Most mucinous carcinomas of the ovary (80% of cases) show a continuum from benign to borderline to frankly malignant areas that are often found in close proximity to each other. About 20% of cases are composed entirely of malignant areas. This image shows borderline areas on one side of the cyst wall (top) and intramucosal carcinoma on the other side of the cyst wall (bottom). The borderline areas show multilayering and tufting of epithelium as well as branching papillary structures and glandular formations. The papillae are either stroma-free or have thin fibrous cores. There is mild to moderate cytologic atypia consisting of nuclear enlargement and hyperchromasia with occasional prominent nucleoli. The intraepithelial carcinoma component shows complex papillary architecture, tufting as well as multilayering of cells. The cells are cytologically malignant with nuclear enlargement and hyperchromasia as well as prominent nucleoli. There is no evidence of invasion of the underlying stroma.