Carcinoma in situ (CIS) is an early form of carcinoma defined by the absence of invasion of surrounding tissues. In other words, the neoplastic cells proliferate in their normal habitat, hence the name 'in situ' (Latin for 'in its place'). For example, carcinoma in situ of the skin, also called Bowen's disease is the accumulation of neoplastic epidermal cells within the epidermis only.
For this reason, CIS will usually not form a tumor. Rather, the lesion is flat (in the skin, cervix, etc) or follows the existing architecture of the organ (in the breast, lung, etc). Some CIS, however, form tumors, for example colon polyps or papillary cancer of the bladder.
Many forms of invasive carcinoma (the most common form of cancer) originate from a CIS lesion. Therefore, CIS is considered a precursor or incipient form of cancer that may, if left untreated long enough, transform into a malignant neoplasm.
When explaining a laboratory report to a patient, most doctors will refer to CIS as pre-cancer, not cancer. However, because most forms of CIS have a real potential to turn into invasive carcinoma, doctors will usually recommend that the lesion be completely removed. Therefore, CIS is usually treated much the same way as a malignant tumor.
In the TNM classification, carcinoma in situ is indicated as TisN0M0 (Stage 0).
