Pseudomyxoma peritonei is a very rare type of cancer that usually begins in your appendix as a small growth, called a polyp. Or, more rarely, it can start in other parts of the bowel, the ovary or bladder. This polyp eventually spreads through the wall of your appendix or wherever else it starts. It then spreads cancerous cells to the lining of the abdominal cavity (the peritoneum). These cancerous cells produce mucus, which collects in the abdomen as a jelly like fluid called mucin.