Carcinoma in situ

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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 cancer originate from a 'carcinomas in situ' (CIS) lesion. Therefore, CIS is considered a precursor that may, if left untreated long enough, transform into a more malignant form of neoplasm, invasive carcinoma or, in common language, "cancer".

Many doctors will not refer to 'carcinoma in situ' as "cancer" when explaining a laboratory report to a patient. However, because most forms of CIS have a real potential to turn into invasive carcinoma, CIS is usually treated much the same way as a malignant tumor.

In the TNM classification, carinoma in situ is indicated as TisN0M0 (Stage 0).

[edit] Dysplasia vs carcinoma in situ vs invasive carcinoma

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

Dysplasia, Latin for 'bad 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. Treatment is usually easy.
Carcinoma in situ is synonymous with high-grade dysplasia in most organs. The risk of transforming into cancer is high. Treatment is still usually easy.
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 will probably kill them. It can often, but not always, be treated successfully.

[edit] Examples of carcinomas in situ

Many bladder cancers are CIS.
Cervical cancer is often predated by cervical intraepithelial neoplasia (CIN), also called dysplasia. 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.
Bronchioalveolar 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.