Amyloidosis of the rectum mimicking collagenous colitis.

Gastrointestinal involvement in cases of systemic amyloidosis is very common. In the colorectal mucosa, amyloid deposition is ordinarily seen around vessels or diffusely in the lamina propria. We report two cases in men aged 69 and 29 years, whose rectal biopsies revealed prominent subepithelial amyloid deposits mimicking collagenous colitis. The amyloid deposits were composed of AA protein. A review of the literature has yielded only one previously reported case. Pathologists should be aware of this deposition pattern of amyloid to prevent misdiagnosis. Our cases underline the importance of the routine use of histochemical stains for amyloid in all cases of colorectal biopsies showing histologic changes suggestive of collagenous colitis.