| Carcinoma in situ | |
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| Classification and external resources | |
| ICD-O: | M8010/2 |
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.[1] 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).
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These terms are related since they represent the three steps of the progression toward cancer:
Carcinoma in situ is, by definition, a localized phenomenon, with no potential for metastasis unless it progresses into a "true" cancer. Therefore, its removal eliminates the risk of subsequent progression into a life-threatening condition. This concept is in some ways analagous to uprooting a tree - easy when a young sapling, and much more difficult later.
Some forms of CIS (e.g. colon polyps and polypoid tumours of the bladder) can be removed using an endoscope, without conventional surgical resection. 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 require major surgery, the best known being intraductal carcinoma of the breast (also treated with radiotherapy). One of the most dangerous forms of CIS is the "pneumonic form" of bronchioloalveolar carcinoma of the lung, which can require extensive surgical removal of large parts of the lung. When too large, it often cannot be completely removed, with eventual disease progression and death of the host.
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