The authors present a case-review of a patient with a large retroperitoneal tumor. A right-sided hemicolectomy with a removal of a large retroperitoneal cyst extending behind the liver up to the diaphragm on the right side, was conducted. Histology confirmed a mucous cystadenocarcinoma of the appendix. Furthermore, the authors discuss this fairly rare type of the GIT carcinoma. This tumor is low-invasive, mucus producing and rarely metastazing. During the tumor progression, the tumorous cells enter the peritoneal cavity and cause, so called, pseudomyxoma peritonei. The treatment principle is to complete a radical surgical removal of the tumor and, furthermore, in case of the pseudomyxoma periotenei, also application of the local peroperative chemotherapy.

        Key words: appendix – retroperitoneal tumor – cystadenocarcinoma