All cancers can cause personality changes both before and after diagnosis. Firstly, the tumor may affect the cognitive systems and change how the patient processes different emotions. This is something that may happen subconsciously and only their loved ones will notice.
After diagnosis, they may also experience personality changes because it’s a stressful situation. Increased aggression and depression are known results that impact many victims. They must have a strong and understanding support network that tries to help them through their battle. (via Science Direct)