Tumor genomic landscape of older patients with metastatic breast cancer☆.

Metastatic breast cancer (MBC) in older patients has distinct clinical and histologic characteristics. Elucidating the genomic basis of MBC helps identify potential therapeutic targets to improve outcomes for older patients with MBC. Using a prospective database and targeted DNA sequencing (OncoPanel), we examined MBC's genomic landscape in older patients (age ≥70 years at MBC diagnosis) and compared findings with those in younger (aged <50 years) and middle-aged (aged 50-69 years) patients. After classifying single nucleotide variants (SNVs) and copy number variations (CNVs) as oncogenic (via OncoKB), the frequencies of SNVs and CNVs, tumor mutational burden (TMB), and oncogenic signaling pathways were compared by age group using Fisher's exact tests. We estimated the association between continuous age at MBC diagnosis and mutations via multivariate logistic regression analysis, adjusting for race, stage at initial diagnosis, subtype, histology, and sample tested (primary versus metastatic). Our study included 2379 patients [853 (35%) younger, 1311 (55%) middle-aged, and 215 (9%) older] who underwent OncoPanel testing between 2013 and 2020. The most frequent tumor alterations in older patients were SNVs in PIK3CA (44%), TP53 (33%), CDH1 (22%) and amplifications in CCND1 (18%). After adjustment, older age was associated with higher frequency of SNVs in CDH1 [odds ratio (OR) = 1.43, 95% confidence interval (CI) 1.21-1.68, q < 0.001], MAP3K1 [OR = 1.30, 95% CI 1.10-1.54, q = 0.008], and PIK3CA [OR = 1.21, 95% CI 1.11-1.31, q < 0.001] and fewer SNVs in TP53 [OR = 0.84, 95% CI 0.77-0.91, q < 0.001]. Patients in the older group were more likely to have tumors with ≥10 mutations/megabase than the youngest patients (26% versus 17%, P = 0.003). In this large cohort of patients with MBC, the tumor genomic landscape differed between older and younger patients even after accounting for tumor subtype. Older patients were more likely to have high-TMB and PIK3CA-mutated tumors, highlighting the importance of genomic testing for treatment applications in this population.