Impact of FES-PET imaging in the management of estrogen receptor–positive breast cancer: Real-world experience.

1080 Background: F18 labeled estradiol PET imaging (FES-PET) is endorsed by NCCN guidelines to identify advanced/recurrent estrogen receptor positive (ER+) breast cancer, especially invasive lobular disease. As an adjunct to conventional imaging, FES-PET may provide information about estrogen dependence in lesions that are not readily biopsied. We reviewed the use of FES-PET imaging at City of Hope, Southern California and how it impacted treatment. Methods: This is a retrospective chart review study of pts undergoing FES-PET imaging in the management of ER+ breast cancer. Demographic data, indications for FES-PET imaging, and impact on subsequent care were extracted from the electronic medical record. Results: Between 1/21/22 and 12/31/25, 191 patients with a median age of 52 (Range 32-89) underwent an initial FES-PET scan at City of Hope and its Southern California Network. All had a diagnosis of estrogen receptor + (ER+) breast cancer; 124 were Invasive Ductal Carcinoma (IDC), 59 Invasive Lobular Carcinoma (ILC), 7 mixed IDC with ILC features, and 1 adenocarcinoma. FES-PET was used to stage “high-risk” local disease in 85, 76 continued their initial treatment plan, 8 were found to have metastatic disease, and 1 underwent additional surgical resection for residual disease in the breast. Of 102 pts. with metastatic disease, 58 were FES positive; 44/50 responded to endocrine therapy, 7 received chemotherapy and 1 refused treatment; 44 were FES negative and 23 received chemotherapy, 10 had no response to endocrine therapy, and 11 received no further treatment. Four patients had synchronous primary tumors and FES was used to distinguish lesions that were attributable to breast cancer. Of the patients with metastatic FES+ disease, 33 underwent serial FES-PET imaging; 3 were part of a clinical trial assessing radiation therapy in oligometastatic disease, 3 had been on prolonged treatment with chemotherapy and FES-PET was obtained to assess potential for reintroduction of endocrine therapy, 27 had response evaluation to aromatase inhibitor therapy where conventional staging did not adequately assess the measurable/evaluable disease as well as FES-PET. Conclusions: Our data support the use of FES-PET which altered treatment in 11% of patients with high-risk localized and identified patients for whom chemotherapy was the appropriate choice of systemic therapy. In select pts, serial FES-PET imaging was more helpful in monitoring disease response to AI-based therapy than conventional imaging and should be considered in future guideline recommendations.