Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help for a seizure, sudden severe headache, fainting, new one-sided weakness, facial drooping, difficulty speaking, sudden loss of vision, or trouble breathing.
Contact your healthcare team urgently for worsening headaches, repeated vomiting, new confusion, increasing drowsiness, new balance problems, or a change in your usual neurological symptoms.
Contact the team sooner if these effects are getting worse rather than improving.
And for anyone, someone who:
Mild fatigue, a manageable headache, and tenderness at pin sites can often be discussed during routine follow-up.
The article also says
Call the treatment centre about persistent bleeding, spreading redness, swelling, pus, or fever after frame placement.
Every sign above is quoted from the sofidex article on Stereotactic radiosurgery - gamma knife. If you are not sure, err upward — it is always reasonable to be checked.