Hormonal receptor positive human epidermal receptor 2 negative (HR⁺/HER2^-) is the commonest molecular subtype of breast cancer (BC). Patients with HR⁺/HER2^- BC may manifest clinically a late recurrence whose BC metastasizes 10-15 years post-operatively. We report one case who presented with pulmonary mass in upper lobe of lung and Horner syndrome 16 years after BC surgery. FDG PET/CT suggested pulmonary malignancy but could not differentiate between primary or metastatic cancer when invasive biopsy was quite risky. Novel ¹⁸F-FES PET/CT facilitated the non-invasive functional diagnosis of estrogen-receptor positive (ER+) pulmonary metastasis of BC, and the patient experienced partial response (PR) after CDK4/6 inhibitor and aromatase inhibitor as endocrine therapy. This article reviews the diagnosis and treatment process of this case, to provide guidance for non-invasive global evaluation of ER status among metastatic HR⁺/HER2^- BC patients with ¹⁸F-FES PET/CT.