Radiation therapy has become a mainstream therapy for brain metastasis, especially, for patients with multiple lesions. With this treatment, median survival is 3-6 months (some data says 2-3 months) depending on number of lesions (patients with solitary lesions have a better prognosis), their radiosensitivity, and the status of systemic disease.
I found some information about case similar to your mum.
It's about a patient with stage IIIb non-small cell lung cancer and brain metastasis, who has received Gefitinib, a drug used in the treatment of certain types of cancer.
The patient experienced a dramatic clinical improvement within 1 week.
Follow-up magnetic resonance imaging of the brain 1 month later showed decreases in both the size and number of brain metastases and the patient remained well 9 months after initiation of gefitinib.