Quantitative imaging of estrogen receptor expression in breast cancer with PET and 18F-fluoroestradiol

The PET compound F-18-fluoroestradiol (F-18-FES) has been developed and tested as an agent for the imaging of estrogen receptor (ER) expression in vivo. F-18-FES uptake has been shown to correlate with ER expression assayed in vitro by radioligand binding; however, immunohistochemistry (IHC) rather than radioligand binding is used most often to measure ER expression in clinical practice. We therefore compared F-18-FES uptake with ER expression assayed in vitro by IHC with both qualitative and semiquantitative measures. Methods: Seventeen patients with primary or metastatic breast cancer were studied with dynamic F-18-FES PET; cancer tissue samples, collected close to the time of imaging, were assayed for ER expression by IHC. For each tumor, partial-volume-corrected measures of F-18-FES uptake were compared with ER expression measured by 3 different ER scoring methods: qualitative scoring (0-3+), the Allred score (0-10), and a computerized IHC index. Results: There was excellent agreement (r = 0.99) between observers using IHC as well as the different methods of measuring ER content (P < 0.001). ER-negative tumors had F-18-FES partial-volume-corrected standardized uptake values of less than 1.0, whereas ER-positive tumors had values above 1.1. Correlation coefficients for the different measures of ER content and the different measures of F-18-FES uptake ranged from 0.57 to 0.73, with the best correlation being between the computerized IHC index and F-18-FES partial-volume-corrected standardized uptake values. Conclusion: Our results showed good agreement between F-18-FES PET and ER expression measured by IHC. F-18-FES imaging may be a useful tool for aiding in the assessment of ER status, especially in patients with multiple tumors or for tumors that are difficult to biopsy.