Adenocarcinoma in situ of the lung

by Jason Wasserman MD PhD FRCPC
September 20, 2022

What is adenocarcinoma in situ of the lung?

Adenocarcinoma in situ (AIS) is a type of non-invasive lung cancer. It is called non-invasive because the tumour cells have not spread beyond the surface of the small airspaces called alveoli in the lung. In order for a tumour to be called AIS, it cannot be larger than 3.0 cm in size.

What causes adenocarcinoma in situ?

The leading cause of AIS is tobacco smoking. Other less common causes include radon exposure, occupational agents, and outdoor air pollution.

How do pathologists make the diagnosis of adenocarcinoma in situ?

The diagnosis of AIS can only be made after the entire tumour has been surgically removed and sent to a pathologist for examination under the microscope. The diagnosis cannot be made after a small sample of the tumour is removed in a biopsy or when cytology is performed. Your pathologist will carefully examine the tumour to make sure there are no areas of invasion before making the diagnosis of AIS.

What does adenocarcinoma in situ look like under the microscope?

The inside of the lung is made up of many small airspaces called alveoli. When examined under the microscope, the tumour cells in AIS are seen covering the inside surface of the alveoli. Pathologists use the term lepidic to describe this pattern of growth. The tumour cells are typically larger and darker than the specialized pneumocytes that normally line the alveoli. Unlike minimally invasive adenocarcinoma, there is no evidence of invasion into surrounding tissues.