Transformation of low-grade mucinous neoplasm of the appendix with pseudomyxoma peritonei to high-grade sarcomatoid carcinoma.

A 66-year-old man initially underwent appendectomy and cytoreductive surgery for a low-grade appendiceal mucinous neoplasm with pseudomyxoma peritonei. One and a half years later, multiple disseminated lesions developed in rectus abdominis muscle and peritoneal cavity. Biopsy showed histopathological transformation to sarcomatoid carcinoma. This case illustrates that evolution of low-grade pseudomyxoma peritonei to high-grade carcinoma truly develops in some patients. The development of this dedifferentiation appears associated with aggressive behavior and poor clinical outcome.