Do you offer definitive radiotherapy for recurrent carcinoma in situ of oral cavity in a patient who is not a surgical candidate?   

There are insufficient data to support such morbid treatment. Moreover, this population typically has field cancerization and will develop invasive disease over time, and RT should be "saved" for when it is clearly indicated.

RT is very toxic so typically I like to wait until I know for sure there is frank invasion.We can borrow from CIS of the larynx but that morbidity is way lower versus oral cavity RT. Why is patient not a surgical candidate? I would push surgery if possible, no RT

I agree w @Nancy Y. Lee!  There is generally no role for RT in situ disease.   This is a surgically managed disease, as is invasive cancer of oral cavity in general.  Sometimes, an obvious fungating or ulcerative mass may yield an unrepresentative sample and called in situ cance...