Mucinous cystadenocarcinoma of the appendix is a rare malignancy. This is a report of a 74-year-old man who presented with recurrent pneumonia which turned out to be a postobstructive pneumonia complicating a large mucinous cystadenocarcinoma of the appendix with massive retroperitoneal and intrathoracic extension. Mucinous cystadenocarcinoma of the appendix is a low-grade malignancy characterized by expansive growth due to progressive accumulation of mucinous fluid produced by the cancer cells. The tendency of this tumor to expand massively is well demonstrated by this case. The unusual retroperitoneal location of appendix in this patient probably allowed the tumor to expand massively in the retroperitoneal space and the thoracic cavity. In addition to computed tomography, [(18)F]fluorodeoxyglucose positron emission tomography ((18)F-FDG PET) was used as an ancillary method for staging in this patient. The value of (18)F-FDG PET in the diagnosis of mucinous cystadenocarcinoma of the appendix has not been determined yet, but it might be promising. The most common presentation of this tumor is abdominal pain or a palpable ileocoecal mass. To the knowledge of the authors, this is the first report of an appendiceal mucinous cystadenocarcinoma with expansion into the thoracic cavity presenting with recurrent pneumonia.
