Rectal ameboma: A new entity in the differential diagnosis of rectal cancer

Abstract

We examined the case report written by Ke et al, describing a rare clinical case. In this editorial, we would like to emphasize the differential diagnosis of rectal masses through a rare case. We describe a case of ameboma, which manifested itself as a mass in the rectum in terms of imaging and rectoscopic features, in an immunocompetent patient who had complaints of constipation and rectal bleeding for weeks. The initial diagnosis suggested malignancy due to imaging and rectoscopic features, but the pathology report reported it as amoebiasis. After ten days of metronidazole and oral amebicide (diloxanide furoate) treatment, the patient’s symptoms and radiological findings were successfully regressed.

Key Words: Ameboma; Amoebic colitis; Rectal mass; Immunocompetent patient; Imaging findings

Core Tip: Entamoeba histolytica is commonly observed in impoverished regions and can induce a mass-like manifestation known as ameboma. In populations with a low incidence of amoebiasis, there can be clinical and radiological misinterpretation between ameboma and malignancy. Ameboma frequently affects the cecum, ascending colon, and rectum. Differentiating between ameboma and colon malignancy prevents exposing the patient to unnecessary medical and surgical interventions.

INTRODUCTION

The protozoan Entamoeba histolytica (E. histolytica) is the cause of the infectious illness amoebiasis. Amoebiasis, it is an important problem for those living in regions where the epidemic is high, for those who travel with immunosuppression, and for patients with acquired immunodeficiency syndrome or organ transplants in developed countries. The clinical manifestations of colorectal amoebiasis can range from an asymptomatic carrier to severe fulminant necrotizing colitis that has perforations and bleeding. Although uncommon, infection may progress to aggressive colitis and ameboma development, which bears a striking resemblance to colorectal malignancy (Table 1). Differentiating malignancy from ameboma is crucial for determining the appropriate treatment for the patient and avoiding unnecessary surgical procedures[1]. The imaging features of amoebic colitis and ameboma have been described in several authoritative sources. According to Tanaka et al[2], the spectrum of computed tomography (CT) findings in amoebic colitis includes wall thickening, mucosal enhancement, and mesenteric fat stranding, which can be pivotal in distinguishing it from other gastrointestinal diseases[3]. Additionally, the review by Morán et al[1] emphasizes that amoebic abscesses in the colon can mimic carcinoma, highlighting the importance of integrating clinical and pathological findings for accurate diagnosis.

Table 1 Clinical and imaging findings of amoebic colitis.

	Ameobic colitis
Clinical features	Presents with abdominal pain and dysentery. Spectrum of colonic manifestations include: (1) Acute protocolitis (dysentery); (2) Fulminant colitis leading to colonic wall perforation; (3) Toxic megacolon; (4) Chronic colitis (non-dysenteric); and (5) Ameboma
Imaging findings	Ulceration; most commonly affects cecum and rectum; ileum is spread helps to differentiate it from Crohn or tuberculosis

The ameboma formation is typically linked to undiagnosed or inadequately treated colonic amoebiasis. After amoebiasis, there is a 1% to 2% chance of developing an ameboma[4,5]. Ameboma is characterized as a thickening of the colon tissue that is proliferative and fibrotic, resulting from the invasion of amoebae and synergistic bacteria. It can affect any part of the colon. The cecum is the most often affected area by ameboma with a rate of about 40%. The rectosigmoid region is the second most prevalent site. Ameboma in the rectum is uncommon, nonetheless[4-7]. Clinically, symptoms of right lower quadrant pain, distension, or intestinal obstruction are frequently observed. Given that tumours are the predominant lesions responsible for obstruction in the colon, colonic wall thickening observed on imaging in patients with colonic ileus may be misdiagnosed as neoplasm. Literature indicates that it may be misdiagnosed as a neoplasm (carcinoma, lymphoma and Kaposi), inflammatory bowel diseases, tuberculosis, or abscess in the differential diagnosis. Diagnosis can be precisely achieved through colonoscopy and histopathological examination[7,8]. The ameboma clinic may mimic colon carcinomas as it may cause lower gastrointestinal tract bleeding and colonic type ileus findings. In addition to the clinic, it has been reported as case reports in the literature that it can be misdiagnosed as colon carcinoma in radiological and endoscopic images by forming intraluminal granulation tissues in the colon[8-10]. We examined the case report written by Ke et al[11], describing a rare clinical case. Similarly, this editorial intends to increase awareness about the differential diagnosis of a rectal tumor by presenting a rare instance of amoeboma in an immunocompetent patient that mimics a mass in an unusual location.

CLINICAL PRESENTATION

A 46-year-old female patient with no known disease applied with complaints of long-term constipation and rectal bleeding. She was employed as a healthcare worker in a tertiary medical centre. It is believed that the potential risk factor for the development of ameboma is the use of communal toilets. There was no previous record of any other illnesses or use of medication in his medical history. No significant pathology was detected in the blood tests performed. The fecal occult blood test resulted positive. Thereupon, the patient underwent contrast-enhanced abdominal CT. CT was reported as “massive wall thickening was observed in the distal rectum, starting from the anorectal junction and reaching a thickness of approximately 10 mm on the left posteriolateral wall”. Mesenteric hyperemia was detected in the perirectal area, suggesting lymphovascular invasion. Additionally, the mesorectal fascia was observed to be thick (Figure 1). In the subsequent magnetic resonance imaging examination, findings similar to CT were detected (Figure 2). Due to the indications of malignancy in the radiological images, the patient was referred for a rectoscopy examination. The visual characteristics of the lesion observed during rectoscopy also indicated the presence of malignancy. Nevertheless, the histopathological assessment of several biopsies obtained during the procedure did not reveal any abnormal cells suggestive of malignant conditions; instead, protozoa consistent with amoeba were observed. At this point in time, our assessment of the patient underwent a complete transformation, leading us to exclude the possibility of cancer and move towards a diagnosis of amoebiasis. The cause of our misdiagnosis thus far is the absence of anticipated symptoms of intestinal amebiasis, including pain, bloody diarrhoea, abdominal tenderness, weight loss, and weakness, in the patient’s clinical presentation[1,4]. Moreover, stool microscopy, culture, and rapid antigen tests were not conducted due to the absence of diarrhoeal symptoms, which postponed the diagnosis of amoebiasis. Abdominal CT and magnetic resonance imaging examinations conducted for carcinoma, bypassing these procedures, are excluded from the guidelines for diagnosing intestinal amebiasis. Nevertheless, it has been reported that they may be executed in instances of ameboma. These cross-sectional imaging methods perform a crucial role in diagnosing liver amoebic abscesses. Rectosigmoidoscopy and colonoscopy, subsequently conducted, are advanced imaging techniques recommended by the guidelines for diagnosing intestinal amebiasis[1,5]. The histopathological evaluation of the colonoscopy biopsy ruled out malignancy and confirmed the diagnosis of intestinal amoebiasis, prompting the initiation of specific treatment. Treatment for amoebiasis varies based on its forms and symptom severity, using different amebicides. The most endorsed medications in contemporary guidelines are categorised into two groups. Luminal amebicides, including paromomycin, diloxanide furoate, and iodoquinol, function exclusively within the intestinal lumen and are used in the treatment of amoebic colitis. The second option group comprises tissue amebicides, including chloroquine, emetine, tinidazole, and metronidazole (the most frequently utilised in treatment protocols), which are advised for symptomatic patients suffering from dysentery[1]. Following the guidelines’ recommendations, our patient presenting with hematochezia and ameboma received a 10-day course of metronidazole and paromomycin. Regression was observed in follow-up abdomen and pelvis tomography (Figure 3). In the distal rectum, there was a thickening observed in a 3 cm segment, measuring 15 mm in a single wall prior to treatment, and 10 mm in the thickest part of a 2 cm segment post-treatment. During the monthly post-treatment follow-up, the patient’s complaints ceased entirely in the third month, and no further radiological imaging was conducted thereafter. Figure 4 shows a flowchart that presents a concise summary of our diagnostic process and the treatment we administered.

Figure 1 Contrast-enhanced pelvic computed tomography images of a 46-year-old female patient. A: Lesion suggestive of malignancy on the rectal wall, ameboma (orange asterisk), increased density in the pararectal mesenteric tissue (green arrow); B: Only one of the lymph nodes in the pararectal region increased in number (yellow arrow); C: Sagittal image shows increased thickness of the rectal wall and calcified myoma (blue star) in the uterus; D and E: It is the magnified image of coronal plane image (D). This magnified image shows increased thickness of the rectal wall (orange asterisk) and lymph nodes in the coronal section (yellow arrow).

Figure 2 Contrast-enhanced pelvic magnetic resonance images of a 46-year-old female patient. A: Increased wall thickness in sagittal sections on T2-weighted magnetic resonance imaging, almost causing obstruction of the rectal wall (orange circle); B: Heterogeneity in mesocolic adipose tissue (white arrow) and asymmetric significant wall thickening in the rectum (orange asterisk) on T2-weighted axial magnetic resonance imaging; C: Lymph nodes in the perirectal region on diffusion weighted imaging (yellow arrow); D: T1-weighted axial post-contrast magnetic resonance images showing homogeneous contrast enhancement in the rectum with increased wall thickness (orange asterisk) and heterogeneity in mesorectal adipose tissue (white arrow).

Figure 3 Contrast-enhanced pelvic computed tomography images of a 46-year-old female patient after treatment. A: On the axial plane image, the thickness of the rectal wall appears to regress (orange asterisk); B: In the sagittal plane image, calcified myomas in the uterus (blue star) are seen in addition to regression of rectal wall thickening; C: Shrinkage of lymph nodes after treatment (yellow arrow).

Figure 4 The flowchart summarises our diagnostic and treatment process. CT: Computed tomography; MRI: Magnetic resonance imaging.

AMEOBIC COLITIS

Amoebic dysentery and amoebiasis among individuals are caused by the intestinal protozoan parasite E. histolytica. Amoebic infection ranks as the third most lethal parasitosis globally, following malariosis and schistosomiasis. It is widely found in the tropical and subtropical areas of South America, Asia, and Africa. It is a disease of epidemic proportions that affects millions of individuals in impoverished nations worldwide and causes between 40000 and 100000 deaths every year[2]. Most cases of transmission occur through consumption of contaminated food or drink, but there is also the possibility of venereal transmission via the fecal-oral route. They are cysts and trophozoites with an infective formula. Cyst formulas are the most affordable and non-invasive. The trophozoite formula is invasive. Intestinal invasion is disrupted by colonic mucosal ulceration, leading to classic bottle pox. The base of the ulcers is usually clean, and it is normal for neighbors to get along well with each other. Therefore, it may be confused with Crohn’s disease (CD). To differentiate amoebiasis from other inflammatory conditions like CD and tuberculosis, it is necessary to consider all clinical symptoms, imaging findings, laboratory test results, and often histopathological evidence. Colon wall thickening and bottle-bottom-shaped ulcers containing E. histolytica trophozoites are commonly seen in amoebiasis. On the other hand, CD is characterized by segmental intestinal wall thickening and noncaseating granulomas. In tuberculosis, intestinal wall thickening with caseating granulomas is observed. Supplementary tests, such as serological and stool examinations, support in confirming the diagnosis, ensuring that patients receive the appropriate treatment. Trophozoites have the ability to phagocytosis, and the presence of phagocytosed erythrocytes within trophozoites indicates invasive disease[1,2].

While the majority of amebiasis cases are asymptomatic, numerous patients infected with E. histolytica exhibit a range of clinical manifestations. Complaints vary from colic-like abdominal pain and watery diarrhoea to bloody diarrhoea accompanied by mucus. Some patients may manifest systemic disease. The predominant systemic manifestation is an amoebic liver abscess. It occurs in fewer than 4% of patients. Liver abscess typically manifests as pain in the right upper quadrant, fever, and tenderness upon palpation. Sometimes ameboma may develop in cases with long-term infection and untreated. These are ulcerative, exophytic and inflammatory masses. Amebomas most commonly occur in the cecum and ascending colon and arise from colonic granulation tissue. It is very rare for it to occur in the rectum, as in our case. It can involve a single or multiple regions. The diameters of amebomas are between 2-15 cm[12]. Clinical symptoms usually include constipation, diarrhea, abdominal pain, tenesmus, rectal bleeding, and fever. It may cause extraintestinal symptoms such as amoebic liver abscess, brain abscess, peritonitis, pleuro-pulmonary disease, genitourinary disease, and pericarditis. Life-threatening conditions such as perforation, peritonitis and fulminant necrotic colitis may develop. Ameboma is confused with carcinoma due to its appearance. It may strengthen the preliminary diagnosis of carcinoma due to its metastatic appearance, especially in cases where it causes amoebic abscess in the liver[12,13]. Even on endoscopy, the lesion has a confusing picture, appearing as a growth in the colon. In cases where clinical, radiological, and endoscopic findings are misinterpreted as malignancy or inflammatory bowel diseases, the literature suggests that non-ameboma aetiologies should be prioritised in patients residing outside endemic regions (South Asia, Middle East, South Africa, and South America). Because while the prevalence of amebiasis is 4% in the United States, this rate reaches 50% in developing countries[14,15]. Therefore, in the majority of cases in Western nations, the diagnosis of colorectal carcinoma or inflammatory bowel disease is deemed more probable than that of a parasitic infection. Abdominal pain and hematochezia in individuals with a history of travel to an endemic area should indicate amoebiasis. Some patients may manifest systemic disease. The predominant systemic manifestation is an amoebic liver abscess. It occurs in fewer than 4% of patients. Liver abscess typically manifests as pain in the right upper quadrant and fever[5,16-18].

In our patient, the initial complaints of colic abdominal pain during episodes of chronic constipation and intermittent hematochezia should have initially indicated amebiasis, albeit with a low probability, given that we resided in an endemic region. Due to the exhibited symptoms, we initially contemplated inflammatory bowel diseases and utilised radiological cross-sectional imaging, consequently misdiagnosing rectal cancer by interpreting wall thickening in the rectum as indicative of malignancy. The diagnosis is made based on serological analysis, stool culture, sigmoidoscopy, or a combination of these three. Antigen sensitivity tests can distinguish E. histolytica from other species. These tests are extremely sensitive, fast, specific, and easy. Since the antibody can remain positive for years, its contribution to diagnosis is limited. Stool culture cannot distinguish between species and is less sensitive than antigen tests[13].

Amoebiasis is typically managed using amebicides. These drugs are categorised as luminal amebicides (such as paromomycin and diloxanide furoate) or tissue amebicides (chloroquine, tinidazole, and metronidazole), based on where they act in the body. Metronidazole is the foremost recommended and extensively utilised drug for treating invasive amoebiasis among all available medications. According to the literature, metronidazole can be administered for a duration of 5-10 days to treat amoebiasis. However, in cases involving ameboma, like the one we are discussing, this duration can be prolonged to a period of 1 month. Luminal agents are administered for a duration of 5-20 days with the objective of eradicating colonisation. Patients who experience complications may necessitate surgical intervention[13,18].

CONCLUSION

E. histolytica is often seen in poor communities and can cause a mass-like appearance called ameboma. Therefore, in populations where amoebiasis is rare, it may be confused with malignancy. Ameboma often involves the cecum, ascending colon, and rectum[4]. This editorial highlights a rare occurrence of rectal ameboma in an immunocompetent patient, initially misdiagnosed as malignancy due to similar imaging features. It underscores the importance of considering parasitic infections in differential diagnosis and the necessity of appropriate pathological evaluation.

Amebiasis, though infrequently observed in developed nations, must be considered when making the differential diagnosis of patients exhibiting bloody diarrhoea and a colonic mass, particularly those with previous episodes of dysentery and travelled to endemic regions. We recommend that clinicians observe symptoms such as pain, bloody diarrhoea, and abdominal tenderness in these patients, as they may indicate amoebiasis rather than carcinoma. Furthermore, clinical suspicion of amebiasis necessitates serological and histopathological assessments. It is important to note that there are no pathognomonic radiological findings for amebiasis that can distinguish it from carcinoma. Clinical improvement may be observed in patients undergoing treatment for amebiasis when differentiation is not possible, and intestinal improvement can be quantitatively assessed through cross-sectional radiological examinations.

In summary, it is necessary to be aware that in countries where amoebiasis is not common, in patients with normal immune systems, the mass-like appearance in the rectum may belong to parasitic diseases. The following are the recommendations for these patients: (1) In situations where there is strong suspicion, it is important to conduct serology and pathological confirmation, as there are no specific radiological or endoscopic findings that definitively indicate invasive amoebiasis; (2) It is important to note that confirming the diagnosis may often require multiple biopsies; and (3) After treatment, rectoscopy should be performed to ensure complete recovery and rule out the presence of an underlying malignancy.

ACKNOWLEDGEMENTS

We express our gratitude to our patient, who, through written consent, enabled us to contribute to the literature by providing her clinical information and radiological images.