Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Use local emergency services rather than waiting for a routine radiation appointment.
Contact your healthcare team promptly for persistent vomiting, inability to drink, worsening pain, new difficulty swallowing, significant skin breakdown, or symptoms specific to the treated area.
Seek urgent medical help for severe breathing difficulty, a new seizure, sudden weakness, confusion, uncontrolled bleeding, or another rapidly developing serious symptom.
Mild fatigue and skin changes can usually be discussed during routine treatment reviews.
The article also says
Report side effects that are worsening, interfering with eating or drinking, or not controlled by the medicines and skin care recommended by your team.
Every sign above is quoted from the sofidex article on Proton therapy. If you are not sure, err upward — it is always reasonable to be checked.