Distribution of 21-Gene Recurrence Scores Among Breast Cancer Histologic Subtypes

Context.-The 21-gene recurrence score (RS) provides a probability of distant recurrence for estrogen receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancers. The utility of RS for rarer histologic subtypes of breast cancer is uncertain. Objective.-To determine the distribution of RS among various histologic subtypes using a population database. Design.-Women between the ages of 18 and 75 with estrogen receptor-positive, HER2-negative breast cancer and known RS results were identified using the Surveillance, Epidemiology, and End Results database. Recurrence scores were categorized into risk groups using both traditional and Trial Assigning Individualized Options for Treatment cutoffs. Multivariable logistic regression was used to determine factors associated with high-risk RS. Results.-We identified 45 618 patients with stage I to III, estrogen receptor-positive, HER2-negative breast cancer who had RS available. Overall, 3087 (7%) and 6337 (14%) of cancers were classified as high risk based on traditional and Trial Assigning Individualized Options for Treatment RS cutoffs, respectively. The proportion of highrisk RS ranged from 1% (tubular, 2 of 225) to 68% (medullary, 13 of 19) and 4% (tubular, 10 of 225) to 79% (medullary, 15 of 19) for traditional and Trial Assigning Individualized Options for Treatment cutoffs, respectively. Based on multivariable logistic regression (excluding medullary), subtypes other than invasive ductal carcinoma and papillary carcinoma were significantly associated with lower RS. The strongest predictors of a high-risk RS were higher tumor grade and negative progesterone receptor status. Conclusions.-We identified distinct distributions of RS among different histologic subtypes of breast cancer. Excluding medullary carcinoma, histologic subtypes other than invasive ductal carcinoma and papillary carcinoma all predict lower RS.