Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek emergency help for:
A first seizure or a seizure lasting several minutes
Sudden weakness, facial drooping, or difficulty speaking
Sudden severe confusion, collapse, or loss of consciousness
A severe headache with repeated vomiting or increasing drowsiness
Sudden major loss of vision
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Arrange a prompt medical review if you develop persistent or worsening headaches, unexplained changes in memory or behaviour, new vision problems, increasing weakness, poor balance, or difficulty speaking.
3
Does any of this apply?
Book an appointment
If you have completed treatment, report new neurological symptoms before your next routine follow-up.
The article also says
During chemotherapy, follow the treatment unit's instructions for fever or signs of infection.
Infection can become serious quickly when white blood cell levels are low.
Every sign above is quoted from the sofidex article on Primary lymphoma of the brain. If you are not sure, err upward — it is always reasonable to be checked.