Head-to-Head Evaluation of F-FES and F-FDG PET/CT in Metastatic Invasive Lobular Breast Cancer.

Invasive lobular carcinoma (ILC) demonstrates lower conspicuity on F-FDG PET than the more common invasive ductal carcinoma. Other molecular imaging methods may be needed for evaluation of this malignancy. As ILC is nearly always (95%) estrogen receptor (ER)-positive, ER-targeting PET tracers such as 16α-F-fluoroestradiol (F-FES) may have value. We reviewed prospective trials at Memorial Sloan Kettering Cancer Center using F-FES PET/CT to evaluate metastatic ILC patients with synchronous F-FDG and F-FES PET/CT imaging, which allowed a head-to-head comparison of these 2 PET tracers. Six prospective clinical trials using F-FES PET/CT in patients with metastatic breast cancer were performed at Memorial Sloan Kettering Cancer Center from 2008 to 2019. These trials included 92 patients, of whom 14 (15%) were of ILC histology. Seven of 14 patients with ILC had F-FDG PET/CT performed within 5 wk of the research F-FES PET/CT and no intervening change in management. For these 7 patients, the F-FES and F-FDG PET/CT studies were analyzed to determine the total number of tracer-avid lesions, organ systems of involvement, and SUV of each organ system for both tracers. In the 7 comparable pairs of scans, there were a total of 254 F-FES-avid lesions (SUV, 2.6-17.9) and 111 F-FDG-avid lesions (SUV, 3.3-9.9) suggestive of malignancy. For 5 of 7 (71%) ILC patients, F-FES PET/CT detected more metastatic lesions than F-FDG PET/CT. In the same 5 of 7 patients, the SUV of F-FES-avid lesions was greater than the SUV of F-FDG-avid lesions. One patient had F-FES-avid metastases with no corresponding F-FDG-avid metastases. There were no patients with F-FDG-avid distant metastases without F-FES-avid distant metastases, although in one patient liver metastases were evident on F-FDG but not on F-FES PET. F-FES PET/CT compared favorably with F-FDG PET/CT for detection of metastases in patients with metastatic ILC. Larger prospective trials of F-FES PET/CT in ILC should be considered to evaluate ER-targeted imaging for clinical value in patients with this histology of breast cancer.