22 Implications of SOUND Trial on Adjuvant Radiation Therapy in Early Stage Breast Cancer Patients

Background

Breast-conserving surgery followed by adjuvant radiation therapy (RT) is a cornerstone of early-stage breast cancer management. Recent trials support further de-escalation of care. The SOUND trial demonstrated that omission of sentinel lymph node biopsy (SLNB) in patients with early-stage breast cancer and a normal axillary ultrasound was noninferior to SLNB in the setting of whole breast radiation. Alternatively, the APBI-IMRT-Florence trial demonstrated no increased risk of ipsilateral tumor recurrence, fewer symptoms of treatment-related toxicity, and improved cosmetic outcomes with partial breast irradiation (PBI). In light of recently published trials, patients with small, early-stage breast cancers face complex tradeoffs between axillary surgical staging and adjuvant radiotherapy. How these findings translate into clinical practice, particularly with respect to adjuvant RT utilization, remains unclear. We evaluated institutional treatment patterns to assess implications of de-escalation strategies on adjuvant therapy decisions.

Materials and Methods

We conducted a retrospective chart review of patients aged 40 to 70 years with clinically node-negative breast cancer measuring less than or equal to 2 cm treated at a single academic institution between 2000 to 2024. Patient demographics, tumor characteristics, SLNB results, endocrine and adjuvant RT details were abstracted from the medical record. Eligibility for SOUND trial criteria was assessed; patients with incomplete data were excluded. Descriptive statistics were used to summarize baseline characteristics and treatment patterns.

Results

Eleven patients met inclusion criteria, with a mean age of 61 years. Among patients meeting SOUND trial criteria, 85.7% (n = 695) were node-negative following SLNB, while 3.5% (n = 28) had nodal micrometastases (pN1mi) and 10.9% (n = 88) had nodal macrometastases (pN1–pN3).

Regarding adjuvant RT, 75.3% received whole-breast irradiation, 22.4% received PBI, and 2.2% received chest wall irradiation. Adjuvant endocrine therapy was administered in 83.2% of patients.

Conclusion

All patients underwent SLNB, with 85.7% found to be node-negative, and, therefore, eligible for partial-breast radiation. Of clinically node negative patients, 14.4% were found to have nodal metastases, similar to rates observed in the control arm of the SOUND trial. For these patients, whole breast with consideration of regional nodal irradiation (RNI) remains current guideline-concordant care, with RNI correlating with a survival advantage. Furthermore, nearly one-fifth of patients never received adjuvant endocrine therapy and adherence among treated patients is challenging to document. These findings highlight the interconnected nature of axillary staging, radiation treatment volume, and systemic therapy decisions. As SLNB omission becomes standard for selected patients, careful multidisciplinary and shared decision-making is essential to ensure safe and coordinated de-escalation of breast cancer treatment plans. Further study is warranted to define optimal integration of SOUND trial findings into practice.