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Purpose: The National Comprehensive Cancer Network (NCCN) guidelines recommend a variety of drug combinations with specific administration schedules for the treatment of early-stage breast cancer, allowing physicians to deliver treatments recognizing individual patient complexities, including comorbidities, and patient-physician preference. While use of guideline regimens has shifted over time, there is little data to describe changes in how treatment for early-stage breast cancer has evolved over time., Methods: In a cohort of 34,109 women treated for stage I-IIIA breast cancer between 2006-2019 at Kaiser Permanente Northern California and Kaiser Permanente Washington, we present the changes in chemotherapy regimens over time, and explore use of NCCN-guideline regimens (GR), guideline regimens used when said regimens were not included in guidelines, referred to as time-discordant regimens (TDR), and non-guideline regimens (NGR). Results are presented by drug combination and over time., Results: Among 12,506 women receiving chemotherapy, 77.4% (n = 9681) received GRs, 9.1% (n = 1140) received TDRs, and 13.5% (n = 1685) received NGRs. In 2006, AC-T (cyclophosphamide-doxorubicin, paclitaxel) was the most common regimen, with TC (cyclophosphamide-docetaxel) becoming the most prevalent by 2019. NGRs were more common in cyclophosphamide-methotrexate-5-fluorouracil (CMF); cyclophosphamide-doxorubicin-paclitaxel-trastuzumab (ACTH); and paclitaxel-trastuzumab (TH). The use of GR has increased over time (p-trend < 0.001), while use of NGR (both in terms of administration schedule and drug combination) and TDR have decreased, although patterns vary by drug combination., Conclusion: Chemotherapy delivery has changed markedly over time, with a move toward more use of GR. These data are important for understanding the landscape of chemotherapy delivery in community healthcare settings., (© 2024. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)