Primary tumor cells can metastasize to local, regional, and distant sites through four main mechanisms: direct invasion, lymphatic spread, hematogenous spread, and celomic spread. In most solid tumors, the initial form of tumor spread is lymphatic metastasis to regional lymph nodes.
In many types of cancer, the presence of regional lymph node metastasis is one of the most important predictors of recurrence and survival. Consequently, the presence of regional lymph node metastasis often leads to recommendations for additional treatment, such as more extensive lymph node surgery, radiation therapy, and/or systemic therapy (i.e., chemotherapy, biological therapy, or targeted therapy). Given the clinical relevance of regional lymph node metastasis in many types of cancer, evaluation of the regional lymph nodes is an essential component of the staging of those cancers.
In this chapter, we provide an overview of the importance of the lymphatic system in tumor metastasis, explain the concept of the sentinel lymph node (SLN) as it relates to cancer patients, describe and illustrate how SLNs are identified and assessed in clinical practice, and provide clinically salient examples of the impact of SLN metastasis on recurrence and prognosis.
THE LYMPHATIC SYSTEM
The lymphatic system in humans has three major interrelated functions: (1) transport of interstitial fluid to the lymphoid tissue; (2) absorption of fatty acids and their transport as chyle to the circulatory system; and (3) transport of antigen-presenting cells, such as dendritic cells, to lymph nodes for activation of the immune system.