Perineural invasion: identification, significance, and a standardized definition

Mohs surgeons have expanded the range of cancers treated using the Mohs technique. Mohs surgeons today are expected to diagnose perineural invasion (PNI) when as little as one nerve is involved.

To address the issue of identification and significance of perineural invasion from the perspective of the Mohs surgeon. The experience of other medical specialties dealing with the same issue are reviewed and applied.

This article is based on a review of the entire medical literature regarding PNI.

PNI is a significant complication of cancers, regardless of the organ of origin. The most common complication of PNI is recurrence of the cancer. Leptomeningeal carcinomatosis occurs in neglected or aggressive cancers. The process is indolent and contiguous, lending itself well to treatment with Mohs surgery. There are diagnostic mimics of PNI. Variation of reported incidences and cure rates suggest that diagnostic criteria for PNI may not be consistent from study to study.

We propose the following definition for the minimum histopathologic criteria required to make a diagnosis of PNI: "In the presence of a malignancy, PNI may be diagnosed according to the observation of cytologically malignant cells in the perineural space of nerves. In equivocal cases, the observation of total or near-total circumferential involvement is supportive, as is the presence of perineural tracking in tangential sections and intraneural involvement."