Invasion is the movement of cancer cells from their normal location into the surrounding non-cancerous tissue. Another word for invasion is infiltration.
Invasion is an important feature that pathologists look for when trying to decide if a tumour is benign (non-cancerous) or malignant (cancerous). Non-cancerous tumours can become larger but the cells usually do not invade the surrounding normal tissue. In contrast, the cells in a cancer almost always break away from their normal location and invade the surrounding tissue.
Once cancer cells invade the surrounding tissue they have the ability to spread into other parts of the body. The movement of cancer cells to another part of the body is called metastasis.
Cancer cells that have not yet invaded the surrounding normal tissue are called in situ. In situ cancers are associated with a high risk of turning into an invasive cancer without appropriate treatment.