e22016 Background: Tumor gene expression analysis using the Recurrence Score (RS) assay is frequently used in ER+ breast cancer. Manual microdissection is performed in cases where biopsy cavities (BxC) are present in the submitted specimen. The objective of this was to characterize by quantitative RT-PCR the impact of BxC on 21 gene expression profiles and the RS. Methods: 48 (15 well, 18 moderate, and 15 poorly differentiated) breast cancers were evaluated for gene expression differences between whole sections (WS; containing BxC) and enriched tumor (ET; BxC excluded). Standardized quantitative RT-PCR analysis for the 21 Oncotype DX genes was performed; reference normalized gene expression measurements ranged from 0 to 15, where each 1-unit reflects an approximate 2-fold change in RNA. Analyses of individual genes and the RS were performed on the entire sample set and stratified by tumor grade. Correlation analyses used Pearson's R, concordance analysis used Lin's sample concordance and paired t- tests to characterize differences. Results: There were statistically significant differences in reference normalized gene expression between ET and WS in 6 genes: BAG1 (ET-WS: 0.13 units, p=0.0025), CD68 (ET-WS: -0.64 units, p<0.0001), ER (ET-WS: 0.29 units, p=0.0012), GSTM1 (ET-WS: 0.18 units p=0.0025), STK15 (ET-WS: -0.18 units, p=0.0041) and STMY3 (ET-WS: 0.62 units, p<0.0001). Expression of the macrophage marker CD68 was higher and expression of ER was lower in WS containing BxC. The correlation (0.95) and concordance (0.92) were generally high between WS and ET for RS overall however among moderately differentially tumors, there was a statistically significant mean increase in RS for WS of 3.3 units (p = 0.0012) while among poorly differentiated tumors there was a trend toward a statistically significant decrease in RS for WS of 2.2 units (p=0.0569). Conclusions: Histologic identification of invasive carcinoma and exclusion of BxC is essential for precise RS assessment. Inclusion of BxC in breast cancer specimens is associated with significant changes in the expression of individual genes and impacts the RS. Removal of BxC from breast cancer specimens assessed for gene expression levels is warranted. [Table: see text]