by Jason Wasserman MD PhD FRCPC
September 20, 2022
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.
The leading cause of AIS is tobacco smoking. Other less common causes include radon exposure, occupational agents, and outdoor air pollution.
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.
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.