Sentinel lymph node dissection (SLND) is an important procedure in the staging of patients with cutaneous melanoma. As opposed to breast cancers, which may have lymphatic spread in a random manner, skin melanomas have a straightforward lymphatic flow that can be mapped. The metastases rarely skip to higher lymph nodes; therefore, an SLND can provide the first evidence of metastatic spread of the melanoma. This operation is indicated in patients who do not have palpable regional lymph nodes. The original melanoma on histologic studies following wide excision should be of intermediate or greater thickness (> 1 mm). If thinner, the melanoma should have associated high risk factors such as ulceration. Additional risk factors to be considered are age, site, Clark's level of invasion, and gender. An SNLD that uses both radionuclide and blue dye is highly accurate in finding positive lymph nodes. It allows a focused pathologic examination by the pathologist with both routine hematoxylin and eosin (H&E), plus immunohistochemical staining on the lymph nodes that are most likely to contain metastases. Finally, an SLND should be considered prior to a wide excision of the primary melanoma site. This is especially important if a rotational skin flap is planned for closure, as the resultant scar will alter the dermal lymphatic flow.
In the example shown (Figure 1), the cutaneous melanoma was excised from the midportion of the patient's back. This is considered a watershed area—that is to say, the lymphatic drainage may go to either axilla or groin. Accordingly, a preoperative scintigram is required to demonstrate which lymphatic basin receives the lymphatic drainage from the tumor site. The most common areas are the axillary and inguinal regions for extremity or truncal lesions and cervical or supraclavicular regions for head and neck primaries. Other sites include deep iliac, hypogastric, and obturator regions and the popliteal or epitrochlear regions for legs and arms, respectively. Last, ectopic sites are also possible.
The skin must be cleared of any active infections, as must the excision site for the melanoma. Preparation, inspection, and monitoring of the radionuclide solution must be coordinated with the nuclear medicine staff.
A few hours before operation, the patient is injected with a radionuclide solution intradermally about the perimeter of the surgical site, using sterile technique. This may be done by the radiologist or the surgeon. The commercially available human serum albumin or sulfur colloid solution tagged with technetium 99m is filtered and sterilized. Two separate syringes are each loaded with 1 mL of solution containing about 500 μC for a total dose of about 1 mC. The area for injection is prepared with an antiseptic solution. Disposable paper drapes are widely placed and the physician is gloved. Extensive shielding for radioactivity is not required, but the site and supplies are monitored with a radiation survey meter. The gloved physician injects the radionuclide in an intradermal pattern about the incision (Figure 2). The area is washed and all the disposable items are surveyed and disposed of in a radiologically safe manner.
The lymphatic drainage area or basin is noted on a large or whole-body scintigram; a hand-held gamma detector is used to identify the hottest area. This spot is marked with indelible ink as a temporary tattoo and the patient is transported to the operating room.
Deep sedation plus local or a general anesthesia may be used.
The patient is placed in a comfortable supine position. If an axillary SNLD is planned, that arm should be out at a 90-degree angle on a padded arm board. If the dissection is planned in the neck, the head of the table may be elevated and the patient's head turned to the opposite side.
The hair is shaved about the tattoo and a routine skin preparation and draping is performed. The surgeon performs another intradermal injection about the perimeter of the melanoma excision site using 1 to 3 mL of isosulfan blue vital dye (Figure 3). The area is massaged for a few minutes, and a faint blue streaking of the dye may be seen in the dermal lymphatics heading toward the SLND site. In this illustration, the sentinel node is within the left axilla. Using a hand-held gamma probe in a sterile cover (Figure 4), the surgeon verifies that the tattoo marks the hottest spot. A small 5-cm transverse incision is made over the tattoo and dissection is carried into the subcutaneous fat (Figure 5). The fat is retracted laterally and the probe explores the open incision to find the area of maximum radioactivity (Figure 6).