Principles of sentinel lymph node identification: background and clinical implications

The management of clinically negative regional lymph nodes in early-stage melanoma has been controversial for many years. While some advocate wide excision of the primary with elective node dissection (ELND), others recommend excision of the primary alone and therapeutic node dissection (TLND) for recurrences in the nodal basin. ELND is based on the concept that metastases occur by passage of the tumor from the primary to the regional nodes and distant sites, in which case early dissection of regional nodes will disrupt metastatic progression and prevent the spread of disease. Advocates of the "wait and watch" approach suggest that regional node metastases are markers for disease progression and that distant disease can occur without node metastases. Four randomized prospective studies comparing ELND and TLND have not demonstrated overall survival advantage for ELND, but suggest that patients with early regional metastases may benefit from ELND. As an alternative, Morton et al., from UCLA and the John Wayne Cancer Institute, devised intraoperative lymphatic mapping and sentinel lymphadenectomy (LM/SL). These minimally invasive operative procedures allow identification of the first and key (sentinel) lymph node (SN). The technique accurately maps the lymphatics by lymphoscintigraphy, and vital blue dye leads the surgeon to the SN. The pathologist then concentrates on seeking metastases in the nodes most likely to contain metastases. Patients with tumor-positive SN undergo completion lymph node dissection (CLND), while those without SN metastases avoid the complications and costs associated with this procedure. Morton et al., in a report on their initial experience of LM/SL, performed CLND in all cases regardless of SN tumor status and demonstrated the precise staging capacity of the procedure. Since this initial report, numerous studies have validated the accuracy and low morbidity of the procedure. Each center must master a learning phase. The procedure is dependent on the close cooperation of nuclear medicine physicians, surgeons, and pathologists. While LM/SL is now almost standard practice in the US, the results of clinical trials are awaited to determine whether LM/SL can replace ELND and TLND in the management of early-stage melanoma.