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

Background and Purpose: To compare ⁶⁸ Ga-fibroblast activation protein inhibitor (FAPI) and ¹⁸ F-FDG PET/CT in imaging locally advanced oesophageal cancer, and evaluate the potential usefulness of ⁶⁸ Ga-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 ¹⁸ F-FDG PET/CT.
Materials and Methods: Twenty-one patients with newly diagnosed oesophageal cancer who underwent both ¹⁸ F-FDG and ⁶⁸ Ga-FAPI PET/CT scans were selected. GTVs of the primary tumours based on CE-CT (GTV _CT ), PET/CT, and CE-CT plus PET/CT were delineated. Gross tumour lengths were measured by GTVs and endoscopy and recorded.
Results: The ⁶⁸ Ga-FAPI PET showed significantly higher radiotracer uptake than ¹⁸ F-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). GTV _CT demonstrated the largest volume (median: 48.80 mm ³ , range: 14.83-162.23 mm ³ ) than PET-based GTVs. For PET/CT-guided complementary contouring of GTV _CT , 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 GTV _CT and planning target volume based CE-CT plus FAPI-PET and meets the organ at risk standard.
Conclusion: The ⁶⁸ Ga-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.
(Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)