My understanding is that most BOT tumours are large at diagnosis.
Surgery would result in quality of life issues and deficits in both speech and swallowing.
Current research and guidelines especially for HPV + ve BOT tumours , show good outcomes with chemo radiation alone.
Surgery is then a backup, or salvage , if treatment fails and /or the tumour recurs.
Please have no regrets. Of course it is Human to do so, especially if you do end up being one of the unlucky ones who do have a recurrence. I do sometimes wonder if we should have initially gone with a total Glossectomy instead of the curative chemo rads offered . But , hey , that's the way the cookie crumbles and we are definitely in the very small majority of people who suffer treatment failure.
Thinking of you and your Dad,
Tammy