The landscape of chemotherapy modifications among women treated for stage I–IIIA breast cancer

Abstract Background In the delivery of cancer care, patients may experience modifications to their intended chemotherapy regimen for reasons such as toxicity, tolerability, and scheduling challenges. Understanding modifications from the intended chemotherapy in the real‐world setting of cancer care may affect treatment planning and inferences of real‐world treatment effectiveness. Methods By leveraging data from the Optimal Breast Cancer Chemotherapy Dosing Study, this research studied 12,558 women receiving chemotherapy for primary stage I–IIIA breast cancer at Kaiser Permanente Northern California (2006–2019) and Kaiser Permanente Washington (2004–2015). Modifications in chemotherapy administered from intended, both overall and by regimen, are described. Prevalence ratios (PRs) were calculated for associations between regimen attributes and treatment modifications via generalized linear models of the Poisson family with a log‐link function and robust standard errors. Results Intent to receive a guideline‐concordant regimen was associated with a lower likelihood of modifications from the intended chemotherapy (vs. non–guideline concordant) (PR, 0.74; 95% CI, 0.69–0.79), as was receiving a dose‐dense regimen (vs. non–dose dense) (PR, 0.84; 95% CI, 0.78–0.91). Those receiving any neoadjuvant chemotherapy (vs. adjuvant only) were more likely to have modifications from the intended chemotherapy (PR, 1.29; 95% CI, 1.19–1.41). Increasing numbers of cycles and drugs administered were associated with a higher likelihood of modifications ( p trend < .001). Conclusions Breast cancer chemotherapy regimens used in the real‐world setting were examined. Chemotherapy modifications varied markedly by regimen and by characteristics of the regimen administered. Given that more than one in three women in this cohort did not receive chemotherapy as intended, these novel findings may be informative in treatment planning and for future research assessing real‐world treatment effectiveness.