When does perineural invasion for a squamous cell carcinoma of the skin of the head and neck merit adjuvant radiotherapy?

In cutaneous squamous cell carcinoma, the presence (vs absence) of perineural invasion is associated with a higher likelihood of local tumor recurrence after a margin negative excision.

However, several recent studies have shown that for small caliber perineural invasion (invasion of a nerve <0.1 mm...

I think @Dr. First Last's answer is perfect. The problem is that Mohs surgeons don't measure the caliber of the nerve diameter; despite the literature on this subject, the reports in my experience remain highly subjective. However I wouldn't radiate a small (presumably <0.1 mm) single focus that has...

Here is a retrospective review that defines extensive versus focal involvement in regards to PNI and helps define the risk of failure + or - RT.

http://www.sciencedirect.com/science/article/pii/S0167814016311677