Carcinoma in situ versus invasive carcinoma
Carcinoma in situ (in place) is malignant-looking epithelium that stays above the basement membrane (the thin supporting layer beneath it), while invasive carcinoma has broken through it.
Remember
While the basement membrane stays intact, malignant-looking epithelium cannot metastasise (spread to distant sites). Invasion through that membrane, not how abnormal the cells look, is what makes a carcinoma invasive.
Memory aid
in situ means 'in place', and the basement membrane is the wall that keeps the cells in place.
The basement membrane acts as a wall: while the cells stay behind it they are in place, and breaking through it makes the carcinoma invasive.
The epithelium is a layer of cells that covers body surfaces, such as the skin. Beneath it sits the basement membrane, a thin supporting layer that separates the epithelium from the tissue underneath. That tissue is the stroma, the supporting connective tissue, and it contains the blood vessels and the lymphatics, which are the small channels that drain lymph.
In carcinoma in situ, malignant-looking epithelial cells stay above the basement membrane, in the place where they began. They have no access to the lymphatics or blood vessels, so they cannot metastasise (spread to distant sites). Detected at this stage, for example through a screening programme, the carcinoma can often be completely excised (removed), with an excellent prognosis.
Invasive carcinoma is the stage in which the cells have broken through the basement membrane, and this means they can metastasise. Invasion is the crossing of malignant cells through that membrane into the stroma. It is the step that separates the two, and it marks a major change in prognosis and treatment.
How abnormal the cells look does not reliably tell the two apart, because carcinoma in situ is already malignant-looking. A high mitotic index (many dividing cells) can be present in carcinoma in situ as well as in invasive carcinoma. So the key is where the malignant cells sit in relation to the basement membrane.
Key facts
- Meaning of in situ
- in place, so the malignant-looking cells are still where they began.
- Carcinoma in situ
- malignant-looking epithelial cells sit above the basement membrane and have not invaded.
- Why it cannot spread
- cells kept above an intact basement membrane have no access to the lymphatics or blood vessels, so they cannot metastasise.
- Outlook
- when detected early, for example through screening, it can often be excised completely and the prognosis is excellent.
- Invasive carcinoma
- the malignant cells have broken through the basement membrane, so they can metastasise.
- Not the distinction
- how abnormal the cells look does not reliably tell the two apart, because carcinoma in situ is already malignant-looking.
Exam traps
- Malignant-looking cells do not give carcinoma in situ the same chance of distant spread as an invasive carcinoma, because while the basement membrane is intact the cells cannot reach the vessels that would carry them to distant sites.
- A high mitotic index (many dividing cells) shows how fast the cells are growing, not whether they have crossed the basement membrane, and it can be present in carcinoma in situ as well as in invasive carcinoma.
- Malignant-looking cells in their original layer are already a carcinoma; it is entering the tissue beneath that makes the carcinoma invasive.
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