[Carcinosarcoma of the colon, one or two tumors?].

Carcinosarcoma is a rare neoplasm that displays morphological features of both an adenocarcinoma and a sarcoma. The question is whether two tumors co-exist or whether the two morphological aspects represent sequential steps in tumor progression. We report a case of carcinosarcoma of the caecum in a young female. To characterize the two tumor cell populations and to gain insight into the pathogenesis of the lesion, we conducted immunohistochemical and ultrastructural analyses of the tumor. The biphasic aspect of the tumor showed an admixture of carcinoma and spindle-cell sarcomatoid areas. Both adenocarcinoma and sarcomatous cells were positive for cytokeratins. Vimentin was undetectable in the epithelial portion, but many of the sarcomatous cells stained for vimentin. Electron microscopic analyses of the sarcomatous portion revealed budding of "retroviral particles" from the rough endoplasmic reticulum cisternae. Our data support the contention that "carcinosarcoma" is a part of a single clinicopathological continuum with "spindle-cell carcinoma", the former being the biphasic expression of the neoplasia, the latter the monophasic expression; the presence of productive retroviral infection in the sarcomatous cells could constitute one of the additional support in tumor progression from the carcinomatous to the sarcomatous phase.