Abstract PO4-20-10: Value of 16α-18F-fluoro-17β-fluoroestradiol PET imaging in ER-positive breast cancer: a case report

Abstract Introduction: An appropriate use criteria workgroup recently recommended the use of PET imaging utilizing the 16α-18F-fluoro-17β-fluoroestradiol (FES) radiotracer as a measure of disease progression and endocrine treatment response for ER-positive breast cancers. Past studies report limitations of decreased FDG uptake, the most common PET radiotracer, when measuring low Ki-67 scoring tumors; FES-PET may noninvasively address this shortcoming in FDG activity detection. Its benefit of detecting low-grade, ER-positive tumor activity was demonstrated in a patient with breast cancer who exhibited tumor growth only on FES-PET while FDG-PET showed no measurable disease. Clinical Case: A 57-year-old postmenopausal woman presented in December 2017 with de novo metastatic invasive ductal carcinoma that was ER+ (100%), PR+ (100%), and HER2- involving the right breast, bone, lungs, and internal mammary, subcarinal, and right hilar lymph nodes. She received nab-paclitaxel and radiation to the right breast, regional lymph nodes, and T11 bone. In March 2019, she underwent a right mastectomy with pathology that noted residual disease in the breast measuring <5 mm and was placed on exemestane in April 2019. She transferred her care to City of Hope and follow-up FDG-PET restaging showed no evidence of new metabolically active metastatic disease. Her cancer antigen 27.29 levels were also within normal limits. In the absence of measurable disease growth, she continued to be treated with single agent exemestane. Due to persistent back discomfort that she attributed to breast asymmetry, she requested the left breast to be removed. One week prior to surgery, both FDG-PET/CT and bloodwork inclusive of tumor markers were normal. She underwent a left mastectomy in March 2023. Pathology identified an incidental 1.4 cm lymph node in the 2000 g specimen that was ER+ (>95%), PR-, and HER2- with a Ki-67 of 1-3%. Somatic gene testing of the tumor revealed an ESR1 D538G mutation. As her progressive disease was not evident on previous FDG-PET imaging, an FES-PET/CT was performed and demonstrated uptake in the ilium, T10, and T12 bones that was not identified on FDG-PET/CT. With objective evidence of evaluable disease, exemestane was discontinued and fulvestrant and ribociclib were initiated. Clinically, she has evidence of early response to therapy with improvement in back pain. Discussion: This case demonstrates the value of FES-PET/CT in identifying disease that was not appreciated on FDG-PET/CT or by cancer antigen 27.29 monitoring. Additionally, ESR1 status has not been consistently reported in studies of patients undergoing FES-PET imaging that investigate the radiotracer’s value in predicting response to subsequent endocrine therapy. Our patient has a documented ESR1 mutation, her disease was clearly identified only on FES-PET/CT, and FES uptake predicted for response to subsequent endocrine therapy. Incorporating FES-PET in routine staging should be considered in patients with metastatic breast cancer that is low-grade and ER-positive. Further evidence of response prediction in patients with ESR1 mutations will enhance the utility of FES-PET in the clinical management of these patients. Citation Format: Megan Wong, Veronica Jones, Dave Yamauchi, Joanne Mortimer. Value of 16α-18F-fluoro-17β-fluoroestradiol PET imaging in ER-positive breast cancer: a case report [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-20-10.