Limitations

There are several reasons why sometimes no sentinel node is identified on the lymphoscintigraphy images. Rarely, there is no drainage of the agent from the injection site [11,23,49]. This risk is greater after a wide therapeutic excision has been performed [3,58], and it is the rule after a skin graft has been applied to cover the defect [59]. Injection of the radiocolloid into the scar of the excisional biopsy may also be associated with no flow from the primary site. Visualization of the liver usually means that some of the tracer was injected intravenously and removed from the circulation by the Kupffer cells.

When the injection site is close to the sentinel node, a different camera angle or a different position of the patient may be needed in order to obtain the required information. For instance, an injection site over the scapula may obscure a sentinel node in the axilla. Dynamic imaging from a lateral angle will then visualize the injection site and the lymphatic duct going around the latissimus dorsi muscle to the sentinel node. An injection site high on the inside of the arm requires the arm to be swung upward in order to avoid overprojection of the injection site and the sentinel node. A primary tumor site on the cheek is best imaged with the head turned to the opposite side. Another option for minimizing the bulk of the radioactivity at the injection site is to cover it with a lead shield.

Lymphatic channels containing obvious tumor can result in blockage of the lymph flow [5,60]. Nodes that are largely replaced by tumor accumulate less of the tracer or sometimes do not accumulate tracer at all [5,12,18,44]. On the other hand, lymphatic channels and nodes are sometimes well delineated despite clinically obvious metastatic tumor deposits [61].

Experience with lymphoscintigraphy has taught us that lymphatic drainage is highly variable. It is now abundantly clear that watershed areas of ambiguous lymphatic flow are much wider than was previously assumed [60]. Drainage sometimes occurs to lymphatic regions other than those expected on the basis of the classic anatomic studies. Drainage across the midline or Sappey's line is not unusual for primary lesions up to 10 cm away. It has been suggested that lymphoscintigraphy is not needed when the primary lesion is located on an extremity because the drainage pattern is predictable [62], but melanomas of the arm can drain directly to a supraclavicular node [63], to an interpectoral node [64], or even to the opposite axilla [65]. Lesser known small lymphatic basins can contain a sentinel node. Hot epitrochlear nodes are sometimes seen in patients with a melanoma of the forearm [66]. A melanoma on the skin of the calf may drain to a sentinel node in the popliteal fossa (Fig. 5). A strong point in favor of lymphoscintigraphy is that such—occasionallyZizarre—drainagepatterns can be identified.

We find sentinel nodes in strange places, outside the known lymphatic basins. It is understandable that melanoma surgeons call these nodes ''in-transit''

Epitrochlear Lymph Node

Figure 5 Anterior (a) and right lateral (b) static images of a 67-year-old woman with a melanoma of the right foot. Three sentinel nodes are depicted in the popliteal fossa.

Figure 5 Anterior (a) and right lateral (b) static images of a 67-year-old woman with a melanoma of the right foot. Three sentinel nodes are depicted in the popliteal fossa.

nodes. They are also known as ''interval nodes'' or ''extra-anatomic'' nodes. No matter what name they are given, they are sentinel nodes. The work of Uren and co-workers at the Sydney Melanoma Unit deserves to be mentioned [67]. They encountered actual sentinel nodes outside the recognized lymphatic fields in 22% of their patients [44]. A review of our own series revealed this phenomenon in 12%. Certain sites turn up repeatedly and patterns are beginning to emerge. In a number of patients, we have seen sentinel nodes just lateral to the areola of the breast receiving drainage from lesions in the epigastric region. A sentinel node is sometimes found high in the flank, as if having dropped from the axilla (Fig. 6). This can be seen in patients with melanoma on the abdominal wall, on the back, or lower down on the flank [68,69]. Melanomas in the epigastric region and periumbilical skin sometimes drain directly to the internal mammary nodes [44,70]. Lymphatic ducts from melanomas on the back occasionally go directly to the mediastinum [14,44]. Drainage to nodes in the triangular intermuscular space occurs in 30% of the patients with melanomas on the upper part of the back [71,72]. This anatomical entity is situated just lateral to the scapula. It is formed inferiorly by the teres major muscle, medial-superiorly by the infraspi-natus and teres minor muscles, and laterally by the long head of the triceps muscle. A static posterior view can give the misleading impression that such a node is located in the axilla. The dynamic study and the lateral view will declare its true location (Fig. 7). More abnormal routes of drainage to watch out for have been described [73]. Without lymphoscintigraphy, these in-transit nodes would elude us.

As has been indicated, it is important to appreciate that not all nodes showing tracer uptake are sentinel nodes. The images can be most difficult to interpret when multiple nodes light up (Fig. 3). One cannot always clearly distinguish first-tier nodes from subsequent (nonsentinel) nodes, even with dynamic imaging. This problem is faced fairly frequently. Despite the fact that the average number of counts in a first-tier node is four to five times as high as the average uptake

Abdominal Wall Melanoma Sentinel Node
Figure 6 Anterior lymphoscintigraphy in a 43-year-old man with a melanoma on the back in the midline. Sentinel nodes are shown in the right axilla, the left groin, and the right flank. A second-tier node is depicted in the right epigastric region.

in a secondary node [40], it is not true that the node with the highest number of counts is always the sentinel node. We have occasionally seen a large second-tier node accumulate more of the radioactive tracer than a small first-tier node. Also, it has been suggested that the lymph node closest to the primary lesion site is always the sentinel node [74]. This notion has proven to be wrong [75]. Uncertainty about the hierarchy of hot nodes is best expressed in the lymphoscintigra-phy report and discussed with the surgeon. Intraoperative mapping will usually solve the problem. The lymphatic channel can be exposed with the aid of blue dye to actually prove whether a node is a first-tier node, receiving drainage directly from the site of the primary lesion, or a second-tier node, receiving drainage from a first-tier node.

Not every hot spot represents a lymph node. A hot spot may be caused by a drop of the tracer spilled on the skin (Fig. 8). Surgical exploration of a hot spot

Figure 7 Posterior (a) and right lateral (b) lymphoscintigram of a 49-year-old woman with a melanoma on the back just to the right of the midline. The posterior image at 30 min shows a lymphatic duct and a sentinel node that could be situated in the right axilla. The lateral image establishes that the sentinel node is situated in the triangular intermuscular space on the back (arrow). A second-tier node is located in the right axilla.

on a scintigram sometimes reveals what appears to be a lymphangioma (''lymphatic lake'') in a lymphatic channel instead of a lymph node. Circumscribed hyperplasia of lymph vessels is known to occur in major lymphatic trunks in the absence of other morbid changes in the surrounding tissues [76]. For no apparent reason, the radioactive tracer is sometimes retained for more than 24 h in such a lesion. We have encountered this phenomenon in the head and neck region, on the back, and on the thigh (Fig. 9).

Lymph Nodes Back

Figure 8 Contamination of the skin by a drop of the radiopharmaceutical (arrow) may suggest the presence of a sentinel node (left). This hot spot was easily wiped off (right).

Figure 8 Contamination of the skin by a drop of the radiopharmaceutical (arrow) may suggest the presence of a sentinel node (left). This hot spot was easily wiped off (right).

Sentinel Lymph Node Groin

Figure 9 Lymphoscintigram (a) of a 46-year-old man with a melanoma just below the right knee. A hot spot suggestive of an interval sentinel node is depicted on the right thigh. Surgical exploration revealed a lymphangioma in this location. The true sentinel node was identified in the groin. The pathology slide (b) shows a conglomerate of lymphatic ducts without glandular tissue.

Figure 9 Lymphoscintigram (a) of a 46-year-old man with a melanoma just below the right knee. A hot spot suggestive of an interval sentinel node is depicted on the right thigh. Surgical exploration revealed a lymphangioma in this location. The true sentinel node was identified in the groin. The pathology slide (b) shows a conglomerate of lymphatic ducts without glandular tissue.

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