Microscopically, metastatic adenocarcinoma (composed of malignant gland-like structures) is seen in a lymph node here. It is common for carcinomas to metastasize to lymph nodes, traveling via lymphatic channels from the site of origin. The first nodes involved are usually those receiving lymphatic drainage from the site of the primary neoplasm. Hence, if this is a left axillary lymph node, then the primary site of the cancer is likely in the left breast.

The initial lymph node receiving lymph drainage may trap the cancer cells to prevent further spread. This is the concept behind sentinel lymph node biopsy. In patients who do not have advanced breast cancer, a small volume of cancer cells detected in a sentinel node suggests that local and systemic adjuvant therapy will adequately treat any remaining cancer, and no additional surgery is required.