Targeting fibroblast activation protein in newly diagnosed squamous cell carcinoma of the oral cavity - initial experience and comparison to [18F]FDG PET/CT and MRI

Purpose While [F-18]-fluorodeoxyglucose ([F-18]FDG) is the standard for positron emission tomography/computed tomography (PET/CT) imaging of oral squamous cell carcinoma (OSCC), diagnostic specificity is hampered by uptake in inflammatory cells such as neutrophils or macrophages. Recently, molecular imaging probes targeting fibroblast activation protein alpha (FAP), which is overexpressed in a variety of cancer-associated fibroblasts, have become available and might constitute a feasible alternative to FDG PET/CT. Methods Ten consecutive, treatment-naive patients (8 males, 2 females; mean age, 62 +/- 9 years) with biopsy-proven OSCC underwent both whole-body [F-18]FDG and [Ga-68]FAPI-04 (FAP-directed) PET/CT for primary staging prior to tumor resection and cervical lymph node dissection. Detection of the primary tumor, as well as the presence and number of lymph node and distant metastases was analysed. Intensity of tracer accumulation was assessed by means of maximum (SUVmax) and peak (SUVpeak) standardized uptake values. Histological work-up including immunohistochemical staining for FAP served as standard of reference. Results [F-18]FDG and FAP-directed PET/CT detected all primary tumors with a SUVmax of 25.5 +/- 13.2 (FDG) and 20.5 +/- 6.4 (FAP-directed) and a SUVpeak of 16.1 +/- 10.3 ([F-18]FDG) and 13.8 +/- 3.9 (FAP-directed), respectively. Regarding cervical lymph node metastases, FAP-directed PET/CT demonstrated comparable sensitivity (81.3% vs. 87.5%; P = 0.32) and specificity (93.3% vs. 81.3%; P = 0.16) to [F-18]FDG PET/CT. FAP expression on the cell surface of cancer-associated fibroblasts in both primary lesions as well as lymph nodes metastases was confirmed in all samples. Conclusion FAP-directed PET/CT in OSCC seems feasible. Future research to investigate its potential to improve patient staging is highly warranted.