Clinician- and Facility-Level Factors Associated With Receipt of Nonguideline Chemotherapy Regimens in Women With Stage I-IIIA Breast Cancer.

PURPOSE: We examined clinician- and facility-level factors associated with selection of nonguideline chemotherapy at treatment initiation in women with stage I-IIIA breast cancer (BC). METHODS: The Optimal Breast Cancer Chemotherapy Dosing Study collected information on chemotherapy delivery in an integrated health care delivery system. This analysis included 9,758 women treated with chemotherapy for stage I-IIIA BC between 2006 and 2019 at Kaiser Permanente Northern California (KPNC). Prevalence ratios (PRs) and corresponding 95% CIs were estimated for the clinician and facility factors in relation to nonguideline regimen (NGR) use. Analyses were stratified by time (pre- and post-2015) to reflect the period before and after KPNC's transition to a subspecialized care model. Secondary outcomes focused on nonguideline drug combinations (NGDCs) and nonguideline administration schedules (NGASs). RESULTS: Women treated by clinicians with substantially greater time since medical school (30+ years v <10 years since medical school) were more likely to receive NGR (PR, 1.38; 95% CI, 1.01 to 1.88; P trend = .01) with significant associations observed for both NGDC and NGAS. While not associated in the primary analysis, larger practice size (10+ oncologists) was associated with a lower likelihood of NGDC use compared with smaller practices (<5 oncologists; PR, 0.43; 95% CI, 0.26 to 0.70; P trend = .01). Time stratification revealed that, in the study's early years, clinician sex and years since medical school were associated with NGR use, but neither association remained post-2015. CONCLUSION: Clinician and facility characteristics significantly influenced the use of NGR in stage I-IIIA BC treatment. Notably, associations diminished over time, suggesting that health system changes in care delivery may enhance guideline adherence and reduce the impact of nonclinical factors on treatment decisions.