Soft tissue tumors of the urinary bladder Part II: malignant neoplasms.

Most bladder tumors arise from the urothelium. However, there are several uncommon but significant malignant bladder lesions that must be differentiated from urothelial carcinomas and from benign lesions of the bladder. The second half of this two-part review will describe rare nonurothelial malignant tumors of the urinary bladder including leiomyosarcoma, rhabdomyosarcoma, angiosarcoma, malignant fibrous histiocytoma (undifferentiated sarcoma), primitive neuroectodermal tumor, malignant peripheral nerve sheath tumor, hemangiopericytoma, and alveolar soft-parts sarcoma. Common clinical presentations, morphologic characteristics, and immunohistochemical features are described to aid the practicing pathologist in the identification of these entities. Because the distinction between malignant and benign lesions has significant therapeutic and prognostic implications, key factors for differentiating them are presented.