Partial breast irradiation after sentinel node omission remains an evidence gap

KEY POINTS

• PBI and omission of sentinel lymph node biopsy are both supported by randomized evidence in selected early breast cancer, but they were developed in different treatment contexts. The major PBI trials routinely incorporated pathological axillary staging, whereas contemporary SLNB-omission trials predominantly used whole-breast irradiation.
• Long-term randomized PBI data remain reassuring when pathological nodal information is available. Ten-year ipsilateral breast recurrence was 3.0% with PBI versus 2.8% with whole-breast RT in IMPORT LOW, 3.7% versus 2.5% in Florence, and 3.5% versus 1.6% in GEC-ESTRO; NSABP B-39/RTOG 0413 reported 4.6% versus 3.9%, although formal equivalence was not demonstrated.
• SLNB omission is also supported by randomized evidence, but not specifically with PBI. SOUND reported five-year distant disease-free survival of 98.0% without axillary surgery versus 97.7% with SLNB, while INSEMA reported five-year invasive disease-free survival of 91.9% versus 91.7%.
• The crucial radiotherapy distinction is that approximately 80% of SOUND patients received whole-breast irradiation, while INSEMA mandated whole-breast irradiation and explicitly prohibited PBI. Whole-breast tangents can deliver incidental dose to axillary levels I–II, whereas PBI deliberately minimizes dose outside the tumor-bed region.
• Occult nodal disease is not negligible even among patients otherwise eligible for SLNB omission: approximately 14–15% of patients meeting SOUND or INSEMA-type criteria harbor sentinel-node metastases when surgical staging is performed. A negative axillary ultrasound lowers this probability but does not reproduce pathological staging.
• The authors propose that the first population for prospective evaluation should be patients at the lowest end of recurrence risk, particularly those who otherwise resemble contemporary radiotherapy-omission cohorts. Examples include older/postmenopausal patients with small, hormone receptor-positive, HER2-negative tumors and favorable pathological or genomic features.
• The review does not recommend routine PBI after SLNB omission. Instead, it proposes moving from nodal status as a single absolute eligibility criterion toward integrated assessment incorporating clinical features, axillary imaging, tumor biology, molecular profiling and estimated risk of occult nodal disease, with prospective validation required before practice change.

CLINICAL TAKEAWAY

SOURCE

Cancers