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Primary signet ring cell adenocarcinoma of the breast is a rare neoplasm. Similar to other mucinous neoplasms, it has low FDG avidity and poor prognosis compared with the traditional ductal/lobular carcinomas. We present a case of a 40-year-old woman with signet ring cell adenocarcinoma and skeletal fluorosis showing higher uptake of 68Ga–prostate-specific membrane antigen in the primary lesion and the metastatic sites compared with 18F-FDG on PET/CT imaging. Additionally, diffuse increased prostate-specific membrane antigen uptake was noticed in the entire skeleton, likely due to skeletal fluorosis.