Primary rectal mucosa-associated lymphoid tissue lymphoma masquerading as proctitis.

A 29-year-old male presented with recurrent mucous bloody stools for more than a year. Colonoscopy revealed ill-defined, mildly congested and edematous mucosa with scattered erosion spots in the lower rectum, highly suspicious for proctitis. Histopathology showed diffuse infiltration of small to medium-sized lymphoid cells in the lamina propria. Immunohistochemistry indicated these cells were positive for CD20, CD79a, CD19, kappa and lambda light chains (partial), and negative for CD3, CD5, CD10, cyclin D and BCL-6. These results were consistent with mucosa-associated lymphoid tissue (MALT) lymphoma. Further investigations consisting of upper endoscopy, bone marrow biopsy, and whole-body PET/CT scan did not detect any extrarectal lesions. Based on these findings, the diagnosis of stage I primary rectal MALT lymphoma was made. The patient underwent 15 fractions of radiotherapy with a total dose of 30 Gy. His symptoms were alleviated following the treatment. A follow-up colonoscopy performed 3 months later showed complete resolution of the lesion.