LAUSR

Objectives The expected regional variability in percent human epidermal growth factor receptor 2 (HER2)-positive breast cancers is not currently clear. Methods Data from the 2006 to 2011 California Cancer Registry were examined by county and health service area. The influence of demographic and pathologic features was used in a multivariable logistic regression model to compare expected with observed HER2-positive percentages by region. Results There was significant geographic variation by California counties (11.6%-26%). The reported HER2-positive percentage was higher when the population had higher stage, tumor size, grade, percent estrogen receptor negative, younger age, or lower socioeconomic status. Ethnic distribution of the population also influenced HER2-positive percentages. Using a multivariable logistic regression model, most regions had expected values based on their population characteristics; however, "outlier" regions were identified. Conclusions These results deepen our understanding of population characteristics' influence on the distribution of HER2-positive breast cancers. Taking these factors into account can be useful when setting laboratory benchmarks and assessing test quality.