Health-related quality of life and estimation of the minimally important difference in the Functional Assessment of Cancer Therapy-Endocrine Symptom (FACT-ES) score in postmenopausal ER-positive, HER2- negative metastatic breast cancer with low sensitivity to endocrine therapy

Background: The HORSE-BC study previously demonstrated that second-line endocrine therapy (ET) for patients with acquired endocrine-resistant metastatic breast cancer (MBC) still provided a clinically meaningful benefit. Herein, we investigated the health-related quality of life (HR-QOL) in the HORSE-BC study. Methods Patients with acquired endocrine-resistant MBC who were scheduled for second-line ET were recruited. The HR-QOL was assessed at baseline, and 1 and 3 months after second-line ET initiation. To investigate the minimally important difference (MID) in the Functional Assessment of Cancer Therapy-Endocrine Symptoms (FACT-ES), we evaluated the means and standard deviations for the distribution-based method, and differences in the change in HR-QOL for the anchor-based method. We also investigated the association between FACT-ES total scores and clinical benefit. Results Overall, 56 patients were enrolled. Of these, 47 were analyzed. When defined as 1/3 standard deviation estimates based on the distribution method, the calculated MID was 5.9. The MIDs of the FACT-ES total scores based on the anchor method were 7.7 for decline and 4.1 for improvement. The MID decline proportions were 6.1% and 14.7% lower in patients who experienced clinical benefits than in those who did not at 1 and 3 months, respectively. The ratios of MID improvement in patients who experienced clinical benefits were 18.3% and 3.2% higher, respectively; the mean change in the FACT-ES total score from baseline improved in patients who experienced clinical benefits. Conclusions Maintaining the HR-QOL as determined by FACT-ES may be associated with clinical benefits in patients with acquired endocrine-resistant MBC treated with ET.