Sentinel Lymph Node Biopsy
Daniel C. Thomas and Dale Han
The vast majority of patients newly diagnosed with melanoma do not present with clinically enlarged lymph nodes; however, lymph nodes represent the most frequent sites of metastatic disease for melanoma, and nodal status is a known prognostic factor for melanoma-specific survival (MSS) (1–3). Approximately 15% to 20% of all patients diagnosed with melanoma will develop metastatic disease in the associated draining nodal basin, and the presence or absence of microscopic nodal disease has been found to be the most important prognostic factor for recurrence and death in early stage melanoma (4,5). In addition, microscopic nodal disease that is clinically undetected and left untreated can develop into palpable nodal disease and theoretically may promote the development of distant metastases. Therefore, detection of microscopic nodal disease provides powerful prognostic data and allows for early control of regional disease. Sentinel lymph node biopsy (SLNB) is an efficacious procedure that has emerged as the technique of choice for nodal staging, and plays a critical role in the management of patients diagnosed with localized cutaneous melanoma.
HISTORY OF ELECTIVE LYMPH NODE DISSECTION
When considering nodal staging for melanoma patients who are clinically node negative, several factors need to be considered such as the patient’s risk for nodal metastases, the prognostic and therapeutic benefit of nodal staging, the false-negative rate (FNR) of the technique utilized, and the morbidity associated with the procedure, particularly taking into consideration a patient’s age and comorbidities. Prior to SLNB, patients with melanoma were treated with either elective lymph node dissection (ELND) at the time of wide local excision (WLE) or with clinical observation and lymphadenectomy if nodal metastases later developed. Approximately 20% of patients who underwent nodal observation eventually developed macroscopic nodal recurrences, which could occur as much as 8 to 10 years after the initial diagnosis of melanoma (5,6). ELND was introduced as a way to identify and potentially treat 20% of patients who may harbor microscopic nodal disease. Although ELND was a relatively extensive procedure, it was the only method available at that time to identify clinically occult nodal metastases. However, ELND was associated with potentially significant morbidity, with the majority of patients (approximately 80%) being found to have no evidence of nodal metastases (7–9). These patients saw minimal benefit from the procedure but were exposed to the increased risks associated with lymphadenectomy. Furthermore, subsequent prospective randomized trials showed that ELND provided no survival benefit over nodal observation, and these data challenged the role of ELND in the management of melanoma patients (8,9).
DEVELOPMENT OF SLNB
Due to these issues with ELND and the recognition that nodal status is an important prognostic marker, Morton began extensive work looking into ways to evaluate draining nodal basins without unnecessarily exposing patients to the risks of lymphadenectomy. Dr. Morton subsequently reported on SLNB as a method for nodal staging in melanoma patients using a technique that was associated with lower morbidity. SLNB allows for identification and removal of a select group of lymph nodes, termed sentinel lymph nodes, that are the first to drain the lymphatics of a melanoma primary (10). Dr. Morton’s work on SLNB was based on three principles: (a) specific areas of the skin drain to specific lymph nodes; (b) these specific lymph nodes can be identified and removed; and (c) if these lymph nodes do not contain tumor, then the remaining nodal basin is unlikely to contain metastases, thereby making completion lymph node dissection (CLND) unnecessary (4,10). These sentinel nodes serve as “gatekeepers” for metastases to the draining nodal basin, and by identifying, removing, and examining the sentinel nodes, the nodes most likely to harbor metastatic melanoma could be evaluated to determine the status of the entire regional lymph node basin. SLNB has been shown to predict the negative status of the remaining regional nodes in at least 96% of negative SLNB patients (11–13). Alternatively, approximately 20% of patients found to have metastatic disease in a sentinel lymph node (SLN) are found to have additional disease in the regional nodes or beyond (14,15). Furthermore, SLNB is associated with lower morbidity compared with formal lymph nodes dissection, primarily because SLNB involves removal of a lower number of lymph nodes and use of smaller incisions. Of note, the SLNB technique is a complex procedure and requires expertise and a coordinated multidisciplinary approach involving nuclear medicine, surgery, and pathology.
TECHNIQUE FOR PREOPERATIVE AND OPERATIVE IDENTIFICATION OF SENTINEL LYMPH NODES
Localization of sentinel nodes utilizes several methods including radiotracer detection with or without the use of intraoperative vital blue dyes. The initial technique of sentinel node mapping described by Morton et al. utilized isosulfan blue dye (lymphazurin) infiltrated intradermally at the site of the primary tumor at the time of WLE (Figure 7.1). The SLN identification rate in the initial report by Morton et al. was 82% when blue dye was used alone. The blue dye was found to have adequate lymphatic uptake while being large enough to become trapped inside of the first lymph nodes encountered (10). As shown in Figure 7.1, surgical exploration of the associated nodal basin allows for identification of the blue-colored lymphatics draining toward the blue sentinel nodes. Vital blue dyes currently utilized include isosulfan blue dye, methylene blue dye, and patent blue dye. Lymphazurin is specifically associated with a very low risk of anaphylactic shock (1%); however, the overall risk of complications from using vital blue dyes, such as mild allergic reactions like local swelling and pruritus, is low and varies in the literature (16,17).
A second method of lymph node mapping utilizes injection of radiolabeled colloid, frequently technetium-99m, also infiltrated at the primary tumor site. Use of preoperative radiotracer allows one to obtain a lymphoscintigraphy to map the location of the draining sentinel nodes (Figure 7.2). Preoperative lymphoscintigraphy is particularly helpful for head/neck melanomas where lymphatic drainage can be complex and vary considerably. Preoperative lymphoscintigraphy is also useful for truncal melanomas that may have more than one draining lymph node basin, may drain cranially and caudally, and may drain across the midline to the contralateral side. In addition, preoperative lymphoscintigraphy may help to identify drainage to epitrochlear nodes or popliteal nodes for extremity melanoma, may help to identify direct drainage to deep pelvic nodes, or may help to identify drainage to interval nodes that are nodes not located in standard nodal basins (18). Radiotracer may be injected several days prior to surgery to perform lymphoscintigraphy with a second radiotracer dose given in the operating room on the day of surgery. Another option is to inject radiotracer as a single dose within 24 hours of surgery (lower dose if injection is done on the day of surgery while a larger dose is used if injection is done the day before surgery) with lymphoscintigraphy and surgery performed on the same day.
Intraoperatively, the radiotracer is then detected using a handheld gamma probe. The surgeon identifies the region with the highest radiotracer uptake, and a small incision is made over this area. Of note, the incision should be fashioned so that it can be potentially incorporated into a CLND incision. Upon entering the nodal basin, the lymph nodes with the highest uptake are found and removed. If vital blue dye was also used, any nodes that are stained blue are also removed. The lymph node with the highest radiotracer count is identified and removed, as well as all associated nodes with ≥10% radioactivity of the highest count node. This method optimizes the detection of all nodes that may harbor micrometastases (19). The use of both intraoperative vital blue dye and radiotracer has been found to identify sentinel lymph nodes in 97% to 99% of patients (20,21). Today, the majority of sentinel nodes are identified utilizing intraoperative radiotracer primarily with or without the use of vital blue dye (3,22).
In addition to technetium-99m, other radiotracers have been developed for use during SLNB. Tilmanocept has been Food and Drug Administration (FDA) approved for use during SLNB for patients with melanoma (23,24). Tilmanocept is a radiotracer that binds tightly to both technetium and mannose receptors through its attached mannose molecules. Mannose receptors are expressed in reticuloendothelial cells that are present in lymph nodes. Tilmanocept is readily picked up and retained in these draining lymph nodes. The efficacy of tilmanocept when used for SLNB specifically for melanoma was demonstrated in a phase III study. In this study, tilmanocept identified more sentinel nodes in more patients and also identified more sentinel nodes with melanoma when compared with vital blue dye (23,24).
Single-photon emission computed tomography with integrated CT (SPECT/CT) has also been used as a modality to identify sentinel nodes (25). Radiotracer is injected up to 24 hours preoperatively, and the sentinel nodes are identified using SPECT. The location of these sentinel nodes is then determined by integrating with CT imaging. Several studies have demonstrated a potential benefit of using SPECT/CT for preoperative planning and intraoperative decision making (25–27).
PATHOLOGY EVALUATION OF SENTINEL LYMPH NODES
The gold standard for pathology evaluation for melanoma metastases in a SLN is hematoxylin and eosin preparation (H&E). Metastatic melanoma cells within the SLN commonly resemble the histologic features of the primary lesion and are most frequently located subcapsular as single cells, nests, or clusters of cells and are less frequently located within the parenchyma and fibrous capsule (28).
H&E staining is associated with a FNR of 10% to 15%, and the addition of immunohistochemical staining for melanoma-associated tumor markers increases the sensitivity of detecting microscopic melanoma metastases (28–30). The most widely used immunohistochemistry markers for evaluation of sentinel nodes for melanoma metastasis include S-100, HMB45, MART-1/Melan A, and tyrosinase (see Figure 7.3). S-100 is a highly sensitive marker for melanocyte differentiation; however, it is not specific for melanoma cells as it also stains several other tumor types and normal cells within the lymph node including benign capsular melanocytic nevi and dendritic cells (31). MART-1 antibodies target with high specificity the MART-1/Melan A complex expressed by malignant melanocytes; however, MART-1 can also be expressed by macrophages (28,32). The antibody HMB45 identifies most melanoma cells within a SLN and usually does not stain macrophages (28,33). The use of reverse transcriptase polymerase chain reaction assay for detection of melanocytic messenger RNA expression remains investigational as studies examining its diagnostic and prognostic value have been conflicting (34,35).
The identification of melanoma in the SLN can be confounded by the presence of nodal nevi, which are collections of benign melanocytes thought to result from the dislodgement of nevocytes into the nodes (36,37). Nodal nevi are most frequently located within the capsule, in contrast to the subcapsular location of melanoma metastases, and can be further distinguished by their different cytologic features compared with metastatic melanoma cells (28,37).
Validation of SLNB
The role of SLNB in the management of patients with primary cutaneous melanomas was validated by the groundbreaking Multicenter Selective Lymphadenectomy Trial I (MSLT-I) (3). MSLT-I was a prospective, randomized trial designed to determine whether SLNB performed in clinically node-negative patients conferred a survival advantage when compared with patients who underwent nodal observation alone after WLE (2). The trial initially evaluated patients with intermediate thickness melanomas but was later expanded to include patients with thin and thick melanomas. Patients were randomized to either WLE with SLNB or WLE with clinical observation of the nodal basin. For patients in the SLNB arm, a CLND was performed for patients who had a SLN positive for metastatic disease, while patients in the nodal observation arm underwent a therapeutic lymphadenectomy only if there was clinical evidence of a nodal recurrence in the follow-up period.
The final report of MSLT-I was published in 2014 and analyzed a total of 2,001 randomized patients (14). The positive SLN rate was 16% for patients with an intermediate thickness melanoma and was 32.9% for patients with a thick melanoma. Specifically, in the intermediate thickness group, the 10-year disease-free survival (DFS) rates were significantly higher in the SLNB arm when compared with the nodal observation arm (71.3% ± 1.8% vs. 64.7% ± 2.3%, respectively; hazard ratio (HR) = 0.76; 95% CI: 0.62–0.94; p