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The sentinel node (SN) concept as currently applied to breast cancer and melanoma is predated by the idea that a single lymph node can reflect the tumor status of an entire lymphatic basin. Famous examples include the Virchow node (left supraclavicular node to which gastric cancer spreads), the Sister Mary Joseph node (an umbilical lymph node that represents metastatic intra-abdominal spread), the Delphian node of the thyroid, and the Cloquet node of the groin.1 The concept of the SN technique as first described by Cabanas in 1977 for use in squamous cell carcinoma of the penis was based on detailed penile lymphangiographic studies that demonstrated consistent drainage of the penile lymphatics into a node located near the saphenous/femoral vein junction.2 When this so-called SN was negative for tumor, metastasis to other ilioinguinal lymph nodes did not occur. Cabanas therefore postulated that the status of the SN could be used to decide whether or not regional lymphatic clearance was necessary. Although multiple studies have since found that a fixed-location SN is an unreliable indicator of nodal status in penile cancer, this work paved the way for mapping the SN in patients with solid cancers that drain via the lymphatics.
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Sentinel node biopsy (SNB) for melanoma was first described in 1992 by Morton and colleagues.3 They defined the SN as the first draining node of a tumor. If nodal spread has occurred, it will target the SN before other lymph nodes. Therefore, if the SN is tumor negative, the other nodes should be negative as well. These investigators showed that when a vital blue dye is injected around the site of the primary melanoma, the SN is the first blue-staining node in the lymphatic basin and therefore the first nodal site of lymphatic drainage from a primary cutaneous melanoma. After identification and removal of the SN, Morton et al performed a completion lymph node dissection for that nodal basin.3 They found that the pathologic status of the SN was a highly accurate predictor of the pathologic status of the entire nodal basin. These findings suggested that melanoma could be accurately staged with procedures that were far less extensive than complete nodal dissections.
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The status of the axilla is the most important prognostic factor in breast cancer. Before the development of SNB, an axillary lymph node dissection (ALND) was required to stage the axilla. This procedure can be associated with significant morbidity, including nerve damage or lymphedema. In an effort to spare patients from these potential complications, attempts were made to develop a less invasive technique for identification of positive nodes in the axilla. Giuliano and colleagues4 successfully adapted SNB for breast cancer and began a pilot study in 1991. This study was reported in 1994 after 174 lymphatic mapping procedures were performed using a vital dye injected at the primary breast cancer site. SNs were identified in 114 (65.5%) of 174 procedures and accurately predicted axillary nodal status ...