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Grossly visible features of gastrointestinal histoplasmosis include mucosal ulceration, nodules,
and hemorrhage. Obstructing masses are not common. The mucosa can appear normal even
when involved. Microscopic findings include diffuse lymphohistiocytic infiltration and
ulceration. Often a minimal inflammatory reaction is present. Well-formed granulomas are not
common.[669] Radiographic features of colonic histoplasmosis include segmental colonic
inflammation, apple-core lesions that may mimic primary adenocarcinomas, and stricture.[416]

MALIGNANT LYMPHOMAS AND LYMPHOID LESIONS.-- The GI tract is one of
the most common sites of involvement by non-Hodgkin lymphomas (NHL) in patients with
AIDS. These lymphomas occur most frequently in the stomach, small intestine, and colon
(Table 5). Unlike Kaposi's sarcoma, gastrointestinal lymphomas may be symptomatic from
complications of obstruction, perforation, or bleeding.[667] The high grade NHL’s seen in the
anorectal region, particularly when the risk factor for HIV infection is sex with other males, are
typically associated with Epstein-Barr virus (EBV) infection, which promotes clonal
proliferation of lymphoid cells.[670]
They usually appear grossly as irregular areas of nodularity of the mucosa that on
sectioning have a firm white appearance extending into the submucosa. Superficial ulceration
can occur. Large masses that can obstruct the lumen of small intestine or colon are not common.
One pattern of abdominal involvement with NHL in AIDS is marked omental and/or mesenteric
infiltration, often with a malignant effusion. Rarely, NHL may appear only as malignant cells in
an effusion, without a definable mass lesion (a primary body cavity-based lymphoma).
Radiographically, gastric NHL features include circumferential or focal thickening of the
gastric wall and mural masses with or without ulceration. In the small intestine, features include
diffuse or focal bowel wall thickening and excavated masses.[416]
Microscopically, the bulk of most lymphomatous infiltrates are submucosal, but small
infiltrates of neoplastic lymphocytes may extend into the lamina propria or mucosa, making
endoscopic biopsy diagnosis possible. The appearance of a monomorphous population of large
cells, aided by identification of monoclonality by immunohistochemical staining, helps to
distinguish malignant lymphomas from chronic inflammatory infiltrates.[561,569] The most
common types are diffuse large cell and immunoblastic, with a smaller number of small non-
cleaved lymphomas.
Most AIDS patients with gastrointestinal lymphomas will respond to chemotherapy, but
toxicity is common and requires dose reduction. Intestinal perforation may occur. Median
survival is only about six months, even with treatment.[671]
Other common lymphoid lesions of the GI tract include chronic non-specific colitis with
or without an identifiable infectious agent. These lesions are characterized by diffuse or focal
mucosal and submucosal collections of small lymphocytes, with minimal or no accompanying
necrosis. More pronounced lymphoid collections characterized as nodular lymphoid hyperplasia
may be related to persistent generalized lymphadenopathy (PGL) in persons at this stage of HIV
infection. Mucosa-associated lymphoid tissue (MALT) lesions are lymphoid proliferations that
may occur at extranodal sites such as the gastrointestinal tract.[573,574]

KAPOSI'S SARCOMA.-- The gastrointestinal tract is the second most common site for
KS following skin, and the commonest visceral site, for KS in AIDS patients. In most cases, the
GI tract is involved in addition to the skin and/or lymph nodes. Lesions may occur anywhere
from the oral cavity to the anus. Grossly, the lesions are raised dark red nodules averaging 0.5 to