For oncological management or radiotherapy planning, reliable staging tools are essential. Recent development of quinoline-based ligands targeting cancer-associated fibroblasts demonstrated promising preclinical and clinical results. The current study aimed to evaluate the role of fibroblast activation protein inhibitors (FAPI)-positron-emission tomography (PET)/computed tomography (CT) for primary malignancies located within the lower gastrointestinal tract (LGT) as a very first clinical analysis.Methods: 68 Ga-FAPI-PET/CT was performed in a cohort of 22 patients with LGT including 15 patients with metastatic disease, 1 patient with suspected local relapse and 6 treatment-naïve patients. 68 Ga-FAPI-04 and 68 Ga-FAPI-46 uptake was quantified by standardized uptake values (SUV)max and (SUV)mean. After comparison with standard imaging, changes in tumor stage/ localization and (radio)oncological management were recorded. Results:The highest uptake of FAPI tracer was observed in liver metastases and anal cancer with a SUVmax of 9.1 and 13.9, respectively. Due to a low background activity in normal tissue, there was a high tumorto-background ratio of more than 3 in most lesions. In treatment-naïve patients, TNM was changed in 50% while for patients with metastases new findings occurred in 47%. In total, FAPI-imaging caused a high, medium and low change of (radio)oncological management in 19%, 33% and 29%, respectively. For almost every patient undergoing irradiation, target volume delineation was improved by 68 Ga-FAPI-PET/CT. Conclusion:The present study demonstrated that both primary and metastatic LGT were reliably detected by 68 Ga-FAPI-PET/CT leading to relevant changes in TNM status and (radio)oncological management. 68 Ga-FAPI-PET/CT seems to be a highly promising imaging agent for the diagnosis and management ofLGT, potentially opening new applications for tumor (re-)staging.

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