Key words: peritoneum, malignant mixed mesodermal tumor. Correspondence to: Mahmoud R Hus- sein, Department of Pathology, Facul- ty of Medicine, Assuit University, As- suit, Egypt. Tel +20-93-2581258; fax +20-88-373727; e-mail mrcpath17@gmail.com, mrh17@gawab.com Received June 5, 2008; accepted July 29, 2008. Primary peritoneal malignant mixed mesodermal (Müllerian) tumor Mahmoud R Hussein 1 , Saad Rezk Abudlwahed Hussein 2 , and Ahmad Rezk Abd-Elwahed 3 1 Departments of Pathology, Assir Central (Abha, KSA) and Assuit University Hospitals, Assuit University, Assuit, Egypt; 2 Department of Radiology, Faculty of Medicine, AL-Azhar University, Assuit, Egypt; 3 Departments of Obstetrics and Gynecology, Faculty of Medicine, AL-Hussein University Hospitals, AL-Azhar University, and Ibn Sina National College of Medicine, Jeddah, Kingdom of Saudi Arabia ABSTRACT Aims and background. Malignant mixed mesodermal tumor (MMMT) is a biphasic neoplasm (carcinosarcoma) composed of both epithelial and mesenchymal ele- ments. Extragenital MMMT, including primary peritoneal MMMT, is an extremely rare tumor with features consistent with its origin from the secondary Müllerian sys- tem. The neoplastic elements of extragenital MMMT presumably arise directly from the mesothelium or submesothelial stroma and hence parallel the biphasic pattern of the genital (uterine or ovarian) counterpart. Methods and study design. Here we report on the clinical, pathological, and immuno- histochemical features of a case of peritoneal MMMT in a 65-year-old woman. The pa- tient presented with abdominal fullness and pain. Gynecological examination revealed a huge pelvic abdominal mass. On histology, the tumor consisted of poorly differentiat- ed carcinomatous and sarcomatous (rhabdomyosarcoma) components. Further im- munohistochemical analysis revealed positive reactivity for both epithelial (cytokeratin and epithelial membrane antigen) and mesenchymal (vimentin, S-100, and desmin) markers.The patient refused treatment and died of the disease three months later. Results and conclusions. Based on the present case and on previous studies, primary peritoneal MMMT seems to be a rare but highly malignant neoplasm with an aggres- sive behavior and poor prognosis. Its exact origin, histogenesis and molecular alter- ations are poorly understood. Introduction Primary malignant mixed mesodermal tumor (MMMT, also called malignant mixed Müllerian tumor and designated in theWHO classification of female genital tract neo- plasms as carcinosarcoma) is an infrequent tumor that develops usually in the uterus and more rarely in the ovary. It is seen more frequently in postmenopausal women in the fifth and sixth decades of life 1 . Patients usually present with a bulky pelvic abdom- inal mass, hemorrhagic ascitis and disseminated metastasis.The aggressiveness of this neoplasm is evident from its rapid spread and less than 24 months’ postoperative sur- vival. Interestingly, MMMT may be associated with autoimmune manifestations (thrombocytopenic purpura and myasthenia gravis), other primary tumors (ovarian thecoma and serous adenocarcinoma) and hormonal therapy 1-3 . A recent analysis showed a substantially high risk for MMMT in association with tamoxifen intake. Hubalek et al. reported a case of a 40-year-old multiparous premenopausal woman who received tamoxifen for 2 years after the surgical treatment of breast cancer and subsequent adjuvant chemotherapy.Two years after initiation of tamoxifen treatment, the patient developed uterine MMMT 1 . Histologically, MMMT is composed of both carcinomatous and sarcomatous components.The prominence of epithelial elements quite often leads to the erroneous diagnosis of undifferentiated carcinoma 4 . In the Tumori, 95: 525-531, 2009