carcinoma in situ n carcinoma in the stage of development when the cancer cells are still within their site of origin (as the mouth or uterine cervix) abbr. CIS
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a neoplastic entity whose tumor cells are confined to the epithelium of origin, without invasion of the basement membrane; the likelihood of subsequent invasive growth is presumed to be high. See also ductal c. in situ and lobular c. in situ. Called also noninfiltrating or preinvasive c.Medical dictionary. 2011.