Importance: Guidelines recommend consideration of chemotherapy for most patients with early-stage, estrogen receptor-positive, invasive breast cancer in the absence of additional prognostic information. The 21-gene recurrence score (RS) assay has been shown in limited academic settings to reduce physician recommendations for adjuvant chemotherapy. Associations between the adoption of the assay and receipt of chemotherapy in the general population have not been examined.

Objective: To examine whether adoption of the RS assay in a nationally representative sample of patients with early-stage breast cancer was associated with use of chemotherapy.

Design, setting, and participants: Retrospective cohort study of Medicare beneficiaries who received a diagnosis of incident breast cancer between 2005 and 2009 using Surveillance, Epidemiology, and End Results data set with linked Medicare claims.

Main outcomes and measures: Receipt of chemotherapy within 12 months after diagnosis.

Results: A total of 44,044 patients had low-risk (24.0%), intermediate-risk (51.3%), or high-risk disease (24.6%, lymph node positive) as defined by National Comprehensive Cancer Network (NCCN) guidelines and met the study criteria. We observed no overall association between receipt of the RS assay and chemotherapy (odds ratio [OR], 1.03 [99% CI, 0.88-1.19]). In multivariable analysis, there was a significant interaction between NCCN risk and use of the RS assay, with assay use associated with lower chemotherapy use in high-risk patients (OR, 0.36 [99% CI, 0.26-0.50]) and greater chemotherapy use in low-risk patients (OR, 3.71 [99% CI, 2.30-5.98]), compared with no receipt of the assay (P=.006 for the overall interaction). Results were similar in prespecified subgroup analyses of patients 70 years and younger, with the exception of a shift toward lower chemotherapy use during 2008 (OR, 0.90 [99% CI, 0.77-.1.05]; P=.09) and 2009 (OR, 0.81 [99% CI, 0.66-0.99]; P=.007). In unadjusted analyses, overall chemotherapy use decreased over time in patients 70 years or younger with high-risk disease and those receiving the assay.

Conclusions and relevance: The impact of the adoption of the RS assay on receipt of chemotherapy was strongly population dependent and was associated with relatively lower chemotherapy use in groups with high-risk disease and relatively higher chemotherapy use in patients with low-risk disease. Overall use of chemotherapy decreased during the study period in patients who were most likely to receive chemotherapy.