MRI appearance of Treated lesions: 

external beam: early response to external beam irradiation, case 2 after external beam, but regrowth after external beam here , another case two months after external beam here. Lung cancer brain mets two months after external beam here and here.

gamma knife response after 3 weeks and same case after 4 months, gamma knife mets after two months here and here  another case at 4 months here. Breast mets at two and three month here and here. Breast cancer single met at 2 months here.  Ovary cancer mets at two months here and here. A classic problem with brain mets is the development of new lesions (see here)