Mucinous ovarian tumors (borderline and malignant) sometimes show mural nodules that range in size from microscopic (not apparent grossly) to several centimeters (macroscopically identifiable as raised lesions in the wall of the tumor). They may be single or multiple and are sharply demarcated from the adjacent mucinous epithelium. The mural nodules may be histologically benign or malignant. 1) Benign, reactive sarcoma-like mural nodules – they are usually hemorrhagic and consist of osteoclast-like multinucleated giant cells, atypical spindle cells and inflammatory cells. Mitotic activity is brisk. Immunoreactivity for keratin is weak and focal. They appear to be a reaction to hemorrhage and/or mucin content of the cysts and are associated with better prognosis. 2) Malignant mural nodules can show sarcomatous or carcinomatous differentiation. Sarcomatous nodules are usually solid and greyish-white. They can resemble fibrosarcoma, rhabdomyosarcoma, or pleomorphic undifferentiated sarcoma. The prognosis is poor with a risk of tumor-related death exceeding 50%. Carcinomatous nodules are composed of highly pleomorphic cells with markedly enlarged irregular nuclei, prominent nucleoli, and abundant eosinophilic cytoplasm. Some cases have rhabdoid appearance. The tumor cells are strongly positive for cytokeratin. Different types of mural nodulels may be seen in a given tumor and individual nodules may show mixed morphological features. The image shows a nodule of anaplastic carcinoma in a mucinous carcinoma of ovary. Image courtesy of: Raghavendra Pillappa, MD, Dept. of Pathology, Mayo Clinic, Rochester, Minnesota; Used with permission.