Findings demonstrated substantial unexplained geospatial variation in initial breast cancer care. The variance explained by region/health service area was multifold larger than that attributed to patient factors.

• In this population-based cohort study of 31,571 patients with stage I to III breast cancer, the extent to which geospatial variation in initial breast cancer care can be attributed to region vs patient factors was investigated.

• Region/health service area explained more observed variation (24%-48%) compared with patient factors (1%-4%); the largest share of variation remained unexplained (35%-54%).

• It was found that the largest proportions of total variance due to region/health service area were endocrine therapy commencement and continuation.

• The significance of patient factors such as race and ethnicity notwithstanding, future quality improvement efforts should place emphasis on decreasing unwarranted geospatial variation, particularly including optimizing the delivery of endocrine therapy in low-performing regions.