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Carbon-11 methionine positron emission tomography (¹¹C-methionine PET; Met-PET) is widely used in neuro-oncology for preoperative tumor characterization, differentiating recurrence from radiation necrosis, and determining stereotactic biopsy targets. Although its role in gliomas has been well established, evidence supporting its value in germ cell tumors, particularly teratomas, remains limited. An 18-year-old man presented with diplopia, photophobia, and a headache. Magnetic resonance imaging (MRI) revealed a pineal tumor. Serum AFP was 31.5 pg/mL, serum β-HCG < 1.0 mIU/mL, and CSF PLAP 23.6 pg/mL. After three courses of CARE chemotherapy and local radiotherapy, gross total resection was performed at another hospital. He subsequently developed hydrocephalus and underwent endoscopic third ventriculostomy at our institution. MRI and Met-PET showed no residual lesions. Additional ICE chemotherapy resulted in the normalization of tumor markers. However, MRI performed 4 months after resection demonstrated a new enhancing lesion in the right posterior thalamus, and Met-PET revealed new uptake (SUVmax, 2.64; TNR, 2.08), suggesting recurrence. A second craniotomy achieved gross total resection of the whitish, elastic, well-demarcated lesion. Histopathology showed mainly granulomas with inflammatory changes, but no viable tumor cells. The postoperative course was uneventful, and no recurrence has been observed for 3 years. This case suggests that in germ cell tumors, particularly teratomas, Met-PET findings may be misleading and do not necessarily indicate viable tumor tissue. Further studies are required to clarify the clinical significance of Met-PET for teratomas.