How would you treat an unresectable SCC of the skin local recurrence in the base of skull with clinical and radiographic CN involvement?

I usually target gross disease to at least 70 Gy in 35 fractions if my treatment intent is curative.  I try to avoid fractional doses >2 Gy around optic nerves and chiasm, so would not typically use 55 Gy in 20 fractions for this reason. HyPERfractionaton, proton beam and concurrent systemic therapy are all considerations depending on the specifics of the situation.  

I don't understand the way this question is framed: "around the optic nerves and brainstem." Clinical perineural invasion by cSCC does not typically involve these structures, unless the tumor has invaded into brain directly or traveled via trigem root into brainstem, or unless it is so massive that it has literally taken over the soft tissues of the orbit and face (I have never seen this except with BCC). We routinely treat the V1 and V2 divisions back to Meckel cave and cavernous sinus without necessarily hitting any optics or brainstem. While the control rates are not great depending on the level of nerve involvement, these patients can live for many years and can even be reirradiated. Therefore I would NOT hypofractionate. We use 70 Gy in 35 fractions although with IMRT we often use 69.96 Gy in 33 fractions covering noninvolved "elective" areas such as cavernous sinus or brainstem to 59.4. Concurrent chemotherapy is preferred e.g. cisplatin but cetuximab is not totally unreasonable for very elderly/frail patients as it does show response in the metastatic setting. With concurrent cisplatin, I feel less compelled to use hyperfractionation, but if we could not give chemotherapy then altered fractionation would be a consideration. I am excited about the possibility of immunotherapy.

Considering this patient's history, this treatment has to be considered palliative.  Therefore we like to treat them in as rapid a fashion as possible to minimize the amount of time that they must spend receiving treatment.  I would treat this patient with SBRT, five fractions of high-dose treatment, highly conformal covering only the CTV with a margin no greater than .5 CM.  The dose per fraction would depend upon proximity of critical structures and the overall size [volume] of the tumor.  Minimum dose per fraction six Gy, maximum dose 8 Gy, total dose 30-40 Gy.  The use of concurrent chemotherapy would be at the discretion of medical oncology and would not affect my treatment.