Fibroblast Activation Protein (FAPI) PET for diagnostics and advanced target volume delineation in head and neck cancer

Ziel/Aim Head and neck cancers mostly grow diffusely, making differentiation between tumor and healthy tissue extremely challenging. This analysis aims to introduce an approach of tumor detection and contouring for radiotherapy using visualization of cancer associated fibroblast: PET-CT with ⁶⁸Ga-radiolabeled inhibitors of Fibroblast Activation Protein (FAPI).

Methodik/Methods FAPI PET-CT was performed prior to radiotherapy on fourteen patients with head and neck cancers. First, for tissue biodistribution analysis, volume of interest was used to quantify SUV_mean and SUV_max in tumor and healthy parenchyma. Secondly, using four thresholds of 3, 5 and 7-fold increase of FAPI enhancement in the tumor as compared to normal tissue, four different gross tumor volumes (FAPI-GTV) were created automatically. These were compared to GTVs created with conventional CE-CT and MRI (CT-GTV).

Ergebnisse/Results The biodistribution analysis revealed that primary tumorous lesions showed high FAPI avidity (SUV_max 14.6 ± 4.4). In contrast, low background uptake was measured in healthy tissues of the head and neck region, (e.g. salivary glands: SUV_max 1.7 ± 0.3). Considering radiation planning, CT-GTV was of 27.3ml (range 9.1 – 266.5ml), whereas contouring with FAPI resulted in significantly different GTVs of 67.7ml (FAPI x3, p = 0.01), 22.1ml (FAPI x5, p = 0.04), 7.6ml (FAPI x7, p = 0.0001). Secondly, we merged FAPI-GTVs with CT-GTVs. This resulted in median volumes, that were, as compared to CT-GTVs, significantly larger with FAPI x3 (54.7ml, +201 % relative increase, p = 0.0005) and FAPI x5 (15ml, +54.9 %, p = 0.01).

Schlussfolgerungen/Conclusions We present first evidence of diagnostic and therapeutic potential of FAPI-ligands in head and neck cancer radiation oncology. This novel, imaging modality provides crucial information regarding tumor spread and generates high contrast images with eminent tumor-to-background ratios. Consequently, automated, precise and comprehensive target volume delineation for radiotherapy of head and neck cancers could be improved by adding FAPI PET-CT information.