Use of adjuvant chemotherapy in hormone receptor-positive breast cancer patients with or without the 21-gene expression assay

Purpose We assessed the use of chemotherapy in breast cancer patients to investigate the factors that changed trends in chemotherapy following the adoption of the 21-gene expression assay in tumor genomic profiling. Methods Our study used 2033 patients from the National Cancer Center in Korea diagnosed with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer (tumor size of 0.5 cm or larger and 0–3 node metastases) from 2010 to 2015. We analyzed use of the 21-gene expression assay, changes in frequency of adjuvant chemotherapy use, and clinicopathological factors related to adjuvant chemotherapy to assess the impact of the 21-gene expression assay. Results Adjuvant chemotherapy use declined from 33.33% (2011) to 13.59% (2015) [relative risk (RR), 0.71; 95% CI 0.56–0.89; p trend = 0.004] in patients with 21-gene expression assay data. Among patients without assay data, adjuvant chemotherapy use decreased from 76.79 to 40.17% between 2010 and 2015 (RR 0.87; 95% CI 0.84–0.91; p trend < 0.001), especially for patients with node-negative/micrometastasis (RR 0.85; 95% CI 0.81–0.89; p trend < 0.001). The frequency of adjuvant chemotherapy was significantly decreased after introduction of the 21-gene expression assay ( p < 0.001). Tumor size ( p < 0.001), progesterone receptor (PgR) status ( p = 0.001), and proliferation index (Ki-67) levels ( p < 0.001) were important factors for chemotherapy decision-making in node-negative/micrometastasis patients who did not undergo the assay. Conclusions For HR-positive, HER2-negative breast cancer patients with 0–1 node metastases, chemotherapy use declined significantly after the adoption of the 21-gene assay. PgR status and Ki-67 were useful for chemotherapy decision-making in cases without the 21-gene assay.