Is the Oncotype DX assay necessary in strongly estrogen receptor-positive breast cancers?

The 21-gene Oncotype DX recurrence score (RS) assay quantifies risk of distant recurrence and predicts benefit from chemotherapy in tamoxifen-treated estrogen receptor (ER)-positive, node-negative breast cancer. Although clinically useful, the assay costs roughly $4650. Because the assay is weighted heavily towards expression of ER, our objective was to determine its clinical utility in strongly ER-positive tumors. This was a retrospective study of Huntington Hospital patients undergoing an Oncotype DX assay between 2007 and 2010. Data collected included patient age, expression of ER, progesterone receptor (PR), HER2/neu, ki67, and p53, tumor size, node status, lymphovascular invasion, and nuclear grade. Of 133 total patients, 84 (63.2%) had strongly ER-positive tumors (≥90% expression). Only seven of 84 patients (8.3%) had a high risk RS (>30), indicating statistically significant predicted benefit from chemotherapy. All seven had intermediate to high ki67 expression (>20%) and lower PR expression (≤50%). Our study demonstrates that the clinical utility of the Oncotype DX assay in these patients is limited as most patients with strongly ER-positive tumors will have a low or intermediate RS. Future studies are needed to identify additional predictive factors in these patients with otherwise good prognosis.