Improved cancer detection in Waldeyer's tonsillar ring by ⁶⁸Ga-FAPI PET/CT imaging

Area of Science:

• Nuclear Medicine
• Oncology
• Radiochemistry

Background:

• Cancer of Unknown Primary (CUP) diagnosis relies heavily on imaging for primary tumor localization.
• Standard [18F]FDG PET/CT often struggles to differentiate small tumors from inflammation in Waldeyer's tonsillar ring.
• Fibroblast Activation Protein (FAP) is a promising target for cancer imaging due to its high expression in tumor stroma.

Purpose of the Study:

• To evaluate the efficacy of the novel PET tracer [68Ga]Ga-FAPI-4 for detecting primary tumors in the Waldeyer's tonsillar ring in CUP patients.
• To compare the diagnostic accuracy of [68Ga]Ga-FAPI-4 PET/CT with standard [18F]FDG PET/CT for primary tumor and lymph node metastasis detection.

Main Methods:

• Eight patients with suspected Waldeyer's tonsillar ring malignancy or CUP underwent both [18F]FDG and [68Ga]Ga-FAPI-4 PET/CT scans.
• Pre-operative tumor localization was performed using both imaging modalities.
• Post-surgical histopathology and immunohistochemistry were used to validate PET/CT findings.

Main Results:

• Both tracers successfully identified primary tumors in all eight patients.
• [18F]FDG PET/CT showed a higher detection rate for small ( < 7 mm) cervical lymph node metastases compared to [68Ga]Ga-FAPI-4 PET/CT.
• [68Ga]Ga-FAPI-4 PET/CT demonstrated significantly lower uptake in healthy tonsils and a higher tumor-to-background ratio (TBRmax) for primary tumors compared to [18F]FDG PET/CT.

Conclusions:

• Non-invasive [68Ga]Ga-FAPI-4 PET/CT imaging offers improved visual detection of malignant primaries in CUP, potentially reducing false positives and unnecessary biopsies.
• The detection rate of small lymph node metastases was inferior with [68Ga]Ga-FAPI-4, likely due to lower FAP expression in these lesions.
• [68Ga]Ga-FAPI-4 PET/CT holds promise for enhancing the diagnosis of CUP, particularly for primary tumor localization in the Waldeyer's tonsillar ring.