A 48-year-old woman was diagnosed with a brain tumor after experiencing gait disturbance and headache. A craniotomy was performed, and the brain tumor was discovered to be poorly differentiated adenocarcinoma or pleomorphic carcinoma. The pathological examination suggested primary lung cancer but whole-body computed tomography and positron emission tomography failed to detect the primary lesion. Three months after surgery, a local recurrence of the brain tumor was found, and the patient underwent reoperation that led to a similar histological diagnosis that revealed a high PD-L1 tumor proportion score (100%). She was treated with cisplatin + pemetrexed + pembrolizumab combination chemotherapy. After four courses of chemotherapy, a small nodule was resected from the upper lobe of the right lung, and no malignant tissue was detected on pathological examination. Pembrolizumab maintenance therapy was discontinued after two courses of chemotherapy due to adverse events. She has been well with no evidence of recurrence for 41 mont