Endometriosis is characterized by endometrial stroma and glands found external to the uterus within the pelvis. Rectosigmoid colon is the most common site and occurs in up to 12% with bowel endometriosis . It is often co-incident with endometriotic deposits at other more characteristic sites such as the ovaries and associated ligaments . Bowel endometriosis may also present like a mass simulating a neoplasm, particularly if the wall thickening is circumferential. A definite diagnosis of endometriosis is made histologically .
Clinical and imaging perspective:
In our patient with altered bowel habit and haematochezia, the diagnosis of colonic neoplasm was considered. Colonoscopy is a useful diagnostic tool because it can visualise the lesion, obtain tissue biopsy and detect synchronous neoplasms. Since the stenosis encountered during colonoscopy prevented complete assessment of the colon, CT colonography can be used as a sensitive alternative to colonoscopy to evaluate rest of the colon. CT of the abdomen and pelvis is useful not only in characterizing the lesion but also in assessing the possibility of any local or regional involvement.
CT scans demonstrate the narrowing of the sigmoid colonic lumen. However, it remained equivocal whether the stenosis was caused by malignancy or from an extrinsic serosal process such as endometriosis. In light of these limitations, the differential diagnosis of colonic neoplasm cannot be ruled out. However, with the benefit of hindsight, the presence of subtle tiny cystic foci at the Pouch of Douglas may have represented endometriotic deposits. MRI is a valuable diagnostic tool in further characterizing these lesions as they can clearly depict haemorrhagic products (i.e. high T1 signal) which are commonly seen in endometriosis and can determine depth of bowel wall infiltration.
With radiological exclusion of any significant bowel obstruction or perforation, her lower abdominal pain can conceivably be contributed by endometriosis.
Lesion localization helps to differentiate between differential diagnoses. Mucosal and submucosal implantations of endometriotic tissue are rare  while colon carcinoma typically originates from the mucosa; with the potential of outward spread. Nevertheless, a high index of suspicion should be maintained for a colonic neoplasm regardless of whether these lesions are encountered in the context of endometriosis.
In view of the high suspicion for sigmoid colon carcinoma, the patient underwent a laparoscopic high anterior resection of the rectum and sigmoid colon. Histopathology of the specimen demonstrated endometrial glands and stroma in the muscularis propria and pericolic tissue of the resected colon.