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

Abstract

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. <b>Methods:</b> 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 <i>P</i> value of less than 0.05 was chosen to determine statistical significance. <b>Results:</b> 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 (<i>P</i> &lt; 0.001). Primary tumors showed significantly increased uptake with ⁶⁸Ga-FAPI PET/CT (<i>P</i> &lt; 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 (<i>P</i> = 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 (<i>P</i> = 0.026). <b>Conclusion:</b> 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.

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