The term "lymphatic" was coined in 1653 by Thomas Bartholin of Copenhagen. In 1692, Anton Nuck⁶³ utilized mercury for delineating the lymphatic vessels and lymph nodes of the uterus. Using this same technique more than 180 years later, that is, in 1874, Sappey, an anatomist, identified the course of lymphatic channels in the skin of cadavers.⁶³ He showed that those channels on the trunk traveled in different directions depending on the location of the lymphatics in the skin. He demonstrated further that those channels could be grouped according to the direction of the flow of lymph within them. The groups could be separated from one another by lines of demarcation that later became known as "Sappey's lines," (Figure 7) which run from just above the umbilicus, curve around the flank, and then head back to the second lumbar vertebra. It was believed that a primary malignant neoplasm in a region of the skin above Sappey's line drained to the ipsilateral axillary nodes and those below the line drained to the ipsilateral inguinal nodes. For many years, Sappey's lines served as the basis for identifying which regional nodes were to be extirpated in a patient diagnosed as having primary cutaneous melanoma.

Anticipating precisely the flow of lymph from the skin and thereby selecting the nodal basin to be dissected often is exceedingly difficult, a reality expressed in statements like that of by Pack, et al.⁷ in 1945 as follows: "This [selection of a nodal basin] requires considerable ingenuity because the malignant melanomas are so variable in their location and drain into so many different groups of regional nodes . . . [and therefore] necessitates careful study of the anatomy of the lymphatics . . . " Sugarbaker, et al.,⁶⁴ in 1976, used anatomical guidelines to predict the route by which a metastasis of melanoma traveled to nodes and wrote about the subject in this sentence: "The anatomy of the lymphatics of the trunk as described by Sappey is an excellent guide to the site of first nodal metastasis." Despite the assertion by Sugarbaker, it was well known at the time that the direction of flow of lymph as set forth by Sappey and memorialized by his "lines" was inaccurate.

When Sugarbaker made his statement in support of Sappey's lines, a new technique already had been developed for the purpose of predicting more consistently the flow of lymph. The new method came about as a consequence of experimental work of Sherman, et al.⁶⁵ published in 1953. Those investigators injected radioactive colloidal Au¹⁹⁸ into subcutaneous tissue of the anterior abdominal wall and into the peritoneum of rats. They then dissected the inguinal, common iliac, and preaortic nodes and found that radioactivity was increased in them. This prompted them to comment that "The results of these experiments demonstrate the effective and rapid drainage of the colloid from the site of injection to the regional lymph nodes." They believed that radioactive colloid was taken up by macrophages which proceeded to transport the material to the nodes closest to the site of injection. It was hoped that the macrophages would deliver radioactive colloid to nodes that harbored cells of a malignant neoplasm, thereby enabling the neoplastic cells to be irradiated. As an offshoot, the technique was refined and used to detect those nodal basins to which lymph flows, that particular modification becoming known as "lymphoscintigraphy."

In 1977, Holmes, et al.⁶⁶ were among the first to use lymphoscintigraphy, employing colloidal gold as the radioactive material for selecting the nodal basin that was to be removed by END. Seventeen of 56 nodal basins detected by this method contained metastatic melanoma. No metastasis of melanoma was found in a nodal basin not identified by lymphoscintigraphy. The authors' enthusiasm for the new technique was undisguised as is apparent thus: "Radionucleotide scanning techniques for identifying drainage patterns for truncal melanomas can reduce the incidence of multiple lymph node dissections and we feel that the procedure has great promise."

Fee and collaborators, ⁶⁷ in 1978, made available information they had gleaned about radionucleotide scanning, demonstrating convincingly that colloidal gold radionucleotide injected in the dermis around the periphery of a melanoma came to be localized in the nodal basin that housed metastases. In their study, 27 patients underwent a total of 36 dissections of nodal basins; nine regional nodes contained melanoma, and each of the metastases was in a region of uptake of radionucleotide. The patients were followed for 55 weeks and over that period of time none of them developed metastases to a region that did not show uptake of radionucleotide. All of this prompted Fee, et al. to conclude the following: " . . . the direction of lymph shed [the flow of lymph], as evaluated by colloidal gold scanning, correlates well with the flow of metastatic disease."

The usefulness and importance of lymphoscintigraphy was put in perspective better in 1991 by Norman and coworkers⁶⁸ who included in their analysis results of reports published previously that concerned efficacy of lymphoscintigraphy. In toto, 532 patients with primary melanoma were studied by lymphoscintigraphy, 123 of whom were excluded for a variety of reasons, among those being failure to record accurately the exact site of the melanoma, limitation of the technique itself because of deficient drainage from the site of the neoplasm, and exclusion of a melanoma on an extremity because lymphatic drainage from that site was predictable. Also excluded were those patients who, following excision of a melanoma, had undergone wide local excision that required a skin graft in order to insure closure, the reason for the elimination of them being that the previous surgery could have resulted in either no drainage from the primary site or drainage whose direction could not be predicted accurately. Using both their own data and that culled from the literature, Norman and colleagues drew a map (Figure 8) that demonstrates the expected flow anatomically of lymph from the site of the melanoma to the nodal basin. The map shows clearly how marked is the variation in flow of lymph in different persons and how difficult it is to predict with any degree of surety which nodal basin serves a particular region of skin. It confirmed that "Sappey's lines" not only are simplistic, but are inaccurate in regard to determining flow of lymph, a correlate of that observation being legitimacy accorded to the use of a radioactive tracer to detect that nodal basin to which lymph from the skin flows. The map of the lymphatics also shows that lymph flowing to a nodal basin situated in an axilla may originate from contralateral cutaneous lymphatics for up to a distance of 11cm from the midline and from well below the anterior and posterior extent of Sappey's lines; lymph from the skin of the ipsilateral supraclavicular region, posterior cervical triangle, and shoulder also may flow to an axillary nodal basin. Lymph from up to 11cms from the contralateral anterior midline region and 5cms from the contralateral posterior midline regions may flow to an inguinal nodal basin, and lymph from the skin of the shoulder, face, and posterior neck moves to the anterior nodes of the cervical chain. The nodes of the posterior cervical chain receive lymph from skin on the upper part of the back, across the upper part of the shoulders, and across the midline of the face. In some regions (Figure 9) it is impossible to predict clinically the direction of flow of lymph precisely, the reason being that lymphatics may actually travel to several nodal basins. In skin located close to the midline, lymph in 89% of patients flows to both the left and right sides of the trunk. The frequency of flow of lymph bilaterally decreased the further a melanoma was situated from the midline. It was possible to determine clinically direction of flow of lymph from skin at the lateral aspect of the trunk; the pattern of flow from the head, neck, and shoulders defied attempts at prediction dependably. Melanoma of the head and neck metastasized to three separate groups of nodes and the particular direction of flow of lymph could not be predicted accurately in more than 60% of patients. Fifty percent of melanomas on the shoulder drained in a fashion that was unexpected, most often to two or three distinct nodal basins. Skin near the sternal notch and at the base of neck posteriorly was served by six possible nodal basins, but no single cutaneous site drained to more than four nodal basins.

The authors compared accuracy of assessment clinically of the location of nodal basins preoperatively with the actual location of nodal basins as it was certified by lymphoscintigraphy. In 59% (48 of 82) of patients, lymph from the site of the primary neoplasm did not flow to nodes expected on the basis of classic guidelines constructed anatomically. Moreover, the prediction clinically underestimated both the number of paths possible and the direction of the flow of lymph from the primary site. A mean follow-up of 2.1 years showed that in only one patient did a metastasis go to a nodal basin which had not been demonstrated by lymphoscintigraphy. In that one instance, the reason hypothesized was that a large in-transit metastasis blocked flow of the radioactive tracer, preventing thereby the correct nodal basin from being demonstrated in the lymphoscintigram.

The elegant study by Norman and co-workers enabled the direction of flow of lymph from a particular region of skin to nodal basins to be demonstrated beautifully. Futhermore, it showed unmistakably that the precise flow of lymph from the skin to a nodal basin cannot be anticipated on the basis of clinical criteria alone; variation anatomically between patients simply is too great. Last, the work confirmed the necessity of lymphoscintigraphy for detection of a particular nodal basin that was thought to be the object of removal by END.

To the idea of attempting to identify accurately a nodal basin by lymphoscintigraphy was added the concept of trying to detect exactly which node in a basin was the first one to receive lymph. In 1977, Cabanas,⁴⁴ a urologic surgeon, by devising a method for treating carcinoma of the penis set the stage for seeking, successfully, that node to which lymph drained first. This is how Cabanas put it then: "The purposes of this paper are: first, to present evidence of the existence of a specific lymph node center, the so-called "sentinel lymph node," in the lymphatic drainage of the penis." He injected the dorsal lymphatics of the penis with radioactive material and found that that material accumulated in a node close to the superficial epigastric vein. The node, which was designated the sentinel one, was the first always to filter flow of lymph from the penis, as evidenced by the accumulation of the radioactive material in it. In patients who had metastatic carcinoma of the penis, the SN always harbored the metastasis. There was not a single instance in which the radioactive material, and hence the lymph bypassed the SN. Cabanas offered his conclusions thus: "1. Lymphangiograms performed via dorsal lymphatics of the penis demonstrate drainage into a specific lymph node center, the so-called SLN which is located anatomically close to the superficial epigastric vein. 2. Anatomically, clinically, and pathologically, it was found that the SLN is the first site of metastasis and may be the only lymph node involved."

Wong, et al.,⁶⁹ in 1991, introduced the concept of the SN for detection of metastasis of melanoma and identified it as the node at which a metastasis arrived first. From that notion came the idea that metastasis of melanoma to regional nodes could be managed by a combination of SNB and END, the former to detect the metastasis and the latter to treat it. This is how Wong and associates expressed their concept: "It is apparent that immediate lymphadenectomy can potentially benefit only those individuals with metastatic involvement of regional lymph nodes." Despite the complexity of the lymphatic system, those coworkers stated that "it does not seem unreasonable to anticipate that the primary lymphatic drainage of any given site on the skin would be to a single SN." In an effort to test this hypothesis, they employed an animal model, to wit, a cat, because the anatomic structure of the lymphatics of a feline is similar to that of the neck, axilla, and groin of a human. The cat, with its three inguinal nodes, was thought by Wong and coworkers to be an ideal model for attempting to determine where lymph from the skin actually flowed. Of three vital dyes (cyalume, methylene blue, and isosulfan blue) tested by them for this purpose, isosulphan blue proved to be the most effective because it moved rapidly into lymphatics with a minimum of leakage and of staining of fluid which bathed the cells of the skin. After skin flaps were raised above the inguinal nodes, the dye was injected directly into the dermis. Accurate placement of the dye was essential because the subcutaneous lymphatics bypassed the superficial nodes and drained directly to the deep nodes. Different anatomical sites were injected, in particular, hind limb, perineum, and abdominal wall, and the pattern of drainage of lymph from each site was noted. The dye was visualized directly as it moved from the dermis into the lymphatics and through them, in sequence, to the afferent lymphatic channel, primary node (Figure 10), and the adjacent "secondary" nodes, entering eventually the deep lymphatic system of the groin. The inference derived from the study was that by using this technique, flow of lymph could be predicted reliably. Wong, et al. characterized their results in this manner: "The ability to reproducibly map a single lymph node by injection of isosulphan blue into a given anatomic site suggests that lymphatic drainage is highly predictable and drains to a single 'sentinel' lymph node."

Morton, et al., in 1992,⁴⁵ used the study of Wong and collaborators as a springboard for a similar study in humans. This is how they phrased their indebtedness to the work of Wong, et al.: "A new procedure was developed [by Wong and collaborators] using vital dyes that permits intraoperative identification of the sentinel lymph node, the lymph node nearest the site of the primary melanoma, on the direct drainage pathway." Morton's group studied 223 patients with melanoma. Any patient whose primary neoplasm was situated in a region of skin from which the flow of lymph was deemed to be capricious was subjected to lymphoscintigraphy in an effort to identify as accurately as possible which nodal basin most likely would receive the lymph. Once this was accomplished, vital dye was injected in the dermis at the site of the melanoma and the flow of the dye was then visualized as it coursed from there through lymphatics to regional nodes. The SN stained blue, making it easy to recognize before removing it for examination histopathologically. A single SN took the stain in 72% of nodal basins, two SNs in 20% of nodal basins, and three or more SNs in 8% of nodal basins.

A dissection of 237 nodal basins was performed and the tissue removed was submitted for assessment by conventional microscopy. Sections of tissue from the SNs and from the nodes harvested in the basins dissected were examined after having been stained by H&E and also by immunohistochemical stains for S100 protein, NK1C3, and HMB 45. A sentinel node was detected in 194 of 237 nodal basins; 47 of 259 of those nodes contained a metastasis of melanoma. The basins dissected yielded a total of 3079 nodes, two of which harbored melanoma, even though a SN itself did not contain any neoplastic cells. In short, the rate of false negatives was less than 1%. On the basis of these results, the authors were confident of their conclusion that " . . . this technique identifies with a high degree of accuracy, patients with early stage melanoma who have nodal metastases and are likely to benefit from radical lymphadenectomy." They enunciated further their conviction that "The technique of selective lymphadenectomy is presented as a rational and practical technical alternative to ELND or wait-and-watch treatment of patients with CS-I [clinical stage I] melanoma."

Subsequently, there have been several articles^70,71,72 that validated the use of vital dyes for detecting the SN, all of them testifying to a high rate of detection of it. All of the authors of these articles agree that a blue lymphatic channel leading to a blue node is the gold standard for identification of a SN.

In 1993, Alex and Crag³⁸ used cats, as did Wong, et al.⁶⁹ in 1991, to prove that a SN could be localized by utilizing a radioactive tracer in conjunction with a gamma probe, a device, held in a hand, for detection of radioactivity. They injected Technetium-99 sulphur colloid into the dermis of the thigh of the cats and, using the gamma probe, measured the radioactivity above the skin at various distances from the site of injection to the inguinal region. The zones in the inguinal region where the radioactive tracer appeared to concentrate were designated "hot spots," and they correlated with SNs identified following injection of isosulphan blue into the dermis at the very same site the radioactive tracer was injected (Figure 11). The authors promoted this technique in the following fashion: "(a) [the gamma-probe] precisely locate on the surface of the skin the position of an underlying lymph node, (b) intraoperatively guide the surgeon to the lymph node during dissection, (c) verify that the correct node has been biopsied, (d) determine the possible presence of residual lymph nodes, (e) allow lymph nodes to be harvested through a small incision as opposed to raising a skin flap, and (f) be rapidly and easily performed."

In time, radioactive tracers, together with a gamma probe held in a hand, as proposed by Alex and Crag, were adopted as vehicles for detecting a SN, the major advantage of the combination being the ability to locate that particular node when vital dye fails³⁸ to do it. The radioactive tracers used included 198 Au colloid, 99mTechnetium antimony trisulphide colloid, 99mTechnetium colloidal albumin, non-colloidal 99mTechnetium human serum albumin, and 99mTechnetium sulphur colloid. Prior to the operation, a radioactive tracer, like a vital dye, is injected in the dermis. Twenty to 30 minutes after the injection, the SN becomes "hot" and several hours later the radioactive tracer moves through the SN to secondary nodes that then become "hot." The amount of tracer that accumulates in the SN is dependent on several factors, among those being the nature and dose of the radioactive tracer, the distance of the SN from the site of injection, and the type of probe used to detect radioactivity.

A lymphoscintigram performed a few hours to two days after intradermal injection of radioactive tracer is able to demonstrate within a basin the SN and the secondary nodes, but it is not able to outline the route the tracer took to get to those nodes. This limitation is mitigated by a modification called "dynamic lymphoscintigraphy" in which scans are performed immediately after injection of the radioactive tracer and the progress of the tracer is then monitored as it moves from the intradermal lymphatics to the node. This permits identification of the lymphatic that enters the SN, as well as determination of the SN itself. In the process, a map of lymphatics and of SNs is constructed. Two to 20 hours after this maneuver has been conducted and just before the time of operation, another scan is performed, namely, "static lymphoscintigraphy," and another map of the lymphatics and the nodes is constructed. The two maps are then compared and, as a rule, the scans are in agreement with one another in regard to which node represents the sentinel one. In the second scan, however, lymphatics usually are not identified because the tracer already has flowed through them and, at this juncture, already may have accumulated in secondary nodes.

In 1997, Pijpers et al.⁷³ verified the accuracy and utility of "dynamic lymphoscintigraphy." In 15 of 135 patients with melanoma studied by them, the lymphoscintigram showed that lymph flowed to two nodal basins, that is, 150 nodal basins were mapped. In 125 nodal basins, a SN and a lymphatic flowing to it were visualized within 20 minutes of the injection intradermally of radioactive tracer; in 20 basins, only the SN was visualized and in five basins, neither the lymphatic nor the SN was observable. Two to 12 hours later, a "static lymphoscintigram" showed that all lymphatic channels were free of radioactive tracer. The second scan revealed at least one SN in every nodal basin. A single SN was seen in 89 instances and two or three SNs were noted in 56 and five instances, respectively. Overflow to secondary nodes was scant, even 18 hours after injection of the radioactive tracer. The authors declared that it was easy to differentiate between a SN and secondary nodes because the concentration of tracer was higher in the SN. In five patients, nodes could be discerned between the skin and the basin. On the basis of these observations, the authors advised as follows: "Dynamic lymphoscintigraphy allows visualization of lymphatic channels from the injection site to the lymph node and ultimately provides the evidence required to consider them true SNs."

Morton et al.,⁷⁰ in 1999, published the results of a trial, conducted in several centers, in which they sought to validate the accuracy of techniques used for SNB. Of 1135 patients that underwent SNB, a SN was detected in 95.2% when blue dye was used alone and in 99.1% when blue dye and radioactive colloid were employed together.

It became obvious that, on occasion, it may be difficult to determine which of the nodes exhibiting radioactivity, and how many of them, truly are sentinel nodes. At least five definitions have been proposed for "radioactive SNs."⁷⁰ That very issue was addressed by McMasters, et al.⁷⁴ in an article titled, "Sentinel node biopsy for melanoma: How many radioactive lymph nodes should be removed?" Those coworkers utilized data derived from patients in the Sunbelt Melanoma Trial in which both a vital dye and a radioactive tracer were used to detect the SN. The study was based on 1373 nodal basins removed from 1184 patients; 2863 SNs were identified, which represented 2.08 SNs per basin. Metastases were found by histopathologic or immunohistochemical techniques in 306 nodal basins of 288 patients (24.3%). One hundred and seventy of the patients had at least one positive SN. In 40 of 306 basins, the less radioactive node contained neoplastic cells, whereas the hottest node did not. In 20 of the 40 nodes, the positive "less radioactive node" possessed less than 50% of the radioactive count of the hottest node, and in two nodes the count was less than 10% of the hottest SN. In 35 of 40 basins in which the positive SN was not the hottest one, the positive node was the first or second SN identified in the basin. Sixty nine percent of all SNs housed evidence of blue stain. Eighty-six and 3/10ths percent of positive SNs showed blue stain, in contrast to 66.4% of negative sentinel nodes. In positive SNs that were not the hottest ones, 84% displayed blue stain. On the basis of different parameters for defining the SN, the results of the study, coupled with the incidence of metastases elsewhere in follow-up, led the authors to articulate what they believed to be false negative rates, with the proviso that the results might change with further follow-up and evidence of metastasis. These are the figures as set forth by them: if the SN is defined as simply the hottest node, then the false negative rate is 13.9%; if the SN is defined as the hottest node and all nodes are obviously blue, then the false negative rate is 6.7%. If the sentinel node is defined as the hottest node and all blue nodes as well, including the ones that show a subtle hue of blue, then the false negative rate is 2.1%: if the hottest node did not contain neoplastic cells and the positive node was the first and second node removed, then the false negative rate is 1.7%; if the SN is defined as all blue nodes and all nodes associated with radioactivity greater or equal to 10% of the hottest node, the false negative rate is 0.4%. The authors recommended that detection of SNs is achieved best by removing all nodes that stain blue and all nodes that show radioactivity greater than 10% of the count in the hottest node.

Using lymphoscintigraphy, Uren, et al.⁷⁵ studied "interval nodes," referring to them as forgotten sentinel nodes and defining them in this way: "Interval nodes (sometimes called in-transit nodes) are defined as lymph nodes that lie along the course of a lymphatic collecting vessel between a primary tumor site and a draining node field." In-transit nodes were identified in 148 patients (7.2%) and were found more often in patients whose melanoma was situated on the trunk rather than on the limbs. In only 21 of those patients the in-transit node was removed and sections from the specimen examined by conventional microscopy. Three of the 21 patients (14%) harbored a metastasis of melanoma within the in-transit node. In their discussion of in-transit metastasis of melanoma, the authors made this comment: " . . . in-transit metastases found before LS [lymphoscintigraphy] was routinely used were in fact metastases in interval nodes, which were actually sentinel nodes . . . " They advised that when a SNB is performed, in-transit nodes be searched for assiduously. They also indicated that even if a patient has an in-transit node which happens to be a sentinel one, there still may be another SN in an established basin.

In brief, biopsy of a node, specifically a SN, enables diagnosis to be made of a metastasis of melanoma only if neoplastic melanocytes happen to be present in it. A precise definition of a SN has evolved slowly over a period of 30 years. Conceptionally, a SN is the node to which lymph flows first, but it turns out not to be that simple. Cabanas, in 1977, stated that a SN draining the penis is found always in the same location, whereas Morton,³⁷ in 1992, opined that a SN was positioned differently in different persons, and that it could be located only by observing the course of a vital dye as it traveled from the site of injection in the dermis to the node itself. Any node that appeared blue as a result of accumulation in it of the vital dye was considered by Morton to be the SN. The definition of a SN was modified further by the use of radioactive tracers; any node that was "hot" was considered to be the sentinel one. Some authorities refer to level of radioactivity in a node greater than 10% of that in the surrounding tissue as being identifying of the SN, whereas others require the SN to be twice as radioactive as that of the surrounding tissue. Some doubt exists, however, as to whether all radioactive nodes and all blue nodes truly are the first ones in a nodal basin to which lymph flows. In reality, vital dyes and radioactive tracers are capable of passing through nodes and eventually dissipating. The rate of dissipation is sufficiently rapid to require repeated injections of vital dyes into the dermis and monitoring the progress of radioactive tracers through both the lymphatics and the nodes.^45,38 In some instances, a vital dye is not visualized in the SN, yet radioactive material still becomes concentrated in it.

As a consequence of the ever expanding, ever more liberal definition of SN, metastasis of melanoma to nodes can be diagnosed effectively and efficiently before the nodes become palpable clinically. It was inevitable that the number of nodes designated sentinel in a basin would increase—and it has. In 1992, Morton, et al., using a vital dye, detected more than two SNs in only 8% of basins, whereas in 1999, Essner, et al.,⁷⁶ used both a vital dye and a radioactive tracer to find, on average, 3.4 SNs per basin. This increase presumably reflects more refined techniques for detection, greater facility with the technique as a consequence of more experience with it, and even, on occasion, erroneous inclusion of a secondary node that was identified wrongly as a SN.

Leong, et al.⁷⁷ detailed the various courses that lymphatics of the skin may take. According to them, lymph from a region of skin may flow to a single SN by way of a single lymphatic, by way of two or more lymphatics that empty into the same SN, or by way of two or more lymphatics that become joined before flowing into the SN. Alternatively, lymph may flow to different SNs by way of different lymphatics as a result of branching of a single lymphatic or as a consequence of passing first through an interval SN or a contiguous SN in the same basin.

Localization with precision of a SN is not easy and requires skill; a curve of learning is necessary for the technique to be mastered. Before being judged skilled sufficiently to perform a SNB without END, a surgeon who seeks to perform the procedure needs to do a minimum of 20–30 SNBs followed immediately by END in order to confirm that the SN was isolated successfully by demonstrating that a metastasis was not missed. Jansen, et al.⁷¹ described the technique employed by them to identify the SN thus: "A lymph node was considered to be a sentinel node when a blue afferent lymphatic vessel was identified coming from the injection site and leading to this node. If no blue dye was seen during exploration, nodes that had a separate afferent lymphatic vessel on lymphoscintigraphy were sought. If no lymphatic vessel was visualized with either blue dye or scintigraphy, the first hot spot in each basin that appeared on the lymphoscintigraphy images was regarded as the sentinel node."

The accuracy and predictive value of SNB can be assessed by two methods: (1) Performing SNB in conjunction with an END and comparing the findings in the SN with those in the secondary nodes, the presumption being that the SN was identified correctly if both the node believed to be the sentinel one and the secondary nodes were negative for melanoma, and (2) by following patients who had undergone SNB and comparing the findings in the SN with evidence of a metastasis, should it become manifest in the future. The assumption in the latter instance is that if the node isolated is negative for malignant neoplastic cells and, after a reasonable period of follow up, melanoma does not make itself known in the basin from which that node came, then that node was the sentinel one. Numerous publications have been dedicated to validation of the technique of SNB. What follows now is a summary of some of the more important of them.

In the original study of SNB for melanoma published in 1992, Morton et al.⁴⁵ identified 194 SNs in 237 nodal basins and this is one example of what they claimed to have found: "Only two (0.06%) of 3079 nonsentinel nodes from 194 lymphadenectomy specimens were the exclusive sites of metastases, a false negative rate of 1%." Morton and coworkers attributed those false negatives to the fact that dye passes rapidly to and through the SN and this is how they put it: " . . . we learned that the dye passes rapidly to the sentinel node and then on to secondary nodes. We later corrected this problem by periodically repeating the dye injections during the operation."

Gershenwald, et al.⁵⁰ sought to determine the outcome of patients with melanoma whose nodes were negative for neoplastic cells. Of 322 patients whose lymphatics had been mapped, 270 had negative nodes on examination histopathologically and 243 of them were followed for a median period of 35 months. Twenty-seven (11%) of the patients followed for nearly three years developed in-transit, regional, and/or distant metastases within that median period. In 10 patients, a metastasis appeared in a nodal basin that had been mapped previously. Eight of those proved to be false negatives as determined by examination further, that is, histopathologically and immunohistochemically. In eight patients, in-transit metastases became apparent, in nine patients distant metastases, and in four patients satellite metastases. Three patients developed metastases of more than one of those three "kinds." This is how the authors concluded: "These data support the concept that patients with negative SLNs . . . compose the most favourable patient group." They stressed the fact that a false negative SNB most often was due to failure to detect the metastasis histopathologically. This is what Gershenwald and colleagues said and what they advised: " . . . the data provide further evidence that lymphatic mapping and SLN biopsy accurately reflect the status of the regional nodal basin. Specialized pathologic techniques are necessary to reduce further the already low false-negative rates and to improve disease staging."

In 1999, Gadd, et al.⁵¹addressed the matter of "Outcome of patients with melanoma and histologically negative SNs," the results of their investigation being similar to those of Gershenwald, et al. ⁵⁰None of the patients had END. After a median follow-up period of 23 months, 11 of 89 patients (12%) showed manifestations clinically of metastasis of melanoma. Seven patients had a detectable metastasis in regional nodes of 106 basins mapped. In three patients, more extensive examination histopathologically of the SN revealed a metastasis which was missed at the time of the original examination. Two patients developed what was said to be in-transit metastasis and two had distant metastasis.

In 2000, Jansen, et al.⁷¹ wrote of their series of 200 patients on whom they performed SNB of 247 basins, in all of which SNs were identified. One of the SNs was positioned at a site not accessible readily and it was thought prudent not to attempt to dissect it. Forty-eight patients had a positive SN. After a follow-up period that ranged from two to seven years, 18 patients developed additional manifestations clinically of melanoma, one of them being at the local site, six as a satellite or in-transit metastasis, and 11 as distant metastases. None showed evidence of melanoma in the nodal basin dissected. Of the group that had negative SNs, 15 came to have manifestations clinically of melanoma, three of them being persistence of melanoma at the local site, four satellite or in-transit metastasis, six regional metastasis in the basin that harbored the SN, and two distant metastasis. The authors estimated that the sensitivity of SNB in detecting metastasis was 89%, which is more or less similar to the results of other studies reported on.

Clary, et al.⁷⁸ studied, retrospectively, 357 patients with melanoma, concentrating on the pattern of metastasis in patients who underwent SNB. Extirpation of the nodal basin was undertaken in all patients with positive SNs. At the beginning of the study, END was performed in 35 patients for the purpose of validating the technique of SNB consonant with the recommendation alluded to previously, that is, a surgeon inexperienced in performing SNB should do an END after the first 20 to 30 SNBs that he/she undertakes, the purpose being to confirm that the SNB was performed in a proper manner. The SN was detected in 93% of patients and was positive in 17% of those patients. Another metastasis was found subsequently in 14% of patients during a median follow up period of 24 months. Fourteen percent of patients with a negative SN developed metastasis, in contrast with 40% in patients with a positive SN. In patients with a negative SN who subsequently developed metastasis, the distribution of metastasis was as follows: Eighteen percent manifested a metastasis within 2 cm of the surgical scar at the site of excision of the melanoma, that is, a satellite metastasis; 28% showed a metastasis in a basin dissected previously; 18% developed in-transit metastasis, and 36% distant metastasis. Nine percent of patients with a positive SN had additional metastases that became apparent clinically at the local site, 14% in the nodal basin previously dissected, 32% in-transit, and 45% distant. Some metastases occurred in patients who had a SN not believed to contain metastasis at the time of the original examination. By once again examining histopathologically the nodes of the 11 patients with metastases, in the resected basin or in-transit, metastasis of melanoma was discovered in seven of them. In brief, metastases occurred in the resected basin or in-transit in four patients with a true negative SN. SNs that were negative in patients with satellite or distant metastasis were not re-examined.

The study of Clary and coworkers revealed, too, that of 59 basins resected in 58 patients who had a positive SN, only six basins were shown in a subsequent dissection to contain a positive secondary node. Twenty-two of the patients showed signs subsequently of other metastases of melanoma. In commenting on the results of the study, the authors said this: "SLN mapping and biopsy is a very accurate method of staging the draining nodal basins . . . "

Bachter, et al.,⁷⁹ also in 2001, told of their experience with 256 patients with melanoma who had undergone SNB. In the experience of those coauthors, the SNB is an important diagnostic tool because metastases, if present, were found always in the SN. Only one patient with a negative SN developed a manifestation of a metastasis and that in the nodal basin after a mean follow-up period of 18 months, the false negative rate being 0.4%. Because of the success in localizing metastasis of melanoma with SNB, they proclaimed the procedure to be "a milestone in the surgery of melanomas."

According to Gershenwald, et al.,⁷² two main factors are evidence of the accuracy of SNB, namely, (1) almost complete absence of metastasis in secondary nodes if the SN does not possess one and (2) metastasis in SNs only (79% of patients). Those coworkers opined favorably as follows: "Lymphatic mapping and SLN biopsy is highly accurate in staging nodal basins at risk for regional metastases in primary melanoma patients . . . ," and Jansen, et al.,⁷¹ seconded that sentiment when he wrote that " . . . sentinel node status is an important prognostic factor, even with a false-negative rate of at least 11 per cent[sic]."

In short, the concept of SN, introduced first by Cabanas⁴⁴ in 1977 by virtue of studies of carcinoma of the penis and refined by Wong, Cagle, and Morton⁶⁹ in 1990 and by Morton, et al.⁴⁵ in 1992 in studies first on cats and then on humans, gained acceptance quickly and widely. In practice, a SN stains blue and/or concentrates radioactive material within it after a particular essential substance has been injected in the dermis adjacent to the primary melanoma. Detection of the SN depends also on capability to detect the basin to which lymph flows, this being accomplished by lymphoscintigraphy, which first was performed in the 1950's. Examination of the SN in patients with melanoma reveals metastasis in approximately 20% of them. A small number of patients with a negative SN present themselves with a metastasis during the course of follow-up. Other patients with a positive SN and negative secondary nodes may show signs of metastasis during follow up. Statistically, a patient with a positive SN is less likely to live as long as a patient with a negative one.

In 2001, Dessureault, et al⁸⁰ presented data extracted, retrospectively, from the database for staging melanoma of the American Joint Committee on Cancer. They collected information on 14,914 patients, some of whom had a wide excision only, others a wide excision and END, and still others a wide excision coupled with a SNB. If the SN was positive, dissection of the nodal basin followed.

This is what Dessureault and coworkers found in patients whose melanoma was thicker than 1.0mm: Those who underwent SNB had a 90.5% survival for five years, whereas of those who had had END, 77.7% survived for five years, and of those who simply were observed, 69.8% survived for five years. These results were significant statistically, prompting the authors to remark as follows: " . . . [SNB and nodal basin dissection] may contribute to a survival benefit in populations of patients with melanoma." The authors, among whom were Balch and Reintgen, were careful not to be too definitive in their statement, aware as they all were from experience over the last 30 years with studies pertaining to END that analysis conducted retrospectively leads readily to errors in interpretation. Other authors have shown in studies performed retrospectively that SNB is not advantageous. Essner and coworkers,⁷⁶ for example, offered this thought about the matter: "These findings suggest that LM/SL/SCLND [lymphatic mapping/SNB/dissection of nodal basin involved by metastasis of melanoma] is therapeutically equivalent to ELND . . . " The results of studies concerning therapeutic advantage of SNB are in conflict and, therefore, are not illuminating in that regard. Moreover, it is illogical to assert that SNB with dissection of the nodal basin will confer advantage in terms of survival if END does not add any benefit in that respect; the procedure of SNB is merely a modified END and END has been proven, beyond doubt, to confer nothing of worth to a patient with metastatic melanoma. When results of a study conducted prospectively, precisely, and in randomized fashion are published at long last, they will be the coup de grace for the ill-advised concept of SNB.

In 2000, Kelley and Cockerell⁸¹ proposed a novel use for SNB when they advised that SNB could be useful in diagnosing what they termed, fuzzily, "melanocytic neoplasms of uncertain behavior." The idea was this: if the SN were positive, diagnosis of the original melanocytic neoplasm could then be made with certainty. If, however, the SN were negative, it might indicate that the neoplasm would likely behave in benign fashion. This proposal in regard to SNB is as ill-conceived as is the procedure itself. Diagnosis of a melanocytic neoplasm is made on the basis of morphologic criteria, that is, gross and microscopic findings. Of course there are times when even the most competent histopathologists misinterpret a melanoma as a Spitz's nevus (and vice versa) and the actual diagnosis of melanoma becomes apparent by virtue of the biologic behavior of the neoplasm. But that situation is exceptional; when criteria that now are well established are utilized, a distinction by conventional microscopy between a nevus and a melanoma can be made in the vast majority of instances. Parenthetically, there is no such thing as a "melanocytic neoplasm of uncertain biologic potential;" that phrase conveys the idea that the neoplasm itself is uncertain about its character biologically when, in actuality, it is the histopathologist who is uncertain. That being the case, it is in the best interest of a patient for a histopathologist to render a diagnosis of "nevus," "melanoma," "melanoma in association with a nevus," or to admit, directly and forthrightly, "I don't know." In the last circumstance, another opinion may be sought from a respected colleague and, irrespective of it, the referring physician responsible for managing the patient can be advised that the neoplasm should be excised completely with a narrow margin.