Many women who undergo lumpectomy for early-stage breast cancer do not adhere to endocrine therapy long term, and those who skip radiotherapy may be a particularly vulnerable group.

An analysis of insurance claims found that less than 30% of patients consistently adhered to endocrine therapythroughout all 5 recommended years. And two groups were especially likely to have substantial lapses in adherence: patients younger than 60 years and those who did not receive radiotherapy post-surgery.

The findings, presented at the American Society for Radiation Oncology (ASTRO) 2026 Annual Meeting in Boston, have implications for treatment de-escalation strategies that omit radiotherapy with the expectation that patients will continue endocrine therapy for at least 5 years.

“Our findings suggest that maintaining endocrine therapy as prescribed, taking a daily medication for 5 years, is more challenging in routine practice than we often assume,” lead author Shannon Jiang, MD, of Washington University School of Medicine in St Louis, said in an ASTRO news release.

While some patients can safely skip radiotherapy after lumpectomy, such decisions should be individualized, including consideration of “how realistic” it is for a patient to stay on endocrine therapy long term, Jiang told conference attendees.

Study discussant Nancy Keating, MD, MPH, a professor at Harvard Medical School in Boston, said that poor adherence among women who omit radiotherapy is concerning because endocrine therapy is critical in this setting.

“I think a really important thing is to really try to understand what we can do to support women in maintaining adherence to these important treatments,” Keating said.

She noted that patients who omit radiotherapy may face challenges that affect their ability to engage with care in general.

Adherence Erodes Over 5 Years

According to Jiang, it’s become clear that omitting adjuvant radiotherapy is “reasonable” for some older patients, assuming 5 years of daily endocrine therapy. However, long-term adherence is highly variable and hard to assess in clinical practice.

Jiang and her colleagues set out to characterize real-world endocrine therapy adherence patterns among US patients, including whether they vary by age and receipt of radiotherapy.

Using Merative MarketScan commercial and state Medicaid claims databases, the researchers identified 17,025 women aged 23-64 years with hormone receptor-positive, early-stage breast cancer who’d undergone lumpectomy and filled at least one prescription for endocrine therapy. Most (89%) received adjuvant radiotherapy. Adherence to endocrine therapy was assessed using the medication possession ratio (MPR), calculated from prescription fill and refill data.

Over 5 years, only about 28% of patients remained continuously adherent to endocrine therapy at the standard MPR threshold of 80% — meaning they had medication available at least 80% of the time. Annual adherence did appear considerably higher, with 59%-75% of patients meeting the threshold in individual years 2-5. However, longitudinal analysis showed a steady accumulation of patients falling below it over time.

Even with a much less stringent definition — having medication available for only half of the year — about 1 in 9 patients did not meet the threshold, Jiang reported.

No Radiotherapy, Lower Adherence

Skipping radiotherapy was associated with a higher risk for nonadherence to long-term endocrine therapy. Patients who did not receive radiotherapy were twice as likely to fall below the 50% MPR threshold over time and were significantly more likely to fall below the 60% and 70% benchmarks. The association was not significant at the standard 80% adherence threshold.

Jiang said the study was not designed to explain why receiving radiotherapy was associated with better adherence. But she noted that patients who received radiotherapy “might represent a group that’s already more engaged with the healthcare system.”

Similarly, radiotherapy visits, themselves, may be creating more opportunities for support, Jiang said, including more frequent contact with the oncology team and earlier recognition and management of treatment-related side effects.

Endocrine therapy lapses were also more common among younger patients. Those younger than 60 years were about 20%-25% more likely to become nonadherent across MPR thresholds.

That finding, Jiang noted, will grow increasingly relevant as radiotherapy omission is studied in younger patients with favorable tumors.

She said that as de-escalation moves into younger populations, “assessing treatment tolerance early, revisiting adherence regularly, and offering supportive resources when needed will be really important in ensuring that these strategies are successful.”

The findings should also help inform patients’ decisions to undergo or skip radiotherapy, according to Jose G. Bazan, MD, chair of the ASTRO Breast Cancer Resource Panel and a radiation oncologist at City of Hope in Duarte, California.

“When discussing omission, clinicians should consider radiation’s established local-control benefit alongside the practical realities of staying on endocrine therapy for years,” Bazan said in the ASTRO release. “The goal is to choose a treatment plan that patients can realistically maintain over time.”

Jiang acknowledged several study limitations. Pharmacy claims show whether patients had medication available, not whether they actually took it. And because the study was observational, it cannot establish that radiotherapy itself improved adherence.

In addition, the database did not include patients older than 64 years, limiting conclusions about the older population for whom radiotherapy omission is most established. Jiang said the team plans to analyze Medicare data to examine adherence in this group.

The study had no commercial funding. Jiang, Keating, and Bazan had no relevant disclosures.