Why is my father getting radiation to his brain when he has lung cancer?

Brain mets: Lung cancer likes to spread to other organs, one of which it usually like to go to is brain. Radiation is often used to treat the brain mets since chemotherapy does not penetrate well because of a blood brain barrier that exists naturally.

Spread to the brain : Lung cancer often spreads to the brain. So either it has already been seen on scans of the brain and is being treated or in small cell lung cancer we use it prophylactically. That is trying to prevent it from showing up, but that is only the case in small cell lung cancer.

Prevention possible: If he has no tumors known "in" his brain, data have shown that treating people with lung cancer prophylacticly gives a survival advantage. Small cell type is the most common type of lung cancer to get this type of radiation therapy.