Case Report: FAPI PET/CT detected occult contralateral DCIS and supraclavicular metastasis, guiding neoadjuvant therapy in synchronous bilateral breast cancer

Background Accurate staging is critical for breast cancer management. While 18 FFDG PET/CT is the standard for metabolic imaging, it has limitations in detecting tumors with low glucose metabolism and small regional metastases. Fibroblast activation protein inhibitor (FAPI) PET/CT targets cancer-associated fibroblasts and offers a novel diagnostic avenue. Case Presentation A female patient presented with left-sided inflammatory breast cancer. Conventional 18 FFDG PET/CT showed uptake in the left breast but missed regional spread and contralateral disease. Conversely, a supplemental 18 FAlF-NOTA-FAPI-04 PET/CT revealed intense uptake in the left breast (SUVmax 19.45) and detected occult metastases in the left axillary (SUVmax 15.12) and supraclavicular lymph nodes (SUVmax 14.83). Crucially, it also identified a high-uptake lesion in the right breast (SUVmax 15.06), which biopsy confirmed as DCIS, upstaging the diagnosis to synchronous bilateral breast cancer. Following neoadjuvant chemotherapy, follow-up FAPI PET/CT demonstrated that uptake in the supraclavicular and axillary nodes dropped to background levels, predicting a complete response (CR). Postoperative pathology confirmed CR in the lymph nodes. Conclusion This case highlights the superior sensitivity of FAPI PET/CT over FDG PET/CT in detecting low-metabolic malignancies like DCIS and accurately staging regional nodal involvement (including supraclavicular nodes). FAPI PET/CT played a decisional role in upstaging the patient and monitoring neoadjuvant therapy response.