Radiotherapy Boost to Clinically Positive Internal Mammary Lymph Nodes: An Effective Non-Surgical Alternative?

To evaluate the effect of radiation therapy (RT) boost sequences and doses to internal mammary (IM) lymph nodes (LNs) on oncologic outcomes and toxicity in patients with clinically IMLN-positive breast cancer.

Twenty-six patients with positive IMLNs at diagnosis who were treated between 2013 and 2022 were retrospectively evaluated. After systemic therapy and surgery, RT was applied to the whole breast or chest wall, including the axillary, supraclavicular, and IM lymphatics. An RT boost to the IMLNs was administered either sequentially or as a simultaneous integrated boost (SIB). Treatment outcomes, prognostic factors, and toxicity profiles were analyzed.

A complete response (CR) and partial response (PR) in metastatic IMLNs were observed in 17 patients (65.4%) and 5 patients (19.2%) following systemic therapy, respectively. One patient (3.8%) had stable disease, while two patients (7.7%) experienced disease progression in the IMLNs.After a median follow-up of 67 months, regional recurrence, distant metastasis, or both occurred in 1 (3.8%), 1 (3.8%), and 2 (7.7%) patients, respectively. The 5-year overall survival (OS) and disease-free survival (DFS) rates were 86.7% and 76.4%, respectively. The absence of extracapsular extension in axillary LNs was associated with improved OS and DFS. A CR or PR to systemic treatment in IMLNs was associated with improved DFS only. The boost sequence had no significant effect on survival, organ-at-risk doses, or cardiac toxicity.

An RT boost to IMLNs without surgical resection provides excellent oncologic outcomes with acceptable toxicity. While the SIB technique reduces the number of treatment fractions, it did not result in superior oncologic outcomes compared to the sequential boost. Further prospective studies are needed to investigate the role of RT boost sequencing to the IMLNs.