What is adenocarcinoma in situ (AIS)?


Adenocarcinoma in situ (AIS) is a medical term used to describe an early form of gland-forming cancer. The word “adenocarcinoma” refers to a type of cancer that develops from glandular cells. The phrase “in situ” means “in its original place.” Together, adenocarcinoma in situ means that the cancer cells are present but have not yet invaded deeper tissues.

Because the abnormal cells are confined to the layer where they first developed, adenocarcinoma in situ is considered a noninvasive form of cancer. If left untreated, there is a risk that it could grow into surrounding tissue and become an invasive adenocarcinoma.

Where in the body can adenocarcinoma in situ occur?

Adenocarcinoma in situ can develop in different organs that contain glandular cells. Some common examples include:

  • Cervix: Adenocarcinoma in situ of the cervix develops in the glandular lining of the cervix.

  • Lung: Adenocarcinoma in situ of the lung arises from glandular cells in the airways.

  • Colon and rectum: Sometimes, very early colorectal adenocarcinomas are described as adenocarcinoma in situ.

Although the term can be used in different locations, the meaning is the same: Abnormal glandular cells are present but have not invaded beyond their site of origin.

How do pathologists make this diagnosis?

A pathologist diagnoses adenocarcinoma in situ after examining a tissue sample under the microscope. The sample may come from a biopsy (a small piece of tissue taken for testing) or from a larger surgical specimen (a larger piece of tissue removed during surgery).

Under the microscope, the pathologist observes glandular cells (cells that usually form glands and produce fluids) that appear abnormal in terms of size, shape, and organization. These cells may look crowded or form irregular patterns. Importantly, the abnormal cells are only found on the epithelium (the outermost layer of tissue) and have not yet spread into the stroma (the supportive tissue beneath the epithelium). This distinction is what separates adenocarcinoma in situ from invasive adenocarcinoma.

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

The distinction between adenocarcinoma in situ and invasive adenocarcinoma is significant because it directly impacts prognosis and treatment.

  • Adenocarcinoma in situ has an excellent prognosis. Removal of the abnormal area usually cures the condition.

  • Invasive adenocarcinoma has a higher risk of spreading to other tissues and requires more extensive treatment.

For this reason, pathologists are very careful in evaluating whether the abnormal cells have broken through the basement membrane (a thin barrier that separates the epithelium from the stroma).

Questions to ask your doctor

If your pathology report says adenocarcinoma in situ, you may find it helpful to ask your doctor:

  • Where in my body was the adenocarcinoma in situ found?

  • Was it completely removed?

  • Does my report show clear margins (no abnormal cells at the edges of the tissue)?

  • What is the risk that this could turn into invasive cancer?

  • Do I need any additional treatment or follow-up tests?

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