Abstract Background: Guidelines informing chemotherapy regimen selection are based on clinical trials with participants who do not necessarily represent general populations with breast cancer. Understanding who receives nonguideline regimens is important for understanding real-world chemotherapy administration and how it relates to patient outcomes. Methods: Using data from the Optimal Breast Cancer Chemotherapy Dosing (OBCD) study, based at Kaiser Permanente Northern California (2006–2019) and Kaiser Permanente Washington (2004–2015), we use logistic regression to examine the associations between patient characteristics and receipt of nonguideline chemotherapy regimens among 11,293 women with primary stage I to IIIA breast cancer receiving chemotherapy. Results: The use of nonguideline regimens was strongly associated with several factors, including older age [≥80 vs. 18–39 years: OR, 5.25; 95% confidence interval (CI), 3.06–9.00; P-trend = 0.002] and HER2 status (HER2+ vs. HER2−: OR, 3.44; 95% CI, 3.06–3.87) and was less likely in women with larger tumor size (>5 cm vs. 0.1 to ≤0.5 cm: OR, 0.56; 95% CI, 0.36–0.87; P-trend = 0.01) and diagnosed in later years (2012–2019 vs. 2005–2011: OR, 0.80; 95% CI, 0.71–0.90). Factors associated varied by type of nonguideline regimens. For example, women with comorbidity and older age were more likely to receive nonguideline drug combinations in particular, whereas women with larger tumor size were less likely to receive nonguideline administration schedules. Conclusions: Nonguideline chemotherapy regimens are more likely in certain patient populations. Impact: These associations highlight that vulnerable patient populations may be less likely to receive guideline care, and thus, real-world studies are essential for understanding how the use of nonguideline regimens impacts patient outcomes in these groups.
