Relationship of histologic grade and histologic subtype with oncotype Dx recurrence scoreI3/4 retrospective review of 863 breast cancer oncotype Dx results

Oncotype Dx (ODx) is a multigene assay that is prognostic and predictive in estrogen receptor (ER) positive early breast cancer. ODx recurrence score (RS) is reported to be histologic grade dependent. Relationship of RS with breast cancer histologic subtypes is unknown. This study was designed to assess the relationship of histologic subtype with RS. Histologic grade dependence of RS was also investigated. Results of consecutive ODx tests (1/2007-7/2016) from two institutions were reviewed. Histologic subtypes (in: Lakhani et al., WHO classification, IARC Press, Lyon, 2012), combined Nottingham histologic grade, age and tumor size were recorded from pathology reports. Univariate and multivariate analysis was performed to investigate the relationship between RS and ODx risk categories and histologic subtypes, grade, age and tumor size. RS was grade dependent. RS of grade 1 and grade 2 tumors were significantly lower than grade 3 tumors. There was no high-risk grade 1 tumor. In favorable histologic subtypes there was no high-risk tumor. Mean RS of grade 1 lobular tumors was significantly higher than grade 1 ductal tumors. Using newer ODx cut-offs, 5 grade 1 tumors were reclassified as high risk (RS > 25) and grade 3 lobular tumors showed significantly higher rate of reclassification as high-risk than grade 3 ductal tumors. In a multivariate analysis, only grade showed a significant positive correlation with RS. Adding dichotomous histologic subtyping (favorable vs. non-favorable) to grade further improved correlation with RS. The Oncotype Dx result is impacted by histologic grade and histologic subtype. Tumors with favorable histologic subtypes and histologic grade 1 tumors do not have high-risk RS. High RS in a grade 1 tumor or in a tumor with favorable histology is unusual that warrants further investigation. Invasive lobular carcinomas rarely show high-risk RS. Histologic grade and histologic subtype should be considered while ordering ODx testing.