18 F-FAPI-04 PET/CT Reveals Thoracic Duct Tumor Infiltration Secondary to Gastric Cancer.

We report ¹⁸F-FAPI-04 PET/CT findings of thoracic duct infiltration by tumor in a 61‑year‑old man with advanced gastric adenocarcinoma. The patient underwent ¹⁸F-FDG PET/CT for initial staging. Subsequently, an additional ¹⁸F-FAPI-04 PET/CT was performed as part of an exploratory imaging study, which revealed a larger primary tumor and more extensive nodal metastases than FDG. Notably, FAPI showed intense uptake in a subtle linear lesion along the left posterior mediastinum, suggestive of thoracic duct involvement, with unremarkable FDG uptake and CT morphology-an unreported observation in gastric cancer. Despite lacking histopathological confirmation, this finding highlights the superior sensitivity of FAPI PET/CT for detecting occult lymphatic spread, potentially enabling more accurate staging.