June 2025
Background: The introduction of novel treatments and treatment approaches for breast cancer has reduced breast cancer mortality and unnecessary toxicity. As care improvements develop, it is crucial to understand whom they reach. We used insurance claims data to analyze the dissemination of four major trends in breast cancer treatment into the population. Methods: We used SEER-Medicare data to study four changes in the treatment of local and regional breast cancer in patients diagnosed between 2010 and 2018: (1) adjuvant paclitaxel-trastuzumab (APT) for patients with HER2+ local disease, (2) pertuzumab for patients with HER2+ regional disease, (3) increasing use of neoadjuvant chemotherapy for patients with triple-negative or HER2+ regional disease, and (4) decreasing use of adjuvant chemotherapy for patients with hormone receptor-positive, HER2-negative regional disease. Patients who received no systemic therapy were excluded. We used multilevel logistic regression to identify demographic (age, comorbidity, race/ethnicity, rurality, median income of residence census tract [low, middle, high]), tumor (hormone receptor status), and care (NCI vs other academic vs community hospital, breast cancer specialization of treating oncologist) factors associated with receipt of updated care. Breast cancer specialization of a physician was defined by terciles as the proportion of breast cancer patients among their patients in SEER-Medicare with colon, lung, or breast cancer. 95% confidence intervals are shown. Results: We identified 2,385 patients with localized HER2+ breast cancer; 1,937 patients with regional HER2+ breast cancer; 3,742 patients with regional triple-negative or HER2+ breast cancer; and 14,611 patients with regional hormone receptor-positive, HER2-negative breast cancer. In these respective populations, from 2010 to 2018, use of the APT regimen increased from 31% (24-38%) to 73% (67-78%), pertuzumab from 0% to 70% (65-75%), neoadjuvant chemotherapy from 24% (19-28%) to 61% (57-66%), and adjuvant chemotherapy decreased from 37% (34-39%) to 26% (24-28%). In multivariate analyses, higher median income of residence census tract of the patient and greater specialization level of treating oncologist were significantly associated with receipt of updated care for all four trends in breast cancer treatment. Additionally, greater cancer specialization of hospital type was associated with receipt of updated care for the APT regimen, pertuzumab, and neoadjuvant chemotherapy. We develop a model that, after adjusting for patient and clinical characteristics, estimates the probability of receiving updated care, according to tumor subtype and stage, for a SEER-Medicare patient from a low-income area treated by an oncologist with low breast cancer specialization vs one from a high-income area treated by an oncologist with high specialization: for APT, 40% (37-42%) vs 76% (75-77%), for pertuzumab 28% (25-32%) vs 63% (60-66%), for neoadjuvant chemotherapy 24% (23-26%) vs 56% (54-57%), and for omission of adjuvant chemotherapy 62% (60-63%) vs 72% (71-73%). In this model, specialization alone substantially mitigates the disparity: for example, a patient from a low-income area treated by an oncologist with high specialization has an estimated 63% (59-67%) probability of receiving appropriate APT. Conclusions: This study reveals disparities in the dissemination of major advances in breast cancer treatment across the SEER-Medicare population from 2010-2018. Patients from lower-income areas, cared for at community hospitals, and treated by oncologists with less breast cancer focus were less likely to receive updated care. These results provide important context for targeting efforts to ensure all eligible patients benefit from the latest advancements in breast cancer care. Citation Format: Jennifer Caswell-Jin, Marissa Reitsma, Hao Tang, James Dickerson, Shannon Phillips, Allison W. Kurian, Becky Staiger, Jeremy D. Goldhaber-Fiebert. Disparities in the dissemination of new breast cancer treatments in the United States [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P4-04-01.