Can radiation therapy still help me when my lung cancer has spread to the brain?

Yes. Radiation treatment can help treat some metastasis to the brain and assist in palliative therapy.
Yes. Radiation is used to treat spread of cancer to the brain to help neurological symptoms. If the spread is minimal such as one small lesion then radiosurgery with radiation can also help destroy the area of spread. Most of the time the treatment would be palliative but in certain cases disease eradication would be the goal.
Yes. Radiation can be used in a palliative setting when disease has spread from the lungs to the brain. Depending on the clinical situation you may be a candidate for stereotactic radiosurgery to just the site of disease as opposed to having whole brain radiation. You will need to discuss this with your radiation oncologist.