Perineural invasion (PNI) is a term pathologists use to describe tumour cells attached to a nerve. Nerves are the wires that carry signals between your body and your brain. Nerves are found in almost every tissue in the body including the skin, bones, and internal organs. PNI is only very rarely seen in non-cancerous tumours. For this reason, pathologists use PNI as evidence that a tumour is cancer.
Perineural invasion (PNI) is important because cancer cells that have become attached to a nerve are able to spread out of the tumour and into the surrounding normal tissue. This can result in the tumour growing back after treatment and a worse prognosis.