ASTRO: <30 Percent With ER+ Early-Stage Breast Cancer Remain on Endocrine Therapy at Five Years

MONDAY, Oct. 5, 2026 (HealthDay News) -- Less than 30 percent of women with estrogen receptor (ER)-positive early-stage breast cancer (ESBC) treated with lumpectomy remain adherent to endocrine therapy (ET) at five years, according to a study presented at the annual meeting of the American Society for Radiation Oncology (ASTRO), held from Sept. 26 to 30 in Boston.Shannon J. Jiang, M.D., from the Washington University School of Medicine in St. Louis, and colleagues used commercial and Medicaid claims data to explore real world ET adherence and its association with receipt of adjuvant radiation therapy. Data were included for 17,025 women aged 18 years and older with ER-positive ESBC treated with lumpectomy and prescribed ET. Medication possession ratios (MPR) were calculated for individual patients by follow-up year at different levels of stringency (≥80 percent, ≥70 percent, ≥60 percent, and ≥50 percent).The researchers found that adherence varied widely by year, with systematic underestimation at year 1 due to indexing at diagnosis. Overall, 34.6 percent of patients were adherent in year 1 at the strictest cutoff (MPR ≥80 percent); 77 percent of those who remained on therapy met the threshold in years 2 to 5, reflecting early discontinuation among patients who were less adherent. Time-to-event analyses demonstrated a steady decline in the probability of remaining adherent during follow-up; at five years, fewer than 30 percent remained adherent using the MPR ≥80 percent cutoff. Nonadherence was independently predicted by age, with a lower hazard of nonadherence across thresholds for patients younger than 60 years (hazard ratios, 0.81 to 0.84). Receipt of adjuvant radiation therapy was associated with substantially improved adherence among older patients."Our findings are a reminder that doctors prescribing endocrine therapy for five years and patients actually taking the medication for five years are not the same thing," Jiang said in a statement.Press ReleaseMore Information.Sign up for our weekly HealthDay newsletter

ASTRO: <30 Percent With ER+ Early-Stage Breast Cancer Remain on Endocrine Therapy at Five Years

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