Abstract P6-16-01: Refined local recurrence risk estimates based on a multigene expression assay combined with clinicopathological features significantly impacts radiotherapy recommendation in patients with low/intermediate risk DCIS treated with breast-conserving surgery

Abstract Background: Guidelines recommend that breast radiation (RT) can be omitted for patients with a low risk of local recurrence (LR) after breast-conserving surgery (BCS) for DCIS. The inability to identify women at low risk of LR (<10%) at 10 years after BCS has hampered efforts to de-escalate therapy for DCIS. The Oncotype DX Breast DCIS Score® (DS), a 12-gene expression assay, predicts LR risk after BCS. The revised DS report adds information from the DS with clinicopathological features (CPF) (age, tumor size, year) (BCRT 2018;169;358-690) to provide refined estimates of LR risk and can better identify patients with low 10-year LR risk (<10%) after BCS. We evaluated the impact of refined LR risk estimates on its ability to change radiation oncologists’ recommendations for RT in women with low/moderate risk DCIS treated with BCS. Methods: Prospective cohort study of women with low/moderate risk pure DCIS treated with BCS. Cases with age <45 yrs, margins <1mm, tumor >2.5cm, multifocality, or prior breast cancer were excluded. Baseline CPFs, the DS and risk of LR were collected. Pre-assay, the radiation oncologist provided an estimate of 10-year LR risk …