The 21-Gene Recurrence Score in Special Histologic Subtypes of Breast Cancer A Population-Based Study

Context.-Recurrence score (RS) testing was developed and validated in invasive ductal and rare lobular carcinomas, although it is used for all special types of breast cancers. Objective.-To determine association of histologic type (HT) and RS, specifically high-risk RS. Design.-We used RSs linked to Surveillance, Epidemiology, and End Results Program registries of invasive breast cancers diagnosed in 2004 through 2015. Multivariable logistic regression was used to evaluate association between HT and high-risk RS. Relationships between HT and low-, intermediate-, and high-risk RS were compared with chi(2) test. Kaplan-Meier curves were compared using log-rank test. Results.-A total of 110 318 patients had RS testing. Of these, 23 220 (21%) had low, 70 822 (64.2%) intermediate, and 16 276 (14.8%) high RS. Histologic types were 80 476 (73%) ductal, 12 713 (113%) lobular, 12 449 (11.3%) mixed, 2151 (2%) mucinous, 610 (0.6%) tubular, 382 (0.4%) micropapillary, 365 (0.3%) salivary, 208 0.2%) papillary, 49 (0.04%) medullary, 26 (0.02%) metaplastic, 26 (0.02%) neuroendocrine, and 863 (0.8%) unknown. The distribution of low-, intermediate-, and high-risk RS was significantly different among HTs. Higher percentages of high-risk RS were identified in patients with ductal, medullary, and metaplastic types (P < .001). The odds of having high-risk RS were lower for some HTs, including micropapillary, after multivariable adjustment (P < .05). The low number of estrogen receptor-positive medullary and metaplastic carcinomas tested had higher odds of having high-risk RS. In T1 and T2 tumors, when ductal, lobular, mixed, and other types combined were compared, the mortality was different. Conclusions.-This population-based study of RS in HTs showed high-risk RSs are identified in traditionally good prognostic subtypes. Micropapillary carcinoma has lower odds of high-risk RS even after multivariable adjustment.