Patient Selection for Partial Breast Irradiation in the Era of Sentinel Lymph Node Biopsy Omission: A Narrative Review

Partial breast irradiation (PBI) and omission of sentinel lymph node biopsy (SLNB) are major advances in the de-escalation of locoregional treatment for early-stage breast cancer. These strategies have evolved independently under different clinical assumptions. The randomized trials establishing PBI incorporated pathological axillary staging into patient selection, although the permitted nodal burden varied among studies, whereas contemporary trials have demonstrated that sentinel lymph node biopsy (SLNB) can be safely omitted in carefully selected patients. Consequently, radiation oncologists are increasingly encountering otherwise suitable candidates for PBI who lack pathological nodal information because SLNB has been omitted, creating an important evidence gap. This narrative review examines how pathological nodal staging became embedded within PBI eligibility criteria and whether this long-standing requirement remains justified following the introduction of SLNB omission. We discuss the biological and clinical rationale underlying current patient selection for PBI, the limitations of extrapolating evidence from whole-breast irradiation to PBI, and the potential contribution of modern imaging, tumor biology, molecular profiling, and prediction models for occult nodal disease to individualized risk assessment. Current evidence does not justify routine implementation of PBI following omission of SLNB. Pathological nodal information may be considered as one component of integrated risk assessment rather than an isolated determinant of PBI eligibility. Patients otherwise meeting contemporary radiotherapy omission criteria represent the most appropriate population for prospective evaluation of this combined de-escalation strategy. This review defines the resulting evidence gap and proposes a conceptual framework for future clinical investigation.