Effects of fulvestrant on the ERs may be evaluable by molecular imaging using positron emission tomography with the ER-specific FES tracer. In this study we will determine the utility of FES-PET in the prediction of response to fulvestrant 500 mg in women with estrogen positive metastatic breast cancer

More than 70% of patients with metastatic breast cancer present with hormone receptor positive disease and for whom hormonal therapy is the preferred treatment approach. First-line treatment recommendations for women with hormone receptor positive locally advanced or metastatic disease includes a generation aromatase inhibitors or tamoxifen. Fulvestrant, a 17β-estradiol analog, is an antiestrogen that suppresses estrogen signaling by binding to ER and inducing oestrogen receptor degradation and has estrogen antagonistic activity but no estrogen agonistic effect. Fulvestrant has been shown to have superiority over other endocrine therapy in a series of randomized controlled trials.

The whole-body imaging of the availability of ER using FES-PET may prove valuable to evaluate the effects of fulvestrant on the ER non-invasively in individual patients. This potentially allows early prediction of therapy efficacy to fulvestrant in women with estrogen positive metastatic breast cancer.

In this pilot-study we will evaluate 35 patients who received fulvestrant as treatment for metastatic breast cancer. All patients will undergo FES-PET/CT at baseline and FES-PET after 28 days. Whenever possible, tumor biopsies will be performed to correlate to FES-PET results.

Inclusion Criteria:

1. Patients with a history of histological proven ER-positive primary breast cancer and, whenever available, histological proven ER-positive recurrence.
2. No previous fulvestrant treatment
3. ER-antagonists should be discontinued for 5 weeks prior to FES-PET. The use of aromatase inhibitors is allowed.
4. Age > 18
5. ECOG 0-2
6. Life expectancy > 6 months
7. Informed consent obtained
8. Able to comply with the protocol

Exclusion Criteria:

1. Presence of life-threatening visceral metastases
2. Evidence of central nervous system metastases
3. > 3 lines of endocrine therapy for metastatic disease
4. Isolated liver metastasis (high FES uptake by normal liver)