Perineural Invasion of Skin Cancers in the Head and Neck: An Uncommon Phenomenon Revisited

PNI describes a microscopic finding of tumor infiltration along a nerve, and is distinguished from Perineural Spread (PNS), which describes the presence of gross tumor growth along a nerve distinct from the main tumor mass on imaging. PNI is defined as cancer cell invasion in, around, and through nerves where tumor cells are seen within any of the layers (epineurium, perineurium, endoneurium) of the nerve sheath [1]. When tumor cells are not seen in the layers of nerve sheath, it can be difficult to differentiate it from tumor abutting on nerve.