Comparison of ¹⁸F-FDG PET/CT and ¹⁸F-FAPI PET/CT in Systemic Staging of Newly Diagnosed Breast Cancer

The purpose of this study was to compare the performance of ¹⁸F-FAPI PET/CT and ¹⁸F-FDG PET/CT in systemic staging of newly diagnosed breast cancer.

Breast cancer patients with initial clinical stage IIB-IIIC who have consequently underwent both ¹⁸F-FAPI and ¹⁸F-FDG PET/CT from June 2022 to June 2023 were retrospectively analyzed. New clinical stage was assigned to each patient if unsuspected level III axillary and extraaxillary regional lymph node metastases (URNM) and/or distant metastases were disclosed after PET/CT. Sensitivity for both tests was calculated on patient basis and lesion basis using histology or follow-up imaging as reference standard.

38 patients were included. The overall upstaging rate was 47.4% for ¹⁸F-FAPI PET/CT (18/38) and 34.2% for ¹⁸F-FDG PET/CT (13/38). The rate of distant metastases disclosed by ¹⁸F-FAPI PET/CT was 5.5% in stage IIB patients, 30% in stage IIIA patients, 50% in stage IIIB patients, and 75% in stage IIIC patients. On patent-based analysis, the sensitivity of ¹⁸F-FAPI PET/CT was significantly different from that of ¹⁸F-FDG PET/CT in detecting URNM [100% (13/13) vs 53.8% (7/13), (P = 0.031)], but not for distant metastases [100% (10/10) vs 90% (9/10), (P = 1.000)]. On lesion-based analysis, the sensitivity of ¹⁸F-FAPI PET/CT was significantly higher than that of ¹⁸F-FDG PET/CT in detecting URNM [97.6% (41/42) vs 52.4% (22/42), (P < 0.001)] and distant metastases [98.1% (51/52) vs 67.3% (35/52), (P < 0.001)].

¹⁸F-FAPI PET/CT leads to significant upstaging in newly diagnosed breast cancer, in a rate higher than ¹⁸F-FDG PET/CT. The significantly higher lesion-based sensitivity in unsuspected metastases implies a future role of ¹⁸F-FAPI PET/CT in evaluation of metastatic disease burden.