PURPOSE: Patients treated with adjuvant trastuzumab require adequate cardiac 
monitoring. We describe the patterns of cardiac monitoring and evaluate factors 
associated with adequate monitoring in a large population-based study of older 
patients with breast cancer.
PATIENTS AND METHODS: Patients age 66 years or older with full Medicare 
coverage, diagnosed with stage I to III breast cancer between 2005 and 2009, and 
treated with adjuvant trastuzumab-based chemotherapy were identified in the 
SEER-Medicare and the Texas Cancer Registry-Medicare databases. The adequacy of 
cardiac monitoring was determined. Chemotherapy, trastuzumab use, cardiac 
monitoring, and comorbidities were identified by using International 
Classification of Diseases, 9th revision and Healthcare Common Procedure Coding 
System codes. Prescribing physician characteristics were also evaluated. 
Analyses included descriptive statistics and multilevel logistic regression 
models.
RESULTS: In all, 2,203 patients were identified; median age was 72 years. 
Adequate monitoring was identified in only 36.0% of the patients (n = 793). In 
the multivariable model, factors associated with optimal cardiac monitoring 
included a more recent year of diagnosis (hazard ratio [HR], 1.83; 95% CI, 1.32 
to 2.54), anthracycline use (HR, 1.39; 95% CI, 1.14 to 1.71), female prescribing 
physician (HR, 1.37; 95% CI, 1.10 to 1.70), and physician graduating after 1990 
(HR, 1.66; 95% CI, 1.29 to 2.12). The presence of cardiac comorbidities was not 
a determinant for cardiac monitoring. Of the variance in the adequacy of cardiac 
monitoring, 15.3% was attributable to physician factors and 5.2% to measured 
patient factors.
CONCLUSION: A large proportion of patients had suboptimal cardiac monitoring. 
Physician characteristics had more influence than measured patient-level factors 
in the adequacy of cardiac monitoring. Because trastuzumab-related 
cardiotoxicity is reversible, efforts to improve the adequacy of cardiac 
monitoring are needed, particularly in vulnerable populations.