Internal mammary lymph nodes in breast cancer: A comprehensive review of anatomy, imaging, biopsy considerations, clinical implications, and future directions.

Internal mammary lymph nodes (IMLNs) play a pivotal role in breast cancer staging, prognosis, and therapeutic planning, yet challenges in detection and ongoing controversies regarding their management persist. This review consolidates current evidence on the anatomy, lymphatic function, imaging features, biopsy considerations, and clinical implications of IMLNs. These nodes, visualized in 1-48% of patients depending on imaging modality, serve as secondary lymphatic drainage sites, particularly for medially or deeply located tumors. Metastatic involvement, observed in 1-5% of cases, is classified as N3b according to the AJCC 8th edition. Advanced imaging tools such as ultrasound and MRI facilitate differentiation between benign IMLNs (oval with a central fatty hilum) and malignant ones (round with cortical thickening >3 mm). While ultrasound may assist in detecting suspicious IMLNs, percutaneous biopsy is generally not recommended due to anatomical proximity to major vessels and the risk of complications. Internal mammary sentinel lymph node biopsy (IM-SLNB) offers a minimally invasive staging option, with reported visualization rates of 63-72%. Select studies suggest that IMLN irradiation may reduce recurrence and mortality, although survival benefits remain debated. Emerging predictive models-including nomograms and AI-based radiomics-show promise in preoperative risk stratification. This review highlights the need for standardized diagnostic protocols, routine evaluation of IMLNs in high-risk patients, and large-scale prospective trials to validate the utility of IM-SLNB and optimize therapeutic strategies. Integrating advanced imaging with precision medicine may ultimately enhance detection, tailor treatment, and improve outcomes in breast cancer management.