MSH: A malignant cystic or semicystic neoplasm. It often occurs in the ovary and usually bilaterally. The external surface is usually covered with papillary excrescences. Microscopically, the papillary patterns are predominantly epithelial overgrowths with differentiated and undifferentiated papillary serous cystadenocarcinoma cells. Psammoma bodies may be present. The tumor generally adheres to surrounding structures and produces ascites. (From Hughes, Obstetric-Gynecologic Terminology, 1972, p185),NCI: A malignant serous cystic neoplasm usually involving the ovary or the pancreas. It is characterized by the presence of invasive malignant glandular epithelial cells which often form papillary structures.