Infiltration is the movement of cancer cells from their normal location into the surrounding non-cancerous tissue. Another word for infiltration is invasion.
Infiltration is an important feature that pathologists look for when trying to decide if a tumour is non-cancerous (benign) or cancerous (malignant). Non-cancerous tumours can become larger but the cells usually do not infiltrate the surrounding normal tissue. In contrast, the cells in a cancer almost always break away from their normal location and infiltrate the surrounding non-cancerous tissue.
Once cancer cells infiltrate 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 infiltrated the surrounding normal tissue are called in situ. In situ cancers are associated with a high risk of turning into an infiltrative cancer without appropriate treatment.