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The level I axilla is caudal and lateral to the muscle, level II is beneath the muscle, and level III (also known as the infraclavicular region) is cranial and medial to the muscle. A standard axillary lymph node dissection resects the tissue and lymph nodes within levels I and II. It is very unusual to have involvement of level III of the axilla without disease in level I or II. The axillary lymph nodes continue underneath the clavicle to become the supraclavicular lymph nodes, which can be involved in locally advanced breast cancers.

Lymphatics can also drain directly into the internal mammary lymph node chain (IMC), which are intrathoracic structures located in the parasternal space. Although these nodes are not usually visualized on computed tomography (CT), the anatomical region of the IMC can be determined by the internal mammary artery and vein, which are easily visualized by CT (Fig. 53.3), and usually lie 3 to 4 cm lateral to midline. When breast cancer involves the IMC, the majority of cases will have disease that is limited to lymph nodes in the first three interspaces. Regardless of the location within the breast, the axilla is the most common site of lymphatic involvement. However, breast cancers that develop in the medial, central, or lower breast more commonly drain to the IMC (in addition to the axilla) than those occurring in the lateral and upper quadrants.

The use of lymphoscintigraphy, by injecting technetium-99 radiocolloid into the peritumoral region, followed by scintillation scanning, is used now for sentinel lymph node imaging. This technique has helped to delineate primary lymphatic drainage patterns of breast cancer. In a study reported by Estourgie et al. (190) of 700 patients undergoing sentinel node mapping, the distribution of axillary and internal mammary drainage is summarized in Fig. 53.4. Even in inner-quadrant lesions, axillary drainage is more common than internal mammary drainage. However, internal mammary drainage was present in over 50% of lower inner-quadrant lesions.