We report a case of gastric cancer that was incidentally found on 3\u27deoxy-3\u27F-18fluorothymidine (FLT) positron emission tomography (PET). FLT PET has the potential utility for noninva-sively measuring cellular proliferation in vivo in many malignant tumors. A 68-year-old male with squamous cell carcinoma of the left upper lung was treated by carbon ion radiotherapy (CIRT; 52.8 GyE./4 fractions). To evaluate the response of the lung cancer to CIRT, an FLT PET/CT study was performed before and 3 months after CIRT. Very intense focal uptake of FLT in the gastric wall on PET images obtained 3 months after CIRT raised the suspicion of gastric malignancy. Endoscopic examination revealed an advanced gastric cancer (tubular adenocarcinoma, well to moderately differ-entiated) at the site of intense FLT uptake
