carcinoma in situ
Meaning of carcinoma in situ: Abnormal cells that look like cancer under the microscope but have not yet crossed the boundary that would let them spread.
Definition of carcinoma in situ
Carcinoma in situ is a collection of cells that have the appearance of cancer but remain entirely within the surface layer where they arose. They have not broken through the thin membrane that separates that layer from the tissue beneath, so they have no access to blood or lymph vessels and cannot spread elsewhere. It is often labeled stage 0.
Why does the boundary matter so much?
Surface layers of the body, whether skin, the lining of a duct or the lining of the cervix, rest on a thin sheet called the basement membrane. Blood and lymph vessels run below that sheet, never above it. Abnormal cells sitting on top of it can multiply and pile up, but they have no route out of the tissue. Once cells digest their way through the sheet the situation changes completely, because they can now enter a vessel and travel. That single step is what separates in situ disease from invasive carcinoma, and it is why a pathologist examines the membrane so closely.
Where is it commonly found?
| Site | Usual name | How it is found |
|---|---|---|
| Breast ducts | Ductal carcinoma in situ | Calcium flecks on a mammogram |
| Cervix | High grade lesion, stage 0 | Cervical screening |
| Skin | Bowen disease | A persistent scaly red patch |
| Bladder lining | Flat carcinoma in situ | Cystoscopy for bleeding or irritation |
Is it cancer or not?
The honest answer is that it sits on the line, and language reflects the discomfort. The cells are abnormal in the way cancer cells are, so it is classified with cancers and treated seriously. It cannot kill by spreading, so it is not cancer in the sense most people mean. Left alone, some of these lesions would progress to invasive disease over years, some would stay unchanged, and some would regress. There is no reliable way to tell which is which in an individual, so treatment is generally offered to all. That uncertainty is the source of a long running debate about overdiagnosis, particularly for breast carcinoma in situ, since screening finds many of these lesions that might never have caused harm.
How is it managed?
Removal of the affected area is the usual approach, and because the disease is confined, taking it out with a clear rim of normal tissue is generally curative. Lymph nodes are not usually involved, so extensive surgery is often unnecessary. Follow-up matters because new areas can arise nearby in tissue that shares the same exposure and the same accumulated damage.
Also known as
Used in a sentence
The excision specimen showed ductal carcinoma in situ with clear margins and no invasive component.
Good to know
Carcinoma in situ cannot spread while it remains above the basement membrane, and the inability to predict which lesions would ever progress underlies the debate about overdiagnosis from screening.
Word origin
Where “carcinoma in situ” comes from
In situ is Latin for in its original place; carcinoma comes from the Greek karkinos, meaning crab.