“Lésion” generally means “lesion” or ”wound”, but as far as I know, never “tumor”.

“Lésion” can be associated with the word “cerveau” when speaking of “lésion cérébrale”, which means (quite approximately, I’m no doctor) “brain damage”.

“lésion” means something going wrong (damage) with any biological tissue causing it not to functional normally. It’s used with brain, muscles, skin or any tissue that lives.