PNI stands for perineural invasion. Perineural invasion means that cancer cells from a tumour have become 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 very rare in non-cancerous tumours. For this reason, when a pathologist finds perineural invasion, it means that the tumour is more likely to be a cancer.
Perineural invasion is important because cancer cells that have become attached to a nerve are able to travel out of the tumour and int the surrounding normal tissue. This can result in the tumour growing back after treatment and worse prognosis.