Ameboma mimicking metastatic colon carcinoma

Clinical History

A pyrexic 62-year-old male patient presented to the emergency department with blood in stools for the past 7 days. He also reported a history of altered bowel habits and weight loss for the past 2 months.

Imaging Findings

Contrast enhanced CT of abdomen revealed marked annular tumoural thickening of the ascending colon with resultant luminal narrowing. The caecum also displayed mild circumferential wall thickening, however, the ileocaecal valve and terminal ileum were spared. Remaining colon was unremarkable. Associated multiple hypo-attenuating nodular lesions of varying sizes were seen in the liver. No significant loco-regional lymphadenopathy was seen.

The patient was taken up for surgery with an interim diagnosis of carcinoma of the colon. Per-operatively a mass was seen in the proximal-most ascending colon with multiple deep ulcers and skip areas of colonic wall thickening involving the remaining right colon. Histolopathological examination of the right hemicolectomy specimen confirmed a colonic ameboma with patchy areas of amoebic colitis and ulcers elsewhere. Trophozoites of entamoeba histolytica were seen in large numbers.

Discussion

Amoebiasis (caused by protozoan Entamoeba histolytica) is a health hazard worldwide; yet developing nations have significantly higher prevalence rates because of poorer socioeconomic conditions and sanitation levels. A rising incidence is also being reported from the developed nations due to increased travel to endemic areas, emigration of people from affected parts and increase in the prevalence of HIV-AIDS patients.

Amoeboma is a rare presentation of amoebic colitis, characterised by the formation of annular granulation tissue mimicking a colonic carcinoma. According to an estimate, only about 1.5% of the patients of all the cases with amoebiasis develop amoeboma. Amoeboma (also called as amoebic granuloma) implies a hyperplastic response of the colon to the protozoan resulting in a nodular or concentric tumour-like proliferative inflammation of the colon. It usually affects the caecum and/or the ascending colon and rarely the sigmoid, transverse colon and rectum; which are the essential sites of stasis giving the organism an opportunity to burry deep within the colonic wall. Amoeboma due to its morphology is often misdiagnosed as colonic carcinoma, especially in the elderly population. Associated amoebic liver abscess can mimic metastatic disease. Even on endoscopy the lesion has a confounding picture presenting as a growth in the colon. Hence, epidemiologic information, clinical details, stool tests for trophozoites and positive amoebic serology titres, are of utmost importance for arriving at an appropriate diagnosis.

To conclude, although the elderly population has a higher incidence of colonic malignancy, amoeboma should be considered in the differential diagnosis of colonic masses, especially when the patient hails from endemic areas.

Differential Diagnosis List

Final Diagnosis

Colonic amoeboma with amoebic liver abscess

URL:	https://www.eurorad.org/case/9439
DOI:	10.1594/EURORAD/CASE.9439
ISSN:	1563-4086