Identification of patients with DCIS with low-risk clinicopathology who benefit from radiation therapy with and without endocrine therapy after breast-conserving surgery assessed with the 7-gene biosignature.
Authors: Pat W. Whitworth, Chirag Shah, Frank Vicini, Rachel Rabinovitch, Julie A. Margenthaler, Fredrik Warnberg, Michael C. Leo, Sheila Weinmann, Bruce Mann, David J. Dabbs, Jess Savala, Steven C. Shivers, Karuna Mittal, Troy Bremer
Journal: Journal of Clinical Oncology (2024)

528 Background: Ductal carcinoma in situ (DCIS) accounts for 20% of breast cancers. For most patients with DCIS, the standard treatment involves breast-conserving surgery (BCS) followed by radiotherapy (RT). Adjuvant endocrine therapy (ET) is typically recommended for hormone receptor-positive disease. However, prospective trials have failed to identify a low-risk CP group not benefitting from RT with respect to ipsilateral breast recurrence (IBR), with no modern trials evaluating ET. We utilized the 7-gene predictive DCIS biosignature to assess the effects of RT and/or ET on 10yr IBR in women with low-risk CP DCIS. Methods: Women (n=926) from four DCIS cohorts treated with BCS had tissue samples analyzed at a CLIA lab (Laguna Hills, CA). Low-risk CP was defined with RTOG 9804-like criteria (margin negative by no-ink on tumor) or MSKCC-like criteria using nomogram-weighted factors (low-risk score&lt;220, excluding number of re-excisions, using negative margin, and RT treatment). Women with either low-risk CP criteria were classified as molecular Low Risk (DS≤2.8, no Residual Risk subtype, RRt) or High Risk (DS&gt;2.8 +/- RRt) using the 7-gene DCIS biosignature. Ten-year IBR Kaplan-Meier rates and Cox proportional hazard ratios (HR) were calculated for ET and RT. Results: Overall, 66% of 926 women were classified as low risk by one of the CP criteria. Patients with low-risk CP treated without ET (n=315) had 10yr IBR rates of 13% after BCS without RT and 7% with RT (HR=0.51, p=.10), while those treated with ET (n=291) had 10yr IBR rates of 9% after BCS without RT and 4% with RT (HR=0.39, p=.09). Overall, 37% of women were classified as low risk (n=338) by the biosignature and had a 10yr IBR rate of 5.6% after BCS with no significant ET (HR=0.66, p=.41) or RT benefit (HR=0.88, p=.78) in multivariable analysis. Of patients with low-risk CP, 48% of those treated without ET were classified as High Risk by the biosignature and had elevated 10yr IBR rates without RT (21%) and a substantial RT benefit (HR=0.31, p=.03, n=151), while 59% of those treated with ET after BCS were classified by the biosignature as High Risk and had elevated 10yr IBR rates without RT (14%) and a 9% absolute reduction from RT (HR=0.22, p=.02, n=173). Patients with concordant low-risk CP and biosignature Low Risk (n=269) had a 5.5% 10yr IBR rate after BCS and no significant RT (HR=1.10 95% CI 0.33 to 3.7, p=.87) or ET benefit (HR=0.44, 95%CI 0.1 to 1.6, p=.22) in multivariable analysis. Conclusions: The 7-gene biosignature provided better identification of patients with low 10yr IBR rates and no significant RT benefit compared to using CP low-risk criteria. Importantly, the biosignature classified over half of women with low-risk CP treated with ET as High Risk. This group received a substantial benefit from RT, while those with biosignature Low Risk did not.