Tumor mutational burden status and clinical characteristics of invasive lobular carcinoma of the breast.

High tumor mutational burden (TMB-H) is an established biomarker for a favorable response to immune checkpoint inhibitors. However, tumor mutational burden (TMB) in invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) has not been sufficiently investigated. We collected data of patients with ILC or IDC from the Center for Cancer Genomics and Advanced Therapeutics database between June 2019 and August 2023. Furthermore, we examined the clinicopathological factors and TMB status. Patients with ILC (n = 170) had a median TMB score of 4.00 mut/Mb (interquartile range, 2.00-7.14 mut/Mb), whereas those with IDC (n = 2598) had a score of 3.90 mut/Mb (2.00-6.00 mut/Mb). TMB-H was more common in patients with ILC than in those with IDC (18.2% vs. 10.1%, P < 0.001), particularly in the ER+ /HER2- subtype. Multivariate analysis revealed that the pathological diagnosis of ILC (P = 0.006), tissue samples collected from metastatic sites (P < 0.001), and older age (50 years, P < 0.001) were independent factors for TMB-H. Patients with ILC were more likely to have TMB-H than those with IDC. The findings of this study would be invaluable in selecting treatment strategies for patients with ILC.