FAPI-positron emission tomography for radiotherapy in head and neck cancer: applications and future directions

Radiotherapy is a cornerstone in the treatment of head and neck squamous cell carcinoma (HNSCC). Imaging plays an important role in target delineation in order to achieve adequate target dose and on the other hand limit radiation-induced toxicity to surrounding healthy tissues. Advancements in imaging techniques, including functional imaging with positron emission tomography (PET), have improved accurate target delineation, allowed for adaptive radiotherapy, and improved response evaluation. This review explores the potential role of fibroblast activation protein inhibitor (FAPI)-PET/CT in radiotherapy planning for HNSCC, comparing its efficacy to the current standard, fluorine-18 fluorodeoxyglucose (18F-FDG) PET/CT. FAPI-PET/CT is an emerging imaging modality that for pretreatment imaging may increase lesion detection, especially near the skull base. For target delineation FAPI PET/CT may enhance the specificity of tumor delineation, particularly in distinguishing tumor tissue from normal tissue. Current evidence suggests that FAPI-PET increases the GTV volume, but whether this represents more accurate tumor delineation or overestimation remains unclear. For response monitoring after treatment, there is no evidence in head and neck cancer, but for rectal cancer FAPI-PET/CT showed promising results in detection of residual disease after neo-adjuvant radiotherapy. For now, FAPI-PET/CT could be used as additional imaging, and should not be used as replacement for other imaging modalities. There is an urgent need for prospective validation studies to determine the clinical impact of FAPI-PET on radiotherapy planning and response monitoring.