Imaging of Tumor Stroma Using ⁶⁸Ga-FAPI PET/CT to Improve Diagnostic Accuracy of Primary Tumors in Head and Neck Cancer of Unknown Primary: A Comparative Imaging Trial

Box plots of SUV_max (A) and TLR (B) detected on ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT. Primary tumors showed significantly higher semiquantitative uptake of ⁶⁸Ga-FAPI than ¹⁸F-FDG (P < 0.001). ¹⁸F-FDG outperformed ⁶⁸Ga-FAPI PET/CT in detecting lymph node metastases, with significantly higher SUV_max (P < 0.001). In terms of TLR, lymph node and bone metastases presented more favorable uptake of ⁶⁸Ga-FAPI than ¹⁸F-FDG (P < 0.001). ***P < 0.001. NS = no significance.

PET/CT and MR images of 72-y-old woman (patient 30) pathologically confirmed with metastatic squamous cell carcinoma of right neck. (A) ¹⁸F-FDG PET images (left and top) and PET/CT images (bottom), shown in coronal, sagittal, and axial views (from left to right), demonstrated metastatic lymph node of right neck with intensive metabolic activity (black arrow, SUV_max, 30.1) but presented no evidence for primary tumor. (B) ⁶⁸Ga-FAPI PET images (top and right) and PET/CT images (bottom), shown in axial, sagittal, and coronal views (from left to right), also detected metastatic lymph node with high ⁶⁸Ga-FAPI activity (black arrow, SUV_max, 16.3). There was intensive uptake of ⁶⁸Ga-FAPI in palate (red arrow, SUV_max, 11.3). (C) T1-weighted, T2-weighted, and contrast-enhanced T1-weighted MRI also presented no evidence for primary tumor. Subsequent surgery confirmed mucoepidermoid carcinoma of palate. C+ = contrast-enhanced.