A 62-year-old man with a history of locally advanced poorly differentiated adenocarcinoma of the gastroesophageal junction had received induction chemotherapy and concurrent chemoradiation. A 6-month follow-up FDG PET/CT revealed nearly resolved primary malignancy, but new hypermetabolic mediastinal lymph node metastasis and abnormal hypermetabolism in the cerebellum region. Brain MRI and lumbar puncture confirmed leptomeningeal metastasis.