Relative effectiveness of adjuvant chemotherapy for invasive lobular compared with invasive ductal carcinoma of the breast

BACKGROUND Invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) have distinct clinical, pathologic, and genomic characteristics. The objective of the current study was to compare the relative impact of adjuvant chemotherapy on the survival of patients with ILC versus those with IDC. METHODS Women with estrogen receptor (ER)‐positive, human epidermal growth factor receptor 1 (HER2) ‐negative, stage I/II IDC and ILC who received endocrine therapy were identified from the 2000 to 2014 California Cancer Registry. Patient, tumor, and treatment characteristics were collected. Ten‐year overall survival (OS) was estimated using the Kaplan‐Meier method and Cox proportional‐hazards modeling. RESULTS In total, 32,997 women with IDC and 4638 with ILC were identified. The receipt of chemotherapy significantly decreased during the study for both subtypes. For patients with IDC, the 10‐year OS rate was 95% among those who received endocrine therapy alone versus 93% (P