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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).

These terms are related since they represent the three steps of the progression toward cancer:

• Dysplasia (from the Greek δυσπλασία "malformation", δυσ- "mal-" + πλάθω "to create, to form"), is the earliest form of pre-cancerous lesion recognizable in a biopsy by a pathologist. Dysplasia can be low grade or high grade (see CIS below). The risk of low-grade dysplasia transforming into high-grade dysplasia and, eventually, cancer is low.
• Carcinoma in situ is synonymous with high-grade dysplasia in most organs. The risk of transforming into cancer is high.
• Invasive carcinoma, commonly called cancer, is the final step in this sequence. It is a disease that, when left untreated, will invade the host (hence its name) and may be lethal.

• Many bladder cancers are CIS.
• Cervical cancer is often predated by cervical squamous intraepithelial lesion (SIL, previously CIN, a form of dysplasia). The term CIS is not used for the cervix. Instead, the term high grade SIL (HSIL) is used, essentially a synonym. It is this lesion that is detected with the pap smear.
• Ductal carcinoma in situ (DCIS) of the breast is a rather frequent disease with a high probability of transforming into true breast cancer if untreated.
• Bowen's disease is squamous carcinoma in situ of the skin.
• Colon polyps often contain areas of CIS that will almost always turn into colon cancer if left untreated.
• High grade Prostatic intraepithelial neoplasia is equivalent to CIS of the prostate.
• Bronchioloalveolar carcinoma of the lung is the only form of CIS that can kill because it fills the lungs and prevents breathing. Thus, it is malignant, but not invasive. For this reason, this unusual form of carcinoma is seen as CIS by pathologists, not by oncologists or surgeons. Therefore, its inclusion among the CIS category is controversial.

Carcinoma in situ is, by definition, localized, with no potential for metastasis unless it progresses into a true cancer. Therefore, its removal eliminates the risk of cancer. It can be compared to uprooting a tree, easy when a young sapling, more difficult later.

Some forms (e.g. colon polyps and polypoid tumors of the bladder) can be removed using an endoscope, without true surgery. Dysplasia of the uterine cervix is removed by excision (cutting it out) or by burning with a laser. Bowen's disease of the skin is removed by excision. Other forms of CIS require major surgery, the best known being Intraductal carcinoma of the Breast. The most difficult form of CIS is Bronchioloalveolar carcinoma of the Lung, which requires the removal of large parts of the lung. When too large, it cannot be removed and often kills its host.

Intraductal carcinoma of the Breast is also treated with radiotherapy.

• Cancer
• Neoplasia
• Dysplasia
• Carcinoma