The impact of young age at diagnosis (age <40 years) on prognosis varies by breast cancer subtype: A U.S. SEER database analysis

PURPOSE: Women under 40 years old are at increased risk for developing human epidermal growth factor receptor 2 (HER2) positive or triple negative subtype and more advanced breast cancer, yet young age itself has also historically been an independent prognostic factor. METHODS: Using the Surveillance, Epidemiology, and End Results (SEER) Program, we examined data for 271,173 women with stage I-III breast cancer between 2010 and 2015. Using Fine and Gray regression models to account for competing risks, we examined the risk of breast cancer-specific death by age and clinical subtypes, considering grade, hormone receptor (HR) and HER2 status, adjusting for demographic, clinical and treatment variables. RESULTS: Of 271,173 women eligible for analysis, 14,109 were <40 years of age. Women under 40 years old were more likely to be non-white, uninsured, and to have higher stage, higher grade, HER2-positive and triple-negative subtype disease (all, p < 0.001). Compared to women ages 40–60, women ages <40 had higher breast cancer mortality (hazard ratio, 1.8; 95% confidence interval (CI) 1.6–1.9) in unadjusted analysis. In models controlling for demographic, clinical and treatment factors, young age was significantly associated with an increased risk of breast cancer mortality among women with HR-positive, lower grade disease (hazard ratio 1.7; 95% CI 1.4–2.1) but not for women with high grade/HR-positive, HER2-positive, or triple-negative disease. Women age >75 had increased breast cancer mortality in all subtypes. CONCLUSION: With modern clinical subtyping, age under 40 remains independently associated with worse outcomes in 30 months follow-up only in HR-positive, lower grade disease.