Well if you are claustrophobic who cares if anyone else has the problem, you do, so asking others and hearing "no" may help you feel abnormal which you should not.
I just had my first FSR this morning and the mask while uncomfortable, wasn't bad as I focused on breathing, kept my eyes closed and tried to relax as much as possible. I don't have claustrophobia though so....
Did your doctor discuss or is a fewer FSR treatment option available? I still haven't figured out why it goes from 1 with Gamma to 3 to 5 with LINAC/Cyber then jumps to 27-30+ FSR, seems like there should be options in between.
Personally, if you can get through the first treatment session you will be fine imo because then you know what to expect, as the anxiety of it all contributes to your fear. Maybe during your first session you can try to focus on the fact a longer FSR treatment can have fewer side effects and preserve more hearing? Try focusing on the positives of what's going on.
Let us know when you are going in so we can provide words of encouragement and prayer.
Good luck