Carcinoma in Situ

Carcinoma In Situ

Carcinoma in situ (CIS) is an early form of cancer that is defined by the absence of invasion of tumor cells into the surrounding tissue, usually before penetration through the basement membrane. 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, that has failed to penetrate into the deeper dermis.

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, do form tumors, such as in the colon (polyps), in the bladder (pre-invasive papillary cancer), or in the breast (more properly called ductal carcinoma in situ).

Many forms of invasive carcinoma (the most common form of cancer) originate after progression of 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 high probability of progression into invasive carcinoma, doctors will usually recommend that the lesion be completely removed. Therefore, CIS is usually treated in much the same way as a malignant tumor.

In the TNM classification, carcinoma in situ is reported as TisN0M0 (Stage 0).