H Haeri - Pathology department of Cancer Institute, Imam Khomeini Hospital, Tehran University of medical sciences, Tehran, Iran
B Vosooghi - Pathology department of Cancer Institute, Imam Khomeini Hospital, Tehran University of medical sciences, Tehran, Iran
F Asadi Amoli - Pathology department of Farabi Hospital, Tehran University of medical sciences, Tehran, Iran

Background: Primary Retroperitoneal mucinous tumor of low malignant potential (borderline) is a rare neoplasm. Almost always, the reported patients were females. The major issueof this condition is how to distinguish the exact origin of the tumor preoperatively; because itcan change the treatment planning, survival or prognosis.Case presentation: A 26 year old Iranian woman with unremarkable medical, surgical andfamilial histories, complaining of abdominal distention and abdominal pain was referred toCancer Institute of Tehran Imam Khomeini Medical Complex. On clinical examination alarge mass at left lower quadrant (LLQ) of abdomen was detected. Primary abdominopelvicCT scan show pelvic mass measuring about 13*10*8cm with extension to LLQ with noevidence of pancreatic or ovarian origin of the mass. Serum CEA, α-FP, CA19-9 and CA125were in normal limits and was 3.5 U/ml, 5.3 U/ml, 35.17 U/ml and 20 U/ml, respectively.A laparotomy was performed and retroperitoneal mass in LLQ, without attachment to otherorgans, completely resected; No significant changes in other abdominal or pelvic organs suchas pancreas were seen.Pathological findings: Gross examination of the received specimen revealed an ovoid shapeencapsulated creamy-grayish piece of tissue measuring 12*10*7cm in diameters. Capsularsurface was unremarkable but cut sections showed heterogeneous solid and cystic areas withpasty greenish materials in some foci.Microscopic examinations shows multiloculated and capsulated mucinous tumor with mild tomoderate nuclear atypia. In some areas proliferating epithelium shows epithelial bridgingand delicate papillary structures with filigree pattern. There is no evidence of stromalinvasion in prepared slides.Immunohistochemistry (IHC): In order to elucidate the exact origin of the tumor IHC studywas performed. Results were positive for CK7, CK20, Ki67 (about 40%) and CEA andnegative for ER and PR. (Figures 3 – 8, respectively)