Definition of In situ

In situ: In the normal location. An "in situ" tumor is one that is confined to its site of origin and has not invaded neighboring tissue or gone elsewhere in the body.

For example, squamous cell carcinoma in situ is an early stage of skin cancer. It is a tumor that develops from the squamous cells which are flat, scalelike cells in the outer layer of the skin (the epithelium).

The term "in situ" is borrowed from the Romans. It means "in the natural or normal place" and, in the case of cancer, it is good news. It indicates that the tumor cells are still where they originated. They have neither entered neighboring tissues nor have they metastasized afar.