Discussion:

Extraskeletal osteosarcomas are soft tissue sarcomas in which neoplastic cells produce osteoid or bone. They encompass 1-2% of soft tissue sarcomas. Primary bladder osteosarcomas are exceedingly rare and account for <0.04 of all urinary bladder malignancies. Risk factors are not well defined, but it seems that exposure to some chemicals, schistosomiasis infection, and radiation exposure are related.

Three criteria are used to help with diagnosis: (1) uniform morphological sarcomatous pattern that excludes a mixed mesenchymal tumor; (2) production of malignant osteoid and/or bone by the sarcomatous tumor; (3) exclusion of osseous origin. Differential diagnosis includes sarcomatoid variant of urothelial carcinoma, urothelial carcinoma with osseous metaplasia, carcinoma with pseudosarcomatous stromal reaction, malignant fibrous histiocytoma, synovial sarcoma and epithelioid sarcoma and bladder carcinosarcoma. Diagnosis of primary osteosarcoma can be made only after exclusion of other malignancies.

When narrowing down one’s differential, keep in mind that carcinosarcomas and transitional cell carcinomas with osseous metaplasia will have neoplastic epithelial elements. Also, metaplastic bone and neoplastic bone have different morphologies. Metaplastic bone has lamellar architecture organized around hemorrhage or tumor. Unremarkable osteoblasts rim the outer edge of the bone and are distinct from the fibroblasts, capillaries, and interposed collagen composing the surrounding tissue. In contrast, neoplastic bone is usually woven bone haphazardly deposited in the tumor. The cells in contact with the bone are malignant and are cytologically similar to osteoblasts (eccentric nuclei and abundant eosinophilic cytoplasm). The surrounding tissue is almost completely composed of these malignant cells.

In known cases, the tumor is typically positive for vimentin and negative for other stains.

Treatment is typically cystectomy. The tumor responds poorly to chemotherapy and is radiosensitive. Unfortunately, it is locally aggressive and has often metastasized, usually to the lungs, by the time of diagnosis. Prognosis is about 6 months due to local spread with urinary tract obstruction and secondary infection.