68Ga-FAPI PET/CT as an Alternative to18F-FDG PET/CT in the Imaging of Invasive Lobular Breast Carcinoma

Accurate staging of invasive lobular carcinoma (ILC), a subtype of breast cancer, is vital for effective clinical management. Although ¹⁸F-FDG PET/CT is a commonly used tool, its efficacy varies across different histologic subtypes. To mitigate this challenge, our investigation delves into the potential utility of ⁶⁸Ga-fibroblast activation protein inhibitor (FAPI) PET/CT as an alternative for staging ILC, aiming to address a significant research gap using a more expansive patient cohort than the smaller samples commonly found in the existing literature. Methods: In this retrospective analysis, women diagnosed with primary ILC of the breast underwent both ¹⁸F-FDG PET/CT and ⁶⁸Ga-FAPI PET/CT. Both modalities were compared across all lesion locations with the used reference standard. The interval between scans was 1 wk, without any intervening treatments. Lesions were categorized visually, and tracer activity was analyzed using SUV_max, tumor-to-background uptake ratio, and uptake ratios. Both modalities were compared across various parameters, and statistical analysis was performed using SPSS 22.0. A P value of less than 0.05 was chosen to determine statistical significance. Results: The study included 23 female ILC patients (mean age, 51 y) with hormone-positive, human epidermal growth factor receptor type 2-negative tumors. Most (65%) had the luminal A subtype. ⁶⁸Ga-FAPI PET/CT outperformed ¹⁸F-FDG PET/CT, with higher tumoral activity and tumor-to-background uptake ratios (P < 0.001). Primary tumors showed significantly increased uptake with ⁶⁸Ga-FAPI PET/CT (P < 0.001), detecting additional foci, including multicentric cancer. Axillary lymph node metastases were more frequent and had higher uptake values with ⁶⁸Ga-FAPI PET/CT (P = 0.012). Moreover, ⁶⁸Ga-FAPI PET/CT identified more lesions, including bone and liver metastases. Pathologic features did not significantly correlate with imaging modalities, but a positive correlation was observed between peritumoral lymphocyte ratio and ⁶⁸Ga-FAPI PET/CT-to-¹⁸F-FDG PET/CT uptake ratios (P = 0.026). Conclusion: This study underscores ⁶⁸Ga-FAPI PET/CT's superiority over ¹⁸F-FDG PET/CT for ILC. ⁶⁸Ga-FAPI PET/CT excels in detecting primary breast masses, axillary lymph nodes, and distant metastases; can complement ¹⁸F-FDG PET/CT in ILC; and holds potential as an alternative imaging method in future studies.