Purpose: Familial adenomatous polyposis is an autosomal-dominant inherited disease with development of as many as thousands of adenomas within colon and rectum. All untreated patients will develop colorectal adenocarcinoma. A variety of extracolonic manifestations can occur, although malignant tumors are rare. An association of familial adenomatous polyposis and sarcomas was reported in a few cases only.
Methods: We present the exceptional case of a 24-year-old male with genetically verified familial adenomatous polyposis (deletion of 10 base pairs at position 228-237 of exon 15A). The patient underwent prophylactic subtotal proctocolectomy and ileal-pouch rectal anastomosis in 2003. Two years later, an obstruction of the left ureter caused by a retroperitoneal mass was diagnosed.
Results: Histopathologic findings after complete tumor resection showed a low-grade fibromyxoid sarcoma. CT scan and clinical follow-up through 15 months postoperatively revealed no recurrent tumor growth.
Conclusions: To our knowledge, this is the first reported case of familial adenomatous polyposis with metachronous retroperitoneal fibromyxoid sarcoma. Proctocolectomy or total colectomy and complete tumor resection is the treatment of choice in this case. In addition to more common semimalignant retroperitoneal desmoid tumors in familial adenomatous polyposis patients, a malignant soft-tissue tumor also has to be considered for differential diagnosis.