade lesions. Some studies have shown a high level of bcl-6 gene hypermutations in diffuse large B-cell lymphomas independent of a rearrangement of the gene. Epstein-Barr virus is not associated with low-grade lymphomas and has only been seen in some high-grade lymphomas

Mantle cell lymphoma
Mantle cell lymphoma of the stomach is typically a component of multiple lymphomatous polyposis of the  gastrointestinal tract and infrequently encountered outside this clinical context. Morphologically and immunophenotypically, the lymphoma is indistinguishable from mantle cell lymphomas of lymph nodes, with a diffuse and monotonous infiltrate of cells with scanty cytoplasm and irregular nuclei that express B-cell markers together with CD5 and cyclinD1.
Other low-grade B-cell lymphomas
Although the lymphoid tissue in the stomach contains all the B-cell populations encountered in nodal lymphoid tissue,
other low-grade B-cell lymphomas, such as follicle centre cell lymphomas, are very rare and usually indistinguishable
from their nodal counterparts.
Diffuse large B-cell lymphoma
These lymphomas are morphologically indistinguishable from diffuse large B-cell lymphomas that arise within lymph
nodes. There is complete destruction of the gastric glandular architecture by large cells with vesicular nuclei and
prominent nucleoli. Variants of large Bcell lymphoma (e.g. plasmablastic lymphoma) may also be encountered.
Burkitt lymphoma
Although rare, classical Burkitt lymphomas may be encountered in the stomach. The morphology is identical
to that of Burkitt lymphoma encountered elsewhere, with diffuse sheets of medium sized cells with scanty cytoplasm
and round/oval nuclei containing small nucleoli. Within the sheets there are numerous macrophages, giving a ‘starrysky’ appearance. Mitoses are frequent and apoptotic debris abundant. The cells express CD10 in addition to
pan-B-cell markers. Close to 100% of nuclei are immunoreactive for Ki-67.
T-cell lymphoma
Primary gastric T-cell lymphomas are rare. Most have been reported from areas of endemic HTLV-1 infection and
probably represent gastric manifestations of adult T-cell leukemia/lymphoma (ATLL). In these regions, T-cell lymphoma may represent up to 7% of gastric lymphomas. Most of the remainder are similar to peripheral T-cell lymphomas encountered in lymph nodes but occasionally, gastric NK cell lymphomas are also seen. It has recently
been demonstrated that some gastric T-cell lymphoma as display features of intraepithelial T lymphocyte differentiation (e.g. expression of the human mucosal lymphocyte 1 antigen, CD103), similar to those seen in intestinal T-cell lymphomas.
Hodgkin disease
Hodgkin disease may involve the gastrointestinal tract but this is usually secondary to nodal disease. Primary gastric
Hodgkin disease is very rare.

Prognosis and predictive factors
Studies on the regression of low-grade MALT lymphoma through H. pylori eradication have shown remission in 67-84% of cases, but this applies only to low-grade lesions and is most effective for lesions showing superficial
involvement of the gastric wall. Although remission following H. pylori eradication has occasionally been seen
in advanced tumours, the highest success rate of 90-100% is seen in tumours confined to the mucosa and superficial
submucosa. The time taken to achieve remission in these patients varies from 4-6 weeks to 18 months. The stability of these remissions remains to be determined; one study has reported a relapse in 10% of patients after a mean follow-up period of 24 months while others have found sustained remissions for up to six years.
Surgical resection is associated with prolonged survival in many cases. Involvement of the resection margins and
advanced stage are poor prognostic features, but not with the addition of chemotherapy. Irrespective of treatment modality, the only significant independent prognostic variables are stage and tumour