Perineural invasion, abbreviated PNI, refers to the invasion of cancer to the space surrounding a nerve. It is common in head and neck cancer, prostate cancer and colorectal cancer.

COMMON DEFINITION OF PERINEURAL INVASION

The current widely accepted definition is “tumor in close proximity to nerve and involving at least 33% of its circumference or tumor cells within any of the 3 layers of the nerve sheath” 

Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6728584

MODIFIED LIEBIG CLASSIFICATION OF PERINEURAL INVASION

WORST PATTERN OF PERINEURAL INVASION

The traditional PNI is classified into four types:

• 0, tumor cells away from nerves
• 1, tumor cells encircling nerves less than 33%
• 2, tumor cells encircling nerves at least 33%
• 3, tumor cells infiltrating into nerve sheathes.

What is worst pattern of perineural invasion and what is its significance?

When multiple types of PNI exist within one tumor, patients with higher score of PNI types tend to have a relatively worse prognosis. Therefore, to define the status of PNI more precisely, the new variable worst pattern of PNI (WPNI) was proposed, which takes the highest score of PNI types present in each patient no matter how focal.

Further reading: https://pubmed.ncbi.nlm.nih.gov/34912713/

Try to answer this FRCPath Part -1 MCQ– To understand how PNI affects staging

A 72 year old presents with a squamous cell carcinoma of the scalp. An excision biopsy is performed and reported as per the RCPath dataset.
Identify which feature would upstage the tumour to pT3.