How Can a Docter Tell if a Lipoma is Not Cancer?

i got a lipoma on the brain. told it fat tumr. i have blackouts and my heart goes fast and i sweat. i cry on coming around. told, "you having sizures" and then say, "no and it not to do with the lipoma." how can the docter say it not cancer. has they say it not. how will this infect my life. i am always tired after and before i go out of it.