Carcinoma in situ (CIS) is a group of abnormal cells that have not spread from the location where they first formed although there is a chance they may later spread into normal cells and become cancer. They are a form of neoplasm.
There's an ongoing debate regarding its classification, whether to classify it as cancer or not. This disagreement hinges on the specific type of CIS involved, such as cervical, skin, or breast CIS. Some experts refrain from labeling it as cancer, acknowledging its potential to develop into cancer, while others classify select types as a non-invasive form of cancer or use the term "pre-cancer."
These abnormal cells proliferate in their original location, hence the term "in situ," which is Latin for "in its place." For instance, skin carcinoma in situ, known as Bowen's disease, manifests as an accumulation of abnormal epidermal cells confined within the epidermis, without infiltrating the deeper dermis. Consequently, CIS typically doesn't form a distinct tumor but appears either flat (on the skin, cervix, etc.) or conforms to the organ's existing structure (in the breast, lung, etc.). There are exceptions, such as CIS in the colon (polyps), bladder (preinvasive papillary cancer), or breast (ductal carcinoma in situ or lobular carcinoma in situ).
Many types of CIS have a considerable chance of progressing to cancer, leading to recommendations for removal. However, the progression of CIS varies significantly, and not all instances evolve into invasive cancer.
In the TNM classification system, carcinoma in situ is denoted as TisN0M0 (stage 0).