140 - Endocrine Therapy Adherence after RT Omission in Estrogen Receptor-Positive Early-Stage Breast Cancer

05:20pm - 05:30pm ET

Room 109

Presenter(s)

Shannon Jiang, MD - Washington University/B-JH/SLCH Consortium, St. Louis, MO

S. J. Jiang¹, M. Agarwal², C. Bergom¹, L. Hall¹, M. A. Thomas¹, I. Zoberi¹, and J. C. Yang¹; ¹WashU Medicine, Department of Radiation Oncology, St. Louis, MO, ²WashU Medicine, Saint Louis, MO

Purpose/Objective(s):

There is increasing interest in endocrine therapy (ET) alone as adjuvant therapy after lumpectomy for estrogen receptor-positive early-stage breast cancer (ER+ ESBC) especially in older patients. Landmark trials suggest ET alone with omission of radiation therapy (RT) is reasonable in selected older patients but assume ET adherence. In practice, adherence is highly variable due to ET side effects and is hard to assess. We used national commercial and Medicaid claims data to better understand real world ET adherence and its relationship to receipt of adjuvant RT.

Materials/Methods:

We used Merative™ MarketScan® Commercial (2006-2019) and Merative™ MarketScan® Multi-State Medicaid (2011-2019) databases to evaluate women age =18 with ER+ ESBC treated with lumpectomy and who were prescribed ET. Medication fill and refill data were used to calculate medication possession ratios (MPR) for individual patients by follow-up year (indexed from diagnosis) at different levels of stringency: =80% (gold standard definition), =70%, =60%, and =50%. Non-adherence was the first point a patient fell below the MPR cutoff. Time to non-adherence was estimated by Kaplan-Meier analysis and compared using Cox proportional hazards models for each MPR cutoff and by age and receipt of RT.

Results:

We evaluated 17,025 patients (median age 56, 89% received adjuvant RT). Adherence varied widely by year, but year 1 was systematically underestimated due to indexing at diagnosis. At the strictest cutoff (MPR=80%), 34.6% of patients were adherent in year 1; among those who remained on therapy, 77% met threshold in years 2–5, reflecting early discontinuation of less-adherent patients. Although cross-sectional adherence appears higher in years 2–5, time-to-event analyses demonstrate a steady decline in probability of remaining adherent over follow-up, with fewer than 30% remaining adherent at 5 years using MPR =80% cutoff. Age was an independent predictor of non-adherence: patients <60 years had a 16–19% lower hazard of non-adherence across thresholds (HR 0.81 [95% CI 0.75-0.87] to 0.84 [0.78-0.89]). Importantly, among older patients, receipt of adjuvant RT was associated with substantially improved adherence, with 38-48% lower risk of non-adherence at lower MPR cutoffs (MPR =50%: HR 0.52 [95% CI 0.42-0.64]; MPR =60%: 0.62 [0.51-0.74]).

Conclusion:

Medication fill and refill data suggest real world ET adherence patterns for patients with ER+ ESBC in the United States to be widely variable. Year 1 adherence may be underestimated due to ET initiation typically weeks to months following diagnosis; however time-to-event analyses demonstrate decreasing adherence with follow up time. Older age and RT omission were associated with higher risk of non-adherence. This is the first study to show that RT omission after lumpectomy is associated with higher risk of ET non-adherence, suggesting implications for patients considered for RT omission.