Carcinoma in situ (CIS) is a group of abnormal cells that look like cancer under a microscope but haven’t spread beyond the spot where they first formed. It’s classified as stage 0 cancer. The cells are confined to the surface layer of tissue and, at some point, may become fully cancerous and invade deeper structures. Because of that potential, CIS is almost always treated even though it isn’t yet considered invasive cancer.
Why the Basement Membrane Matters
Every organ’s surface tissue sits on top of a thin structural layer called the basement membrane. Think of it as a boundary line between the surface cells and the deeper tissue underneath. In carcinoma in situ, abnormal cells are stacking up and multiplying on one side of that line, but they haven’t broken through it. The moment those cells penetrate the basement membrane and start infiltrating surrounding tissue, the diagnosis shifts from “in situ” to “invasive” cancer. That single distinction changes the stage, the treatment plan, and the overall outlook dramatically.
This is why pathologists examine biopsy samples so carefully. They look for signs like enlarged, darkly staining nuclei, cells that have lost their normal orientation, and abnormal cell division happening in layers where it shouldn’t. If those features are present but the basement membrane is still intact, the diagnosis is carcinoma in situ.
Where CIS Occurs
CIS can develop in many different tissues throughout the body. The most common types include:
- Ductal carcinoma in situ (DCIS) in the breast
- Squamous cell carcinoma in situ in the skin, mouth, and larynx
- Adenocarcinoma in situ in the cervix, lungs, and gastrointestinal tract
- Urothelial carcinoma in situ in the bladder
Each type behaves differently, progresses at different rates, and requires a different treatment approach. Being diagnosed with CIS in the bladder, for instance, is a very different clinical situation than CIS on the skin.
Breast: Ductal Carcinoma in Situ
DCIS is one of the most commonly diagnosed forms of CIS, largely because mammography screening catches it frequently. The abnormal cells are confined inside the milk ducts of the breast and haven’t broken into surrounding breast tissue. If left untreated, some cases of DCIS will transform into invasive breast cancer, but the challenge is that there is currently no reliable way to predict which cases will progress and which will stay put. Because of that uncertainty, DCIS is almost always treated.
Treatment typically involves surgery to remove the affected area, either by removing just the section of the breast containing the abnormal cells or, less commonly, the entire breast. Radiation therapy may follow a partial removal to reduce the chance of recurrence. The prognosis for treated DCIS is excellent, with very high long-term survival rates.
Cervix: CIN 3 and Cervical CIS
In the cervix, carcinoma in situ falls under the umbrella of cervical intraepithelial neoplasia grade 3 (CIN 3), sometimes called severe dysplasia. This means severely abnormal cells cover the full thickness of the cervical surface tissue. CIN 3 is not cancer, but it carries a meaningful risk of progressing to invasive cervical cancer if left untreated.
Screening is particularly effective at catching cervical changes early. A single Pap test detects high-grade lesions with a sensitivity of roughly 70% to 80%. HPV DNA testing performs even better, identifying precancerous changes in 84% to 97% of cases. When combined, these screening tools catch the vast majority of cervical CIS before it has a chance to become invasive. Treatment usually involves removing the abnormal tissue through a minor outpatient procedure.
Bladder: A More Aggressive Form
Carcinoma in situ of the bladder is flat, often invisible to the naked eye during a scope exam, and considered more aggressive than other non-invasive bladder cancers. It tends to recur and carries a higher risk of eventually becoming invasive if not treated promptly.
The standard treatment involves a procedure to scrape away visible abnormal tissue from the bladder lining, followed by a course of immunotherapy delivered directly into the bladder through a catheter. This immunotherapy uses an inactivated form of tuberculosis bacteria to trigger an inflammatory immune response inside the bladder, which helps destroy remaining cancer cells and reduces the risk of recurrence. It’s the most common intravesical (inside-the-bladder) immunotherapy for early-stage bladder cancer, and it is not chemotherapy, even though it’s sometimes confused with it.
Skin: Bowen’s Disease
Squamous cell carcinoma in situ of the skin is commonly known as Bowen’s disease. It appears as a persistent patch on the skin with clear edges that doesn’t heal on its own. The patch is typically scaly or crusty, flat or raised, and can measure up to a few centimeters across. It may be itchy. On lighter skin it looks red or pink, though it can be harder to spot on darker skin tones. These patches appear most often on sun-exposed areas like the lower legs, neck, and head.
If the patch starts bleeding, develops into an open sore, or forms a lump, those are signs it may have progressed to invasive squamous cell skin cancer. Treatment options include freezing the affected skin with liquid nitrogen (which causes it to scab and fall off over a few weeks), applying a prescription cream that stimulates an immune response in the skin, or surgically cutting out the abnormal area under local anesthesia. In cases where the patch is very slow growing, a dermatologist may recommend close monitoring instead of immediate treatment.
How CIS Is Found
Most carcinoma in situ causes no symptoms. It’s typically discovered through routine screening or incidentally during a biopsy done for another reason. Mammograms pick up DCIS as tiny calcium deposits in the breast. Pap tests and HPV testing catch cervical changes. Bladder CIS is often found during evaluation for blood in the urine or during follow-up for a previous bladder cancer.
Because CIS is silent, screening programs are the main reason it gets caught at this early, treatable stage. Without screening, the first sign of trouble would often be the invasive cancer that develops afterward, which is far harder to treat and carries a worse prognosis.
CIS vs. Invasive Cancer: What Changes
The practical difference between CIS and invasive cancer is significant. CIS that is completely removed has a very high cure rate because, by definition, it hasn’t had the opportunity to spread to lymph nodes or distant organs. There’s no need for the systemic treatments (like chemotherapy circulating through the whole body) that invasive cancers often require. Treatment is usually local: surgery, topical therapy, or targeted treatment delivered to the specific organ involved.
That said, CIS does require follow-up. Some forms, particularly in the bladder and breast, can recur in the same location or develop in a new area. Regular monitoring after treatment helps catch any recurrence or progression early, when it’s still highly treatable.