I didn't have larynx, I have tonsil. But there are many similiarites. I recently completed 35 radiation treatments, mask and all.
I was more worried about the stupid mask and being bolted down to not move then I was about the cancer. The mask turned out to be nothing. You can't move. But if you had to because you panicked, you can get out. You can reach the twist knobs and undo yourself. Plus they are watching you all the time so they'd be there in a second. Don't sweat that.
The radiation itself is nothing. The side effects can be bad. I lost 15 lbs. in one month. Eating/swallowing is difficult. Taste buds get destroyed, salivary glands can get killed off. Just ask your dr. or staff what to expect and they'll pretty much hit the nail on the head. We all go through them, the only question is how bad? Everyone is different.
Use your pain meds. Use your Magic Mouthwash, and keep swallowing no matter what.
Good luck, God Bless.
Rick