A mitotically active, cellular tumor stroma and/or inflammatory cells associated with tumor cells may contribute to intermediate or high Oncotype DX Recurrence Scores in low-grade invasive breast carcinomas.

Oncotype DX is an RT-PCR-based 21-gene assay validated to provide prognostic and predictive information in the form of a Recurrence Score in patients with estrogen receptor-positive, lymph node-negative breast cancer. Although the Recurrence Score was shown to correlate with several histopathological tumor features, there is a significant proportion of cases showing an apparent discrepancy between Recurrence Score and risk estimates based on the traditional clinicopathological tumor features. In this study, we tested whether a proliferating, cellular stroma and/or admixed inflammatory cells may result in an artificially increased Recurrence Score in low-grade invasive breast cancers. We analyzed the histopathological features in 141 low-grade invasive breast carcinomas, including 41 special type (tubular, cribriform and mucinous) carcinomas, with available Recurrence Score. The tumor stroma was evaluated for increased cellularity and presence of inflammatory cells. Double immunohistochemical stains for pancytokeratin and Ki-67 was performed to assess the cell proliferation in tumor vs stromal/inflammatory cells. The clinicopathological features of tumors with Recurrence Score <18 (low risk) were compared with those with Recurrence Score ≥18 (intermediate/high risk). Carcinomas associated with Recurrence Score ≥18 showed lower progesterone receptor immunoreactivity, increased stromal cellularity and presence of inflammatory cells associated with the tumor. Double immunohistochemical stains showed significantly increased proliferation in stromal/inflammatory cells compared with carcinoma cells in cases associated with Recurrence Score ≥18. A Ki-67-positive stromal/tumor cells ratio of >1 predicted Recurrence Score ≥18 with an area under the curve of 0.8967 on receiver operator curve analysis (P<0.0001). Our results suggest that the presence of increased stromal cellularity and/or associated inflammatory cells in low-grade invasive breast carcinomas may contribute to an apparently increased risk of recurrence according to Oncotype DX Recurrence Score. Careful assessment and correlation with histopathological features in such cases may help in determining the appropriate patient management.