Synchronous Low-grade Appendiceal Mucinous Neoplasm and Primary Peritoneal Low-grade Serous Carcinoma: A First Description of These 2 Neoplasms Presenting Together as Suspected Peritoneal Carcinomatosis.

Low-grade appendiceal mucinous neoplasm is a neoplasm typically of appendiceal origin, which is characterized by diffuse peritoneal involvement by pools of mucin with mucinous epithelium lacking high-grade cytologic atypia, and clinically presents as suspected peritoneal carcinomatosis. A similar clinical presentation can sometimes be seen with disseminated low-grade serous carcinomas of the peritoneum, fallopian tubes, or ovaries; however, this neoplasm is histologically characterized by tubal-type epithelium and invasive or confluent growth. In this case report, we describe a patient presenting with a clinical examination and radiologic features suggestive of peritoneal carcinomatosis and a prominent pelvic mass; however, after pathologic review, the patient was proven to have peritoneal involvement by both low-grade appendiceal mucinous neoplasm of appendiceal origin and a low-grade peritoneal primary serous carcinoma. In short, we present the first description of low-grade appendiceal mucinous neoplasm and serous carcinoma of the peritoneum presenting synchronously, providing morphologic characterization and immunohistochemical studies supporting the diagnosis, and illustrating a rare instance in which 2 neoplastic processes are underlying clinically suspected peritoneal carcinomatosis.