False-positive cervical lymph nodes on FAPI PET/CT after neoadjuvant chemotherapy for breast cancer: a case report.

A 49-year-old woman with cT3N1M0 stage IIIA HER2-positive invasive breast cancer is described in this retrospective case report. After six cycles of TCHP neoadjuvant chemotherapy, breast MRI confirmed that the primary lesion and axillary lymph nodes achieved radiological complete response. However, newly enlarged lymph nodes in levels II and III of the left neck were detected by neck ultrasound and contrast-enhanced CT. Subsequent 18F-FAPI-04 PET/CT revealed significantly elevated tracer uptake in these cervical lymph nodes, with a maximum standardized uptake value (SUVmax) of 18.2, which was highly suspicious for metastasis. Ultrasound-guided core needle biopsy and postoperative histopathological examination finally confirmed that these cervical lymph nodes were benign reactive lymphoid hyperplasia with focal necrosis and inflammatory cell infiltration, and the patient achieved pathological complete response in both breast and axillary regions without any lymph node metastasis. This case reveals that despite the high sensitivity of FAPI PET/CT in detecting malignant tumor lesions, benign post-treatment inflammatory processes can induce significant FAP upregulation, leading to typical false-positive results. Clinical interpretation of FAPI PET/CT findings must be closely integrated with the patient's treatment history and multimodality examinations, and histopathology should remain the gold standard for definitive lesion characterization to avoid unnecessary misdiagnosis and overtreatment.