Response Assessment of Gastric Adenocarcinoma Using ⁶⁸Ga-FAPI-46 and 18 F-FDG PET/CT : Highlighting the Role of the Tumor Microenvironment in Therapy Resistance.

We report a 71‑year‑old woman with recurrent gastric adenocarcinoma presenting with colonic wall involvement on follow-up contrast-enhanced CT scan. Initial restaging 18 F‑FDG PET/CT demonstrated intense colonic and omental uptake, consistent with recurrent malignancy. After chemotherapy, follow‑up 18 F-FDG PET/CT showed marked metabolic improvement; however, concurrent ⁶⁸Ga‑FAPI PET/CT revealed persistent intense uptake in the colonic wall, peritoneum, and omentum, closely resembling that observed on the prior 18 F-FDG PET/CT, correlating with ongoing clinical deterioration. This discordance suggests differential treatment response between tumor cells and cancer‑associated fibroblasts. Our findings raise the hypothesis that persistent tumor microenvironment activity may underlie therapeutic resistance, highlighting the potential value of early integration of FAP‑targeted strategies alongside tumor‑directed therapies.