68Ga-fibroblast activation protein inhibitor PET/CT on gross tumour volume delineation for radiotherapy planning of oesophageal cancer.

Background and purpose: To compare 68Ga-fibroblast activation protein inhibitor (FAPI) and 18F-FDG PET/CT in imaging locally advanced oesophageal cancer, and evaluate the potential usefulness of 68Ga-FAPI PET/CT on gross target volume (GTV) delineation aimed at radiotherapy planning for oesophageal cancer as compared with contrast-enhanced CT (CE-CT) and 18F-FDG PET/CT. Materials and

Methods: Twenty-one patients with newly diagnosed oesophageal cancer who underwent both 18F-FDG and 68Ga-FAPI PET/CT scans were selected. GTVs of the primary tumours based on CE-CT (GTVCT), PET/CT, and CE-CT plus PET/CT were delineated. Gross tumour lengths were measured by GTVs and endoscopy and recorded.

Results: The 68Ga-FAPI PET showed significantly higher radiotracer uptake than 18F-FDG PET (median SUVmax 16.71 vs. 11.23; P = 0.002) in the primary tumours. SUV thresholds of FAPI ×20%, 30%, 40%, and FDG ×40% showed similar lesion lengths compared with that in endoscopic examination (P > 0.05). GTVCT demonstrated the largest volume (median: 48.80 mm3, range: 14.83-162.23 mm3) than PET-based GTVs. For PET/CT-guided complementary contouring of GTVCT, four patients (19%) were increased by FAPI ×20% and 30%, two patients (9.5%) were increased by FAPI ×40%, and only one patient was increased by FDG ×40%. Furthermore, the volume of GTV based on CE-CT plus FAPI ×20%, 30%, and 40% showed no significant difference with GTVCT and planning target volume based CE-CT plus FAPI-PET and meets the organ at risk standard.

Conclusion: The 68Ga-FAPI PET/CT methodology showed favourable tumour-to-background contrast in oesophageal cancer and might provide additional information for target volume delineation and help avoid tumour geographic misses.