ASTRO 2026 | Study Highlights Lower Adherence to Endocrine Therapy Post-Lumpectomy When Radiation is Excluded

A recent analysis of U.S. insurance claims has highlighted a significant decrease in the adherence rates to hormone therapy among breast cancer patients who forgo radiation therapy. This study, which captures real-world clinical scenarios, reveals that under 30% of patients persist with the prescribed hormone therapy for five years following lumpectomy.

The research demonstrates that patients who excluded radiation therapy from their treatment regimen were more prone to lapsing in adherence compared to those who included it. Younger patients also showed a trend towards lower adherence. These findings were shared at the American Society for Radiation Oncology (ASTRO) Annual Meeting.

Traditionally, patients with hormone receptor-positive early-stage breast cancer are treated with radiation therapy post-lumpectomy to minimize the risk of local recurrence, alongside endocrine therapy aimed at reducing overall recurrence risk. However, for specific older patients with less aggressive disease, clinical trials have supported the omission of radiation therapy without compromising survival outcomes, provided that endocrine therapy is consistently administered.

Endocrine therapy works by blocking or reducing hormones that can drive breast cancer growth. Nevertheless, side effects—such as hot flashes, fatigue, joint pain, and cognitive impacts—pose challenges that may deter long-term adherence.

The study involved data from 17,025 patients aged 23 to 64 years, all diagnosed with hormone receptor-positive, early-stage breast cancer, who underwent a lumpectomy and initiated endocrine therapy. Of these, 89% also underwent radiation therapy. Their adherence was assessed via the medication possession ratio from pharmacy claims, which evaluates whether patients consistently obtained their prescriptions rather than if they took the medication.

Only about 28% of patients maintained continuous adherence to the prescribed therapy over five years. Without radiation therapy, patients were notably twice as likely to fall below the 50% adherence level over time. Younger age was also a predictor of reduced adherence.

The absence of data for patients older than 64 limits this study's ability to reflect adherence patterns in older populations who are frequently regarded for radiation omission. Future studies plan to include Medicare data to understand adherence in this demographic further.

These findings underscore the need for informed discussions between clinicians and patients concerning the potential implications of omitting radiation in treatment plans, highlighting the importance of maintaining long-term endocrine therapy for better outcomes.