Adenocarcinoma In Situ (AIS): Definition

Adenocarcinoma in situ (AIS) is an early form of cancer that develops from glandular cells — the specialized cells that line organs and produce substances such as mucus or hormones. The phrase “in situ” is Latin for “in its original place,” meaning the abnormal cells are confined to the layer of tissue where they first developed and have not yet spread into the surrounding tissue. Because of this, AIS is considered a non-invasive form of cancer — sometimes described as pre-invasive or an early-stage cancer. If left untreated, AIS can progress over time and become an invasive adenocarcinoma.

Where in the body can adenocarcinoma in situ occur?

AIS can develop in any organ that contains glandular cells. The most common locations where this diagnosis is made include:

Cervix — AIS of the cervix develops in the glandular cells lining the cervical canal. It is strongly associated with infection by high-risk types of human papillomavirus (HPV). For more information, see the dedicated guide on adenocarcinoma in situ of the cervix.
Lung — AIS of the lung arises from glandular cells lining the tiny air sacs (alveoli) of the lung. It is a small, non-invasive tumor that carries an excellent prognosis when completely removed. For more information, see the dedicated guide on adenocarcinoma in situ of the lung.
Other organs — AIS can also occur in the endometrium (uterine lining), stomach, pancreas, and other glandular tissues, though it is less commonly diagnosed in these locations.

How is adenocarcinoma in situ diagnosed?

A pathologist diagnoses AIS by examining tissue under the microscope. The sample may come from a biopsy or a larger surgical specimen.

Under the microscope, the glandular cells appear abnormal — larger than normal, darker, and irregularly arranged. However, the critical finding that defines AIS is that these abnormal cells remain confined to the epithelium (the surface layer of tissue). They have not crossed the basement membrane — a thin structural boundary that separates the surface layer from the deeper supporting tissue called the stroma. This distinction between AIS and invasive cancer is one of the most important assessments a pathologist makes.

What is the difference between adenocarcinoma in situ and invasive adenocarcinoma?

The distinction matters enormously for prognosis and treatment:

Adenocarcinoma in situ — the abnormal cells are confined to the surface layer and have not invaded surrounding tissue. Because they cannot access blood vessels or lymphatic channels, they cannot spread to other parts of the body. Complete surgical removal is usually curative, and the prognosis is excellent.
Invasive adenocarcinoma — the cancer cells have broken through the basement membrane and spread into the stroma. From there, they can access blood vessels and lymph nodes, raising the risk of spread to other parts of the body. More extensive treatment is typically required.

Because this distinction has a significant impact on treatment, pathologists examine the boundary between the surface layer and the stroma very carefully before diagnosing AIS.

What does finding AIS in my report mean?

A diagnosis of AIS is serious enough to require treatment, but it also carries an excellent outlook — especially when the abnormal area is completely removed. The goal of treatment is to remove all abnormal cells before they have a chance to become invasive. Your doctor will review the margins of the removed tissue — the edges of the specimen — to determine whether the AIS was fully excised or whether additional surgery may be needed.

For specific information about what AIS means in your situation, the dedicated diagnosis guides linked above provide much greater detail about cervical and lung AIS.

Questions to ask your doctor

Was the adenocarcinoma in situ completely removed, and what do the margins show?
Is there any concern that invasive cancer may also be present?
What follow-up or additional treatment is recommended?