A Case of Invasive Ductal Carcinoma with Axillary Skip Metastasis Confined to the Interpectoral (Rotter's) Lymph Node.

[INTRODUCTION] Axillary skip metastasis is a rare phenomenon in breast cancer and is defined as metastasis to level II or III lymph nodes without involvement of level I nodes. Interpectoral (Rotter's) nodes are situated between the pectoralis major and minor muscles and may occasionally be overlooked during sentinel node (SN) mapping. Reports of isolated interpectoral node metastases are rare. Here, we present a unique case of breast cancer with isolated interpectoral node involvement despite a negative sentinel lymph node, underscoring the clinical implications of preoperative imaging and surgical planning.

[CASE PRESENTATION] A 69-year-old woman was referred to our hospital after an abnormality was detected by mammography. MRI demonstrated a 20-mm enhancing breast mass located in the deep portion of the upper outer quadrant, along with a strongly enhancing 6-mm interpectoral lymph node; no suspicious axillary level I nodes were identified. The patient underwent a mastectomy with sentinel and interpectoral node biopsies. The SN was negative, whereas the interpectoral node was positive, prompting axillary dissection. Histology confirmed a 15-mm invasive ductal carcinoma, with only the interpectoral node being positive among the 12 dissected nodes. Immunohistochemistry showed an ER-positive, PgR-positive, and HER2-negative status. The patient was started on adjuvant endocrine therapy. Her postoperative course was uneventful and she remained disease-free at 54 months of follow-up.

[CONCLUSIONS] This extraordinarily rare case of axillary skip metastasis limited to the interpectoral node emphasizes the potential for false-negative SN biopsies. Careful review of preoperative images, particularly MRI images, is crucial to avoid understaging. Awareness of interpectoral node involvement may help guide appropriate treatment strategies for selected patients.