Positron emission tomographic assessment of "metabolic flare" to predict response of metastatic breast cancer to antiestrogen therapy

We have investigated whether increased tumor uptake of fluorine-18 fluorode oxyglucose (FDG) detected with positron emission tomography (PET) early aft er initiating tamoxifen therapy ("metabolic flare") predicts a hormonally r esponsive breast cancer. Eleven postmenopausal women with biopsy-proved est rogen receptor-positive (ER+) metastatic breast cancer were studied by PET with FDG and 16 alpha[F-18]fluoro-17 beta-estradiol (FES) before and 7-10 d ays after initiation of tamoxifen therapy. FDG and FES uptake was evaluated semiquantitatively in 21 lesions. The PET results were correlated with fol low-up evaluation, continued until the patient became unresponsive to hormo ne therapy (3-24 months). There were seven responders and four nonresponder s based on clinical follow-up. None of the responders had a clinical flare reaction, but all demonstrated metabolic flare, with a mean +/- standard de viation increase in tumor standardized uptake value (SUV) for FDG of 1.4+/- 0.7. No evidence for flare was noted in the nonresponders (change in SUV fo r FDG -0.1+/-0.4; P = 0.008 vs; re-spenders). The degree of ER blockade by tamoxifen was greater in responders (mean decrease in SUV 2.7+/-1.7) than i n nonresponders (mean decrease 0.8+/-0.5) (P = 0.04). The lesions of respon ders had higher base line SUVs for FES than did those of three of four nonr esponders (greater than or equal to 2.2 vs less than or equal to 1.7). The findings of a metabolic flare by FDG-PET and the degree of ER blockade by F ES-PET early after institution of tamoxifen treatment appear to predict res ponsiveness to antiestrogen therapy in patients with ER+ metastatic breast cancer.