99mTc-FAPI-46 Uptake in Simultaneous Occurrence of Benign Thyroid Nodule and Mixed-Mucinous-Invasive Ductal Breast Carcinoma.

We present a case of a 65-year-old woman diagnosed with mixed mucinous-invasive ductal carcinoma, a rare subtype of breast cancer. Utilizing 99mTc-FAPI-46 scintigraphy, we observed a high target-to-background ratio in the breast mass and metastatic axillary lymph nodes. Notably, a benign follicular nodule was also detected in the thyroid which showed absent 99mTc-FAPI uptake. Our findings suggest that 99mTc-FAPI-46 shares similar characteristics with 68Ga-FAPI and may outperform 18F-FDG PET/CT in mucinous breast cancer. This case highlights the potential of FAPI as a predictive biomarker for malignancy and its role in benign findings.