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Purpose: Studies have reported disparities by age and race in the initiation of adjuvant trastuzumab for the initial treatment of older women with early-stage breast cancer, but less is known about its initiation in younger patients. Therefore, we assessed temporal trends and clinical and demographic factors associated with trastuzumab initiation in a large, population-based cohort of patients aged <64 years in 5 states. Methods: Using a cancer registry and claims-linked data set of 13,398 women with incident invasive breast cancer from 2006 to 2011, we identified 934 patients aged <64 years with HER2-positive stage I–III breast cancer. We assessed trastuzumab initiation within the first 9 months after diagnosis and conducted logistic regression analyses to assess sociodemographic and clinical factors associated with trastuzumab initiation. Results: From 2006 to 2011, trastuzumab initiation steadily increased in patients with node-positive (from 65% to 91%) and node-negative (from 39% to 75%) breast cancers. Several tumor-related factors were associated with trastuzumab initiation, including high histologic grades (adjusted odds ratio [aOR], 6.43; 95% CI, 3.27–12.65; and aOR, 3.25; 95% CI, 1.66–6.36, for grades 3 and 2, respectively), node-positive status (aOR, 1.88; 95% CI, 1.28–2.78; P=.001), tumor size >2 cm (aOR, 1.50; 95% CI, 1.04–2.16; P=.03), and hormone receptor–negative status (aOR, 1.51; 95% CI, 1.01–2.26; P=.04). We found a null effect of race. Conclusions: Adjuvant trastuzumab therapy for early-stage breast cancer has been widely disseminated among women aged <64 years. The initiation of this targeted therapy was associated with higher-risk features, consistent with practice guidelines.

Background: A number of practice guidelines incorporate the use of gene expression profiling (GEP) tests for early-stage, hormone receptor–positive, HER2-negative breast tumors. Few studies describe factors associated with GEP testing in US oncology practice. We assessed the relationship between clinical, demographic, and group-level socioeconomic variables and test use in women younger than 65 years. Patients and Methods: Data from 5 state cancer registries were linked with insurance claims data and GEP test results. We assessed rates of testing and variables associated with test use in an incident cohort of 9,444 commercially insured women younger than 65 years, newly diagnosed with stage I or II hormone receptor–positive breast cancer from 2006 through 2012. Results: Rates of testing for women with N0 disease increased from 20.4% in 2006 to 35.2% in 2011. Variables associated with higher rates of testing, beyond clinical factors such as nodal status (P<.001), included being diagnosed from 2008 through 2012 versus 2006 through 2007 (adjusted odds ratio [OR], 1.67; 95% CI, 1.47–1.90), having preexisting comorbidities (adjusted OR, 1.35; 95% CI, 1.14–1.59), and higher out-of-pocket pharmacy costs (adjusted OR, 1.66; 95% CI, 1.40–1.97). Women younger than 50 years were more likely to be tested if they had stage I versus stage II disease (P<.0001). Conclusions: In an insured population of women younger than 65 years, GEP testing increased after its inclusion in clinical practice guidelines and mounting evidence. Additional research is needed to better understand oncologists' decision not to order GEP testing for their patients who are otherwise eligible.