Association of Body Mass Index With 21-Gene Recurrence Score Among Women With Estrogen Receptor-Positive, ERBB2-Negative Breast Cancer.

IMPORTANCE: Body mass index (BMI) may affect the 21-gene recurrence score (RS) in patients with ER-positive, ERBB2-negative breast cancer. If high BMI increases genomic risk in ER-positive, ERBB2-negative breast cancer, weight control will become more important.

OBJECTIVE: To assess the association between RS and BMI according to age groups and address BMI as a factor associated with high RS.

DESIGN, SETTING, AND PARTICIPANTS: This cohort study included 2295 patients with ER-positive, ERBB2-negative breast cancer who had undergone a multigene assay between March 29, 2010, and December 31, 2020, in 2 hospitals. All of the study patients were Korean women, and the median follow-up period was 45 months (range, 1-40 months). The correlations between continuous RS and BMI were investigated. A high BMI was defined as a body mass index greater than or equal to 25. In the younger age group (age ≤45 years), a high RS was defined as an RS of greater than 20.

EXPOSURES: Body mass index.

MAIN OUTCOMES AND MEASURES: The Pearson correlation coefficient was used to estimate the association between RS and BMI. A multivariable binary logistic model was used to identify high RS.

RESULTS: Among the 2295 women included (mean [SD] age, 49.8 [4.00] years; range, 22-81 years), 776 were aged 45 years or younger; RS and BMI were weakly correlated (correlation coefficient, 0.119; P < .001) in this younger group. Among them, the proportion of patients with an RS greater than 20 was significantly higher in the high BMI group than in the normal BMI group (45.5% [46 of 101] vs 27.3% [184 of 675]; P < .001). In the multivariable analysis, high BMI was an associated factor for high RS (odds ratio, 2.06; 95% CI, 1.28-3.32; P = .003). The 21-gene multigene assay-guided chemotherapy rate was significantly higher in patients with high BMI (30.7% [31 of 101] vs 20.2% [136 of 674]; P = .02).

CONCLUSIONS AND RELEVANCE: In this cohort study of women aged 45 years or younger, high BMI was associated with higher RS in those with ER-positive, ERBB2-negative breast cancer; further studies are necessary to examine the underlying mechanisms.