Immunoperoxidase staining of tissue for prostatic acid phosphatase has been useful in confirming the prostatic origin of metastatic deposits. This technique was used on the prostate tumors of 2 patients to differentiate between a true carcinosarcoma and a pure epithelial carcinoma with sarcomatoid changes. Positive staining for prostatic acid phosphatase in both the sarcomatoid element, as well as the area of well-differentiated carcinoma, confirmed the common epithelial cell origin of these components. Electron microscopy further confirmed these findings by demonstrating desmosomes in the sarcomatoid areas. Although each type of tumor is rare, differentiation between true carcinosarcomas and true carcinomas with sarcomatoid changes is important to elucidate further their different clinical behaviors and responses to therapy.
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