Head-to-Head Comparison of [68 Ga]Ga-FAPI-46-PET/CT and [18F]F-FDG-PET/CT for Radiotherapy Planning in Head and Neck Cancer

Abstract

Introduction: In head and neck cancers (HNCs), fibroblast activation protein (FAP) is expressed by cancer-associated fibroblasts (CAFs) in the tumor microenvironment. Preliminary evidence suggests that detection and staging is feasible with positron emission tomography (PET/CT) imaging using [⁶⁸ Ga]-radiolabeled inhibitors of FAP ([⁶⁸ Ga]Ga-FAPI-46) in HNCs. This study aims to compare [⁶⁸ Ga]Ga-FAPI-46 PET/CT and [¹⁸F]-fluorodeoxy-D-glucose ([¹⁸F]F-FDG) PET/CT with a focus on improved target volume definition and radiotherapy planning in patients with HNC referred for chemoradiation.

Methods: A total of 15 patients with HNCs received both [⁶⁸ Ga]Ga-FAPI-46 PET/CT and [¹⁸F]F-FDG PET/CT with a thermoplastic mask, in addition to initial tumor staging by conventional imaging with contrast-enhanced CT and/or MRI. Mean intervals between FAPI/FDG and FAPI/conventional imaging were 4 ± 20 and 17 ± 18 days, respectively. Location and number of suspicious lesions revealed by the different procedures were recorded. Subsequently, expert-generated gross tumor volumes (GTVs) based on conventional imaging were compared to those based on [¹⁸F]F-FDG and [⁶⁸ Ga]Ga-FAPI-46 PET/CT to measure the impact on subsequent radiation planning.

Results: All patients had focal FAPI uptake above background in tumor lesions. Compared to FDG, tumor uptake (median SUVmax 10.2 vs. 7.3, p = 0.008) and tumor-to-background ratios were significantly higher with FAPI than with FDG (SUVmean liver: 9.3 vs. 3.2, p < 0.001; SUVmean bloodpool: 6.9 vs. 4.0, p < 0.001). A total of 49 lesions were recorded. Of these, 40 (82%) were FDG⁺ and 41 (84%) were FAP⁺. There were 5 (10%) FAP⁺/FDG^- lesions and 4 (8%) FAP^-/FDG⁺ lesions. Volumetrically, a significant difference was found between the GTVs (median 57.9 ml in the FAPI-GTV, 42.5 ml in the FDG-GTV, compared to 39.2 ml in the conventional-GTV). Disease stage identified by FAPI PET/CT was mostly concordant with FDG PET/CT. Compared to conventional imaging, five patients (33%) were upstaged following imaging with FAPI and FDG PET/CT.

Conclusion: We demonstrate that [⁶⁸ Ga]Ga-FAPI-46 -PET/CT is useful for detecting tumor lesions in patients with HNCs. There is now a need for prospective randomized studies to confirm the role of [⁶⁸ Ga]Ga-FAPI-46 PET/CT in relation to [¹⁸F]F-FDG PET/CT in HNCs and to evaluate its impact on clinical outcome.

Keywords: FAPI; FDG/PET; Head and neck cancer; PET-based; Radiotherapy planning.