Primary retroperitoneal mucinous cystadenoma with a sarcoma-like mural nodule: an immunohistochemical study with histogenetic considerations and literature review.

Primary retroperitoneal mucinous cystadenomas (PRMCs) are extremely rare tumors and their association with sarcoma-like mural nodules (SLMNs) has not been described thoroughly. The aim of this study is to characterize the gross and microscopic features and the immunohistochemical profile of the first case of PRMC with SLMN and to discuss the differential diagnosis of SLMNs. The literature related to primary retroperitoneal mucinous tumors is reviewed in an attempt to clarify the histogenesis of the epithelial and sarcomatoid components of the associated mural nodules. A 34-yr-old woman presented with a 14-cm retroperitoneal cystic lesion with a 6-cm mural nodule. An immunohistochemical study with a panel of 19 antibodies and a histochemical study for mucin stains were performed. The epithelial component of the PRMC showed positive staining for cytokeratin (CK) 7, CK AE1/3, epithelial membrane antigen, carcinoembryonic antigen, and calretinin. The neoplasm was not immunoreactive for CK 20, CK 5/6, and the other antibodies used in this study. In addition, it stained positively for mucin by mucicarmine, periodic acid-Schiff, and Alcian blue. The stromal cells of the cyst showed estrogen receptor positivity. SLMN cells were negative for all CKs and other epithelial markers used in the study, but they showed diffuse positive staining for vimentin and CD68, and positive staining for Ki-67 was demonstrated in 25% of these cells. The immunohistochemical and histochemical profiles of PRMC were similar to those of ovarian mucinous neoplasms and the mesothelium. The formation of SLMNs seems to be related to subepithelial hemorrhage and some reactive epithelial changes near the mural nodules. The specific immunohistochemical and morphologic features of SLMNs are helpful in differentiating them from malignant mural nodules, including true sarcomas, osteoclast-rich undifferentiated carcinomas, and carcinosarcomas. Such a differentiation is critical in view of its significant impact on the management of these neoplasms, particularly in young patients who desire to preserve their fertility.