18. An unusual rectal mass: mammary type myofibroblastoma

Background Mammary-type myofibroblastoma is an uncommon, benign neoplasm of mesenchymal origin that was first described in the breast. Since then, it has been increasingly recognised to occur in a variety of anatomical sites, mostly along the milk line in the inguinal region. Rarely, it has been described in other sites such as the seminal vesicles and in the soft tissues of the peri-anal region. We hereby report an unusual case of mammary-type myofibroblas-toma affecting the rectum. Although mammary-type myofibroblas-toma has been rarely described in the peri-anal region, to the best of our knowledge, this is the first reported case in the rectum. Method We present a case of a 61-year-old Chinese female who was found to have a rectal mass during sigmoidoscopy. Results and conclusion Computed tomography of the abdomen and pelvis revealed a 2.4 cm midline hypodense mass arising from the rectal wall and protruding into the lumen of the rectum. She subsequently underwent transanal excision of the mass. Grossly, a firm nodular lesion that was partially covered by mucosa, measuring 4×3×2.5 cm was received. Cut sections showed a well-circumscribed mass with whitish to greyish cut surfaces centred with the submucosa. Histological examination revealed a low to moderately cellular neoplasm composed of bland spindle cells with ovoid nuclei and tapering cytoplasmic processes distributed in an oedematous stroma, which contained admixed mature adipocytes, delicate collagen bundles and numerous mast cells. A fascicular arrangement of spindle cells was appreciated in some areas. On immunohistochemical staining, the spindle cells were diffusely positive for CD34 and desmin; and were negative for c-kit, DOG1, cytokeratin cocktail AE1/3 and epithelial membrane antigen. This case highlights the possibility of a mammary-type myofibroblasto-ma presenting as a mass centred within the wall of the rectum and as a potential differential diagnosis of a spindle neoplasm originating from the wall of the gastrointestinal tract. Acknowledgement The authors would like to acknowledge Professor Christopher D. M. Fletcher of Harvard Medical School for his invaluable aid and expert opinion in this case.