For a pathologic Stage I squamous cell carcinoma of the oral tongue with negative margins and adequate neck dissection, is perineural or lymphovascuar invasion alone an indication for adjuvant radiotherapy?  

I think Dr Eisbruch is spot on- patients with PNI do worse in the surgical literature, but the benefit of XRT in this scenario to obviate recurrence is far from robust.  My personal belief is that PNI is not a sign of neurotropism like in melanoma or adenoid cystic ca, but likely another surrog...

PNI or lymphvascular invasion in oral cavity cancer, in the absence of any other risk factor, are indications for post-op RT according to NCCN recommendations (even for small T stages). There is more data about PNI compared with LVI as a factor that increases substantially the risk of local failure....

I add postop RT for PNI but not for LVI as a sole indication.