About 80% of the mucinous tumors are benign, and 5-10% are malignant. About 5% of the benign and 20% of the malignant tumors are bilateral.
Grossly appear as large cystic masses. Often multiloculated. Contain a gelatinous sticky fluid.
Microscopically the tumor consists of cystic spaces lined by tall columnar epithelium with mucinous differentiation. In the malignant form, the epithelium may form papillary projections, show ssolid areas and cytologic atypia, and invades the tumor capsule. The borderline tumors are those that exhibit atypia without the propensity to show invasion.
The borderline and the malignant varieties may seed the peritoneum. In such case, the peritoneum is filled with a gelatinous fluid (pseudomyxoma peritonei).