Extra-nodal non-Hodgkin’s lymphoma involving the gastrointestinal tract is not uncommon, accounting for 5-20% of all non-Hodgkin’s lymphoma. When they do occur, the stomach and small intestines are the commonest sites. The common non-Hodgkin’s lymphoma includes marginal zone lymphoma, small lymphocytic lymphoma, mantle cell lymphoma, diffuse large B cell lymphoma and T cell lymphoma. Common presentations of lymphoma in the gastrointestinal tract includes mucosal erosions, indurations, nodularity, ulcerations and polypoid masses. ^{1 – 3}

However, primary follicular lymphoma involving the gastrointestinal tract is infrequent. Primary follicular lymphoma involving the colon is particularly rare. ^{4 , 5}

The patient was a healthy 63-year-old Caucasian female being followed up with colonoscopy every 2 years for history of polyps and a family history of bowel cancer. A surveillance colonoscopy in May 2022 showed five polyps that were subsequently excised and sent for histopathological assessment.

Leading to the most recent colonoscopy, there were no gastrointestinal complaints, constitutional symptoms, lymphadenopathy or organomegaly. Laboratory findings include Hb 137 g/dL, normal serum ferritin level of 70, and normal blood levels of creatinine, calcium and lactate dehydrogenase (LDH). Serology indicated a non-reactive status of hepatitis B, hepatitis C and HIV. A positron emission tomography (PET) scan showed no concerning sites of fluorodeoxyglucose (FDG) activity.

Microscopically, two of these polyps, located at the hepatic flexure of the colon and descending colon, respectively, showed a polypoid lesion with normal colonic mucosa and submucosal lymphoid infiltrates showing a vague nodularity. The atypical lymphocytes had “centrocytes” features and centroblasts were rare ( Figure 1). By immunohistochemistry, the vague nodularity is accentuated by the CD20-positive B cells and CD23-stained nodular dendritic network. Neoplastic cells were positive for CD20, BCL6, CD10 (weak) and BCL2 with reactive CD3- and CD5-positive T cells ( Figure 2). The Ki-67 proliferative index was 10%. There was no large cell transformation. Fluorescence in situ hybridization (FISH) studies indicated the presence of t (14; 18) translocation. A definite diagnosis of primary follicular lymphoma (FL) involving the colon was made, as image studies did not establish any lymphadenopathy.

Subsequently, the patient was assessed in the hematology clinic where she reported a 12-pound (5.44 kg) weight loss, but denied any fever, chills, or night sweats. There was no palpable lymphadenopathy. Complete blood count and electrolyte levels were performed and were within normal limits. A computed tomography (CT) scan was performed and did not reveal any masses or metastatic disease. The patient feels well and is currently on “watch and see” with no active treatment.

Primary follicular lymphomas are relatively uncommon outside the lymph nodes, with most occurring on the skin, salivary glands, and reproductive system. Gastrointestinal follicular lymphomas are relatively rare, accounting for 1.0 to 3.6% of all lymphomas. Most of the gastro-intestinal follicular lymphomas occur in the small intestine (duodenum, jejunum or ileum) and this is thought to be related to the relative abundance of lymphoid follicles. ^{1 – 3}

FL in the colon is rare with a frequency of 1–2%. ^{4 – 6} It is slow growing and found in older adults with no gender differences in incidence.

We add to the limited literatures a case of primary follicular lymphoma involving the colon presenting as an incidental finding during endoscopy.

Usually a suspicion is made when there is an incidental finding during endoscopy, as in our index patient. The gross pathology and endoscopic appearance are not sufficient to establish a definite diagnosis, requiring histopathological and immunohistochemical examinations. ^{6 , 7} Polyposis in the gastrointestinal tract may be a reactive lymphoid hyperplasia or due to a lymphoma, like marginal zone lymphoma, mantle cell lymphoma, follicular lymphoma or large B cell lymphoma. Since treatment strategies and prognosis are different, a correct diagnosis is important. ^{7 – 9}

As follicular lymphoma arise from the germinal center of B cells, a follicular growth pattern and expression of germinal center markers like CD10 and BCL6 is observed. The positive expression of BCL2 distinguishes FL from a reactive follicular hyperplasia. Lymphoma cells in primary gastro-intestinal follicular lymphomas express a mucosal homing receptor, α4 β7 integrin. This antigen is negative in systemic follicular lymphomas. ^{10 , 11} Mantle cell lymphoma are usually CD10 ⁻ but do express CD5, cyclin D1 and sox11. Marginal zone lymphomas usually lack CD5 and CD10 expressions. Other differential diagnosis includes as adenomatous polyps, familial hereditary polyposis, Peutz–Jeghers syndrome, and B cell chronic lymphocytic leukemia. Thus, we stress the importance of a good correlation with clinical history imaging studies, genetic assessments and pathology. ¹²

Due to its indolence, it is considered more of a chronic disease and rarely progress to large B cell lymphoma. Asymptomatic patients may only require observation. ^{1 , 2}

Though rare, primary follicular lymphoma should be considered as one of the differential diagnosis of incidental colonic polyps.

The patient’s written informed consent has been obtained for publication of this case report, in accordance with this journal’s policy.