PNI stands for perineural invasion. Perineural invasion or 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.
Perineural invasion is important because tumour cells that have become attached to a nerve are able to travel out of the tumour and into the surrounding normal tissue. This can result in the tumour growing back after treatment and worse prognosis.
Perineural invasion is only very rarely seen in benign (non-cancerous or non-malignant) tumours. For this reason, pathologists use perineural invasion as evidence that a tumour is malignant (a cancer).