LAUSR

A 66-year-old male was found with an incidental right pelvic mass. The patient had a previous history of colonic adenomatous polyps and underwent routine follow-up colonoscopy, which showed a rectal wall mass (Fig. 1). On computed tomography (CT) scan of abdomen/pelvis, a 3.3-cm well-circumscribed mass with central calcification was identified on the right posterior pelvic inlet. It appeared to be pushing the adjacent structures, including the branches of the right internal iliac artery and the rectosigmoid, rather than invading into them (Fig. 2). As the mass appeared benign with a clear margin around it, a magnetic resonance imaging scan was not performed and the patient underwent diagnostic laparoscopy. A large 3.2-cm white, firm, completely mobile mass was identified in the right posterior pelvic cavity (Fig. 3), abutting into the rectal wall. There was no evidence of fibrosis within the adjacent mesentery nor mesorectum. Mass was removed in an Endocatch (Medtronic, Dublin) specimen retrieval bag. The patient was commenced on a normal diet and discharged on post-operative day 1. Histopathology showed a fibrous tumour with central fat necrosis and calcification. Immunohistochemistry did not stain for β-catenin nor CD34. The differential diagnosis of such benign fibrous pelvic masses includes: (i) intramural lesions including gastrointestinal stromal tumour (GIST), (ii) fibroma/desmoid tumour, (iii) ovarian tumour in females, (iv) sclerosing mesenteritis, (v) inflammatory pseudotumour and (vi) extrapleural solitary fibrous tumour. A GIST is the most common mesenchymal tumour of the gastrointestinal tract; however, rectal GIST comprises only 5% of all GISTs. It arises from the interstitial cells of Cajal, which act as the pacemaker of the intestine. However, in our case, it was not an intramural tumour, nor was it a GIST. Another differential is intra-abdominal fibroma (desmoid tumour/ mesenteric fibromatosis), which is derived from the myofibroblasts. It usually occurs within the mesentery or abdominal wall, and it is derived from the mutation within the β-catenin pathway (APC gene). Whilst it does not metastasize, it is locally aggressive and causes erosion/obstruction of mesenteric vessels leading to ischaemia and bowel obstruction. The desmoid tumour can appear on CT as welldefined or ill-defined, homogenous or heterogeneous enhancing solid/solid-cystic soft tissue mass with local infiltration giving a stellate appearance. The appearance depends on the extent of fibroblast proliferation, fibrosis, collagen content and vascularity. The immunohistochemical profile of desmoid tumours is negative for cell markers CD34 and CD117, whereas GIST is almost universally positive for CD117 and often positive for CD34. Fig. 1. Colonoscopy showing pelvic mass pushing onto the rectal wall.