Partial Breast Irradiation for High Molecular Risk Early-Stage Breast Cancer

Partial breast irradiation (PBI) is a suitable and well-tolerated alternative to whole breast irradiation (WBI) following lumpectomy for many forms of low-risk, early-stage breast cancer. Molecular risk scores, such as the Oncotype DX recurrence score (ODX RS), are increasingly guiding systemic treatment decisions. However, molecular/genomic profiling for radiation therapy (RT) decision-making remains investigational, and it is unclear whether a high ODX RS should preclude the use of PBI. We compared oncologic outcomes among patients with high ODX RS (>25) treated with PBI versus WBI.

Patients who underwent breast conservation followed by PBI or WBI with ODX RS > 25 were ascertained from a prospectively maintained institutional database. Comparable PBI and WBI cohorts were generated in 1:5 fashion using propensity score matching based on salient clinicopathologic features. We evaluated the incidence of local recurrence (LR) as a function of RT approach.

We identified 968 patients with an ODX RS > 25 who were treated with adjuvant RT, with a median age of 59 years (range, 25-86) and a median 5.3 years of follow-up. In a propensity matched cohort analysis that included 28 patients who received PBI matched to 140 who received WBI, we observed 3 LR events among those receiving PBI (2 of which were in different quadrants from the primary lesion) and 5 events among those receiving WBI. Among this cohort with ODX RS > 25, the 72-month cumulative incidence of LR following PBI was 7.9% (95% CI, 1.3%-23%) compared to 4.8% (95% CI, 1.6%-11%) following WBI (P = .6).

In this cohort of patients with high ODX RS, few LRs were observed, and no statistically significant difference in LR was identified between PBI and WBI. Although these findings suggest that PBI may be considered in carefully-selected high-genomic-risk patients, larger studies with longer follow-up are needed to definitively establish the safety of this approach.