Patient Characteristics
| Characteristic | All patients, n = 91 |
| Median age (y) | 61 (range, 33–83) |
| Sex (n) | |
| Female | 90 (99%) |
| Male | 1 (1%) |
| Median time from primary tumor to metastasis (y) | 5 (range, 0–22) |
| Primary tumor immunohistochemistry receptor status | |
| ER (n) | |
| Positive | 85 (97%) |
| Negative* | 3 (3%) |
| Unknown* | 3 |
| PR (n) | |
| Positive | 64 (83%) |
| Negative | 13 (17%) |
| Unknown | 14 |
| Human epidermal growth factor receptor 2 (n) | |
| Positive | 11 (18%) |
| Negative | 50 (82%) |
| Unknown | 30 |
| Second primary breast malignancy | 17 (19%) |
| ER antagonist use before 18F-FES PET (n) | |
| Median stop duration (wk) | 5.2 (range, 3–11) |
| Receptor discordance primary tumor and metastasis | |
| ER (n)/total patient with known receptor status | |
| Concordant | 29/35 (83%) |
| Positive to negative | 5/35 (14%) |
| Negative to positive | 1/35 (3%) |
| PR (n) | |
| Concordant | 23/29 (79%) |
| Positive to negative | 5/29 (17%) |
| Negative to positive | 1/29 (3%) |
| Human epidermal growth factor receptor 2 | |
| Concordant | 17/18 (94%) |
| Positive to negative | 1/18 (6%) |
| Negative to positive | 0/18 (0%) |
↵* Metastatic lesion or secondary primary breast cancer ER-positive on immunohistochemistry.