Primary retroperitoneal pseudomyxoma masquerading as a benign retroperitoneal abscess from an ascending colon subacute perforation: a case report.

Introduction and importance: Pseudomyxoma peritonei is a neoplastic process characterized by diffuse mucinous implants on peritoneal surfaces arising from mucin-producing tumors. Rarely, pseudomyxoma from a ruptured mucin-producing tumor can be confined to the retroperitoneum, termed "retroperitoneal pseudomyxoma" or "extraperitoneal pseudomyxoma." This entity often masquerades as a retroperitoneal abscess of benign etiology. Case presentation: A 58-year-old man presented with a symptomatic retroperitoneal cystic mass with a fistulous communication to the ascending colon seen at colonoscopy, without evidence of mucosal neoplasm. Operative exploration revealed a fixed mass in the retroperitoneum, resection was aborted, and diverting loop ileostomy and operative biopsy of the mass performed. Biopsy histology revealed "pseudomyxoma peritonei." Of note, the patient had undergone appendectomy 20 years previously. Three months after fecal diversion, the patient underwent further staging which revealed the tumor was abutting the right femoral nerve. En-Bloc resection of the mass was performed and the femoral nerve was spared. Histologic evaluation revealed a mucinous neoplasm of the appendix, with negative margins of resection. Clinical discussion: Primary retroperitoneal pseudomyxoma is a rare entity. The tumors often arise from a retroperitoneal appendix, although our patient had a prior appendectomy, increasing the diagnostic confusion. Histology confirmed that the tumor arose from the remnant appendiceal stump, and the fistula was malignant erosion into the colon. In this case, with no detectable peritoneal disease, primary treatment was radical resection with the goal of achieving negative margins. Conclusion: Primary retroperitoneal pseudomyxoma is rare, often arising from a retroperitoneal appendix. In the absence of peritoneal disease, radical resection may be considered definitive therapy.