Hi,
My dad had this therapy in China ( I am a chinese, I come to the US for graduate study). I am sure you can find proton therapy in the US, cause the machine in china was bought from US.
For my dad, he didn't feel anything during the therapy, and it is very short too, only about 30 mins everyday, it lasts about a month. The only side effect is that he lost some appetite.But overall it is not bad. Since his therapy, he has recovered a lot, gained some weight and has more energy. Keeping his spirit up is absolutely important too.
I found this website very informative.
http://www.proton-therapy.org/howit.htm
Bless and good luck!
Best,
Emily