177Lu-FAPI-2286 Therapy in Metastatic Breast Cancer: A Case Study Highlighting the Potential Risk of Tumor Dedifferentiation and Hormonal Marker Elevation.

This case report presents a 42-year-old woman with metastatic breast cancer refractory to multiple conventional treatments, including surgery, chemotherapy, radiotherapy, hormonal therapy, and bilateral oophorectomy. As a salvage, she received 177Lu-FAPI-2286 radioligand therapy. After the first cycle, an unexpected serum β-hCG elevation was observed-unusual given her history of total hysterectomy and bilateral salpingo-oophorectomy. Three hypotheses were proposed: assay interference (heterophile antibodies), emergence of second neoplasm secreting β-hCG, and tumor dedifferentiation with ectopic β-hCG expression. Unfortunately, rapid clinical progression precluded further evaluation. This case underscores the importance of awareness regarding tumor dedifferentiation and biomarker changes during novel theranostic treatments with 177Lu-FAPI, emphasizing the need for vigilant multidisciplinary monitoring.