Head-to-head comparison of Ga-68 FAPI and FDG PET-CT images in breast cancer patients

In this study, we aimed to evaluate patients who were pathologically diagnosed with breast cancer and had FDG PET-CT for primary staging purposes, but who had Ga-68 FAPI PET-CT imaging to obtain a definitive diagnosis, make a differential diagnosis and contribute to the staging in the presence of breast lesions, suspicious lymph nodes, organ metastases and, rarely, peritoneal metastases that could not be clearly differentiated using FDG PET-CT. Patients who underwent FDG PET-CT studies in our department for primary staging purposes between January 2024 and June 2025 were examined. In these patients, those with primary breast lesions, suspicious lymph nodes, bone and visceral organ metastases, and rarely peritoneal metastases that could not be clearly distinguished with FDG PET-CT were subjected to Ga-68 FAPI PET-CT studies. In both studies, SUV values were compared by manually drawing the patients' primary breast lesions. In addition, the SUV values and tumor background ratio (TBR) of metastases, newly appearing and becoming apparent lesions detected in FDG PET-CT and Ga-68 FAPI PET-CT studies were calculated and compared. At the same time, the total number of breast lesions, metastatic lesions, newly appearing and becoming apparent lesions were calculated and evaluated in both studies. While FDG PET was able to detect the primary lesion in the breast in 27 out of 29 patients in the patient-based evaluation, FAPI PET was able to detect the primary lesion in all 29 patients. SUVmax, SUVmean, TLG and TBR values obtained from the primary breast lesion in FAPI PET were calculated significantly higher than FDG PET (p<0.005). SUVmax, SUVmean, TLG and TBR values obtained from the lymph node in FAPI PET were found to be significantly higher than FDG PET (p<0.005). SUVmax and TBR values obtained from bone lesions in the FAPI PET study were found to be significantly higher than the values in the FDG PET study (p<0.005). Since normal liver involvement was less in the Ga-68 FAPI PET-CT study compared to FDG PET-CT, the TBR rate of liver metastases was higher and the lesions were easier to detect. In the lesion-based evaluation; FDG PET detected 44 lesions in the breast in 30 patients, while 49 breast lesions were observed in the FAPI PET study. While the number of lymph nodes considered metastatic in FDG PET was 108, 122 metastatic lymph nodes were observed in FAPI PET. FDG PET detected a total of 23 bone metastases in 3 patients, while FAPI PET detected a total of 41 bone metastases in 3 patients. 1 metastasis was detected in the liver in 1 patient in both imaging studies. As a result, it was concluded that Ga-68 FAPI PET-CT could be an alternative imaging method to obtain definitive diagnosis, make differential diagnosis and contribute to the staging in breast cancer patients who underwent FDG PET-CT study for staging purposes and in the presence of breast lesions and suspicious metastases that could not be clearly distinguished.