For patients with hormone receptor-positive early-stage breast cancer, the standard approach after breast-conserving surgery has long involved two complementary therapies. Radiation therapy to the remaining breast tissue sharply reduces the risk that the cancer will return locally in the treated breast, while endocrine therapy, which blocks or lowers the hormones that can fuel this subtype of cancer, lowers the risk of recurrence throughout the body. In selected older patients with lower-risk disease, randomized clinical trials have demonstrated that radiation can be omitted without compromising overall survival, although the local recurrence risk is higher. Crucially, every patient in those trials was prescribed endocrine therapy as part of the plan, making sustained medication use the linchpin of the entire de-escalation strategy.

The new study, led by Shannon Jiang, MD, a radiation oncology resident physician at Washington University School of Medicine in St. Louis, examined real-world adherence among 17,025 patients aged 23 to 64 with hormone receptor-positive, early-stage breast cancer. Drawing on Merative MarketScan commercial and multistate Medicaid claims data, the researchers identified women who had undergone lumpectomy and filled at least one endocrine therapy prescription; 89 percent of them also received radiation therapy. Rather than relying on self-reported medication use, the team used pharmacy claims to calculate each patient’s medication possession ratio, a measure of how consistently prescriptions were filled and whether enough medication was on hand to follow the prescribed regimen. The measure cannot confirm that every dose was actually swallowed, but it offers an objective window into whether patients had the means to stay on treatment.

The headline number is striking. Using the standard adherence threshold, under which a patient is considered adherent when medication is available for at least 80 percent of the measured period, only about 28 percent of patients remained continuously above that line across all five recommended years of follow-up. Annual snapshots told a rosier story, with 59 to 75 percent of patients meeting the threshold in any given year, but the longitudinal view exposed a steady erosion: with each passing year, more patients accumulated gaps in medication availability until the continuously adherent group shrank to a small minority.

The most consequential finding concerned radiation omission. Patients who did not receive radiation therapy were twice as likely to fall below the 50 percent medication-possession threshold over time, a level representing substantial and prolonged lapses in treatment. They were also significantly more likely to drop below the 60 and 70 percent benchmarks. Interestingly, the association did not reach statistical significance at the standard 80 percent threshold, suggesting that radiation omission may matter most for identifying patients at risk of the most severe disengagement from therapy rather than modest shortfalls.

The study was not designed to explain why radiation receipt was associated with better adherence, and the authors are careful to note that the findings do not prove radiation therapy itself caused patients to stay on their medication. Several plausible mechanisms deserve consideration. Patients who receive radiation have more frequent contact with their oncology teams during treatment, creating additional touchpoints for prescription renewal, side-effect management and reinforcement of the treatment plan. Differences in health care access, treatment preferences and baseline engagement with the medical system between the two groups may also have contributed. Disentangling these possibilities will require further research, but the association is a warning flag for any clinician counseling a patient toward radiation omission on the assumption that the medication will simply take care of the rest.

Age emerged as a second, independent correlate of nonadherence. Patients younger than 60 had a 20 to 25 percent higher risk of becoming nonadherent across the thresholds studied. That detail may prove increasingly relevant as the research community explores extending radiation omission to younger women with favorable tumor biology, a population in which the durability of five years of daily endocrine therapy has now been called into question by these real-world data. Side effects of endocrine therapy, including hot flashes, fatigue, joint pain, effects on bone health and changes in memory and concentration, are well documented and can make multi-year adherence difficult for patients at any age, but they may weigh differently on younger women managing work, fertility and family life.

The analysis does have an important limitation: the claims database did not include patients older than 64, so it cannot show whether adherence patterns differ among older patients, who currently make up much of the population considered for radiation omission in the first place. The research team plans to incorporate Medicare data to examine adherence in that older group, a step that could either reinforce or complicate the current picture. Until then, clinicians counseling patients over 64 must extrapolate from a younger cohort with acknowledged caution.

Expert commentary from the field underscores the practical implications. “Radiation therapy omission can be appropriate for selected people with lower-risk breast cancer, but endocrine therapy is an important part of that strategy. This study adds real-world context to treatment discussions by showing how difficult long-term adherence can be. When discussing omission, clinicians should consider radiation’s established local-control benefit alongside the practical realities of staying on endocrine therapy for years. The goal is to choose a treatment plan that patients can realistically maintain over time,” said Jose G. Bazan, MD, Chair of the ASTRO Breast Cancer Resource Panel and a radiation oncologist at City of Hope Comprehensive Cancer Center in Duarte, California. His framing captures the central tension: de-escalation strategies are only as strong as the therapies patients actually complete.

For Dr. Jiang, the message is one of realistic conversation rather than alarm. “Radiation therapy after lumpectomy has a well-established benefit in preventing breast cancer from returning in the treated breast. For some patients with lower-risk disease, radiation therapy may reasonably be omitted after an individualized discussion, but this approach assumes that patients will continue endocrine therapy for several years. Our findings suggest that maintaining endocrine therapy as prescribed, taking a daily medication for five years, is more challenging in routine practice than we often assume,” she said. She added that prescribing a five-year course and actually completing it are not the same thing, and that when radiation is omitted, endocrine therapy becomes an especially important part of recurrence prevention, making honest discussions about long-term treatment and quality of life essential. The study’s ultimate goal, the team says, is to give clinicians better real-world information for treatment counseling, so that the evidence guiding de-escalation is weighed alongside what actually happens to patients in the years that follow. As personalized medicine continues to trim unnecessary treatment from cancer care, this analysis serves as a reminder that every therapy removed from a plan raises the stakes for the therapies that remain, and that the success of any streamlined regimen ultimately depends on the patient’s ability to sustain it, one daily pill at a time, for half a decade.

Subject of Research: Real-world adherence to adjuvant endocrine therapy after lumpectomy for early-stage hormone receptor-positive breast cancer and its relationship to radiation therapy omission

Article Title: Real-world data indicate lower adherence rates for breast cancer endocrine therapy when radiation therapy is omitted

Article References: Real-world data indicate lower adherence rates for breast cancer endocrine therapy when radiation therapy is omitted. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: breast cancer, endocrine therapy, radiation therapy, lumpectomy, medication adherence, radiation omission, ASTRO, hormone receptor-positive, cancer recurrence, de-escalation, insurance claims data, oncology