ER status conversion and subsequent treatment: an assessment of negative ER expression detected by 18F-FES PET in metastatic breast cancer patients with ER-positive primary tumors.

Background: The 18F-fluoroestradiol positron emission tomography/computed tomography (18F-FES PET/CT) technique provides a convenient method to evaluate the overall estrogen receptor (ER) expression in metastatic breast cancer (MBC) patients. There are long debates on the characteristics and treatment strategy of patients with positive primary ER lesions but negative ER expression in metastatic disease. 18F-FES PET offers an opportunity to answer this question.

Objectives: This study aimed to characterize the primary ER-positive patients with advanced-stage FES negativity and investigate the real-world treatment decisions made by physicians subsequently, and compare the efficacy between different regimens.

Design: This observational cohort study was conducted at Fudan University Shanghai Cancer Center, enrolling breast cancer patients with ER-positive primary tumors who showed advanced-stage FES negativity.

Methods: Descriptive statistics were used in clinicopathologic characteristics and compared with a chi-square test or -test. In addition, progression-free survival (PFS) was estimated by the Kaplan-Meier method and compared by log-rank test.

Results: 16.6% (52/314) of patients with an ER-positive primary tumor had negative ER expression assessed by 18F-FES for MBC prior to receiving first-line systemic therapy, among whom adjuvant endocrine therapy was prevalently utilized (86.5%, 45/52). The rate of FES negativity in the advanced stage was negatively correlated with levels of ER expression of primary tumors. Chemotherapy (83.3%, 40/48) was the most common treatment strategy afterward, among which capecitabine monotherapy (62.5%, 25/40) was a dominant alternative. PFS was significantly prolonged with capecitabine alone other chemotherapy (median PFS: 13.14 6.21 months, = 0.029).

Conclusion: Negative conversion of ER in MBC detected by 18F-FES occurred frequently. Patients with lower ER expression in the primary lesion were more likely to have negative ER expression in the metastasis. In real-world clinical practice, most physicians primarily opted for chemotherapy, with capecitabine monotherapy being a commonly selected regimen.

Trial Registration: ClinicalTrials.gov identifier: NCT05797987.