Response to Neoadjuvant Chemotherapy as a Potential Indicator of Internal Mammary Node Involvement in Early Breast Cancer.

PURPOSE: Internal mammary lymph node (IMN) irradiation has the potential to improve the overall survival in patients with nodal-positive breast cancer. Nevertheless, achieving these benefits while keeping treatment-related morbidity low is technically complex and requires careful patient selection and treatment planning. For this, detection of IMN involvement is of particular importance. This study aims to investigate the response of visible IMN to neoadjuvant chemotherapy (NACT) as a potential surrogate for initial IMN involvement.

METHODS AND MATERIALS: We analyzed 298 patients with nonmetastatic breast cancer who underwent high-resolution pretreatment magnetic resonance imaging (MRI) and follow-up MRI after NACT. If visible IMNs were present, diameters were measured in axial and coronal planes on both pre- and post-NACT MRIs. Differences in size were assessed, and response was systematically classified and correlated with clinical and histopathological response, axillary lymph node (AXN) metastasis (AXN+), and tumor characteristics.

RESULTS

Out of 298 patients with breast cancer, 175 (58.7%) had visible IMN on pretreatment MRI and 158 patients had a histopathological response of the primary tumor. Among these, 62 patients (39.3%) had a response of visible IMN. In patients with AXN+, the rate of IMN response was particularly high at 48.6%. In patients without AXN metastasis, 21.8% of patients with visible IMN showed a response to NACT. Response of visible IMN correlated significantly with medial tumor location and predominant tumor contact with internal mammary perforator vessels. The large majority (76.3 %) of IMN with response to NACT were initially rated as nonsuspicious during initial staging and in the interdisciplinary tumor board consensus.

CONCLUSIONS

Clinical IMN response to NACT may serve as an indicator of previously undetected IMN involvement in a relevant proportion of patients with early breast cancer. The observation that approximately 20% of patients without AXN metastases demonstrated a radiologic IMN response warrants further investigation, as this subgroup is not routinely considered for IMN irradiation. These findings highlight the need for prospective validation in larger, well-characterized cohorts using standardized imaging protocols to better define the clinical significance of IMN response to systemic therapy.