Stereotactic radiosurgery (SRS) or radiotherapy (SRT) is commonly used to treat brain metastasis (BM). While effective in achieving tumor control, a significant number of BM patients exhibit lesion growth on follow-up MR imaging after SRS which cannot reliably distinguish between tumor growth, radiation necrosis (RN) or a mixture of both. SRS retreatment is a therapeutic option for tumor regrowth but contraindicated for RN. Here, we describe an instructive case of MRI progression of a breast BM previously treated by SRS where [18F] fludeoxyglucose-positron emission tomography (FDG-PET) proved useful to anatomically delineate metabolically active tumor from RN for re-treatment planning. Post-treatment FDG-PET and MRI studies indicated decreased uptake and enhancement respectively following treatment. This case study underscores the biological heterogeneity underlying MRI based BM progression after prior SRS and the potential utility of FDG-PET to guide treatment planning for repeat SRS.