Non-small cell lung cancer (NSCLC) commonly presents with metastasis to the brain. When brain metastases are treated with stereotactic radiosurgery (SRS), longitudinal imaging to monitor treatment response may identify radiation necrosis, metastasis progression, and/or another primary brain malignancy. A 60-year-old female with metastatic NSCLC involving the brain underwent treatment with systemic therapy and SRS. While some brain metastases resolved, two remaining sites evolved to resemble radiation necrosis on magnetic resonance imaging and spectroscopy. One of those sites was later confirmed to be radiation necrosis after receding with steroids and bevacizumab. The other lesion continued to enlarge and was then surgically resected, pathologically proven to be a gliosarcoma. When scan findings diverge among multiple treated disease sites, imaging should be cautiously interpreted in conjunction with clinical information as well as early surgical consultation for biopsy consideration, especially when there is suspicion of unusual or superimposed pathologies.