A mucinous tumor of the mesocolon with features of borderline malignancy.

Primary mucinous cystadenoma of the mesocolon is a rare tumor with an uncertain histogenesis. A 38-year-old woman was diagnosed with a 17-cm cystic lesion in the left abdomen, identified as a mucinous cystadenoma of the mesocolon. This type of tumor appears rarely in extraovarian sites. We believe that metaplasia, either celomic or mucinous, is the most likely pathogenic mechanism. On rare occasions, a borderline or invasive component may be present.