Background: The Oncotype Dx Recurrence Score (RS) predicts distant recurrence risk and benefit from chemotherapy for women with early-stage estrogen receptor (ER) positive (+)/human epidermal growth factor receptor 2 (HER2) negative (-) breast cancer (BC). Due to the independent risk of young age and small proportion of young women in the validation studies, providers may be hesitant to rely on the RS among young women. Methods: Using a multi-center, prospective cohort study of women newly diagnosed with BC at age ≤40 years enrolled from 2006-2016, we identified participants with stage I-III, ER+/HER2- BC. Clinical data were obtained through patient surveys and medical record review. Factors associated with RS testing by univariable analyses (p < 0.20) were used to generate a multivariable logistic regression model. Results: 182 (32%) of 575 eligible women had a RS performed (Table 1). Younger women (OR_{age ≤30 vs 36-40}= 0.49, p = 0.03) and those with larger (OR_{> 2cm vs ≤2cm}= 0.54_, p = 0.007), node positive (OR_{pos vs neg}= 0.14, p < 0.0001) or high grade tumors (OR_{high vs low/intermediate}= 0.37, p < 0.0001) were less likely to have a RS performed. Of women who had a RS performed, chemotherapy usage was 21/88 (24%), 44/77 (57%), and 17/17 (100%) among those with low, intermediate, and high RS, respectively. Most women with low risk RS who received chemotherapy had other low risk features: 67% T1, 67% N0, and 86% low/intermediate grade. Conclusions: Despite the development of multigene testing to assess the benefit of chemotherapy, many young women with node-negative ER+/HER2- BC are not tested, and when tested, a substantial percentage receive chemotherapy despite a low RS. This highlights an opportunity to improve individualized care for young women with BC.

* Missing data and factors not associated with RS testing not included