Dissection of the “sentinel lymph node” (SLN) as identified by lymphoscintigraphy is becoming increasingly important in the treatment of patients with malignant melanoma. The purpose of the current study was to determine whether the SLN also could be identified by ultrasound.
Sixty-seven patients with malignant melanoma (40 females and 27 males, with an average age of 48.8 years) in whom extirpation of the SLN was indicated underwent ultrasonography of the regional lymph nodes prior to preoperative lymphoscintigraphy. The location of the melanoma was the legs in 30 patients, the arms in 14 patients, and the trunk in 23 patients. During regional ultrasonography, the location of the lymph nodes that differed in the cortex/medulla ratio from the surrounding lymph nodes was marked on the skin corresponding to the planes of insonation (M1) when the probe was held vertically to the skin surface. After lymphoscintigraphy using technetium-99m, the position of a gamma probe at which the highest count rate vertical to the skin surface was recorded also was marked (M2).
In the inguinal region, the agreement between M1 and M2 was found to be 100% (40 of 40 SLNs) and was 72.5% in the axilla (29 of 40 SLNs). In patients with melanomas located on the leg, the location of M1 and M2 agreed in 97% of cases (36 of 37 lymph nodes in 30 patients); in patients with melanomas located on the arms, the agreement was 76% (13 of 17 lymph nodes in 14 patients) and in patients with melanomas located on the trunk, the agreement was 75% (21 of 28 lymph nodes in 23 patients). The position documented by ultrasound relative to the neighboring structures of the SLN was confirmed intraoperatively in all cases.
Sentinel lymph node (SLN) biopsy has assumed an important role in the surgical treatment of malignant melanoma.1, 2 The method first was devised in 1977 by Holmes et al.,3 who described regionally draining lymph nodes marked with radioactive colloidal gold in patients with melanoma, and by Cabanas,4 who to our knowledge was the first to use the term “sentinel [lymph] node” in cases of penile carcinoma. Morton et al.5, 6 defined the SLN as the first lymph node that receives lymphatic drainage of the tumor. In systematic studies, the authors investigated the technique as well as the clinical usefulness of SLN biopsy.5, 6
The actual status of regional lymph nodes at the time of surgery for melanoma has been reported to have high prognostic value5 with regard to their clinical significance in the progression of melanoma.1, 7, 8 Assuming that the SLN is part of the drainage pathway to additional regional lymph nodes, it is ascribed an important filter function for potential metastases. The SLN is considered representative of the respective regional lymph node group.
The mapping of the SLN requires appropriate localization. Identification by lymphoscintigraphy with radiolabeled colloids currently is the most widely used method.2, 6, 9 Intraoperative mapping is possible using a probe optimized for the technetium-99m (Tc-99m) label. It occasionally can be helpful to stain the SLN using peritumoral cutaneous injection of Patent Blue V to improve its identification during surgery.
The current study was conducted to evaluate whether the SLN also can be mapped by ultrasound, whether this technique can identify the reference to neighboring structures clearly, and whether ultrasonography of the SLN is of clinical value in patients with malignant melanoma.
MATERIALS AND METHODS
Sixty-seven patients (40 females and 27 males, with an average age of 48.8 years) with malignant melanoma that was located on the trunk or the extremities were enrolled after informed consent was obtained. All patients had tumors with a Breslow thickness of > 1.0 mm (range, 1.08–5.5 mm) and a Clark level > III; therefore, wide excision combined with SLN biopsy was indicated. In 32 patients, SLN biopsy and wide local excision followed as a second procedure after primary excision of the melanoma. Patients with melanoma of the head or neck were not included.
In 30 patients, the primary melanoma was situated on the lower extremity with the SLN located in the inguinal area. In 14 patients, the melanoma was found on the arms and the SLN was in the axilla. In 23 patients, the tumor was situated on the trunk with mainly axillary lymphatic drainage (16 cases in 1 of the axilla basins and 2 cases in both axillary basins). In two cases the pathway was to the ipsilateral axilla and inguinal area, in two cases the drainage was inguinal, and in one case it was nuchal.
Ultrasonography was performed using the color-coded Doppler Apogee 800 ultrasound system (Advanced Technology Laboratories, Solingen, Germany), with a 7.5-megahertz (MHz) and an 11-MHz linear transducer. At these probe frequencies, the axial separation was 0.2 mm or 0.12 mm, respectively; the axillary and inguinal lymph nodes could be visualized from a size of 2–3 mm upward. Pathological changes can be assumed if the sonographic image demonstrates lymph nodes measuring > 3–4 mm.10
The investigation always was performed with the patient in the supine position, with the leg in abduction and in external rotation when visualizing the inguinal lymph nodes, and with the arm elevated and the hand placed under the head for the axillary lymph nodes. Care was taken to ensure that the position of the patient during sonography and lymphoscintigraphy corresponded to the positioning during surgery.
Course of the Investigation
In a first step, 1 day before lymphoscintigraphy and surgery were performed, all lymph nodes within the entire basin were scanned by ultrasound. Asymmetric lymph nodes were conspicuous. The size and the position of these lymph nodes relative to the skin surface and anatomic landmarks were documented. For the inguinal region these structures were: the common femoral vein, the great saphenous vein, and the common and superficial femoral artery. For the axilla the anatomic landmarks were: the major pectoral muscle, the thoracodorsal artery, and the axillary vein and artery. To avoid a shifting of the transducer while marking the skin, the physician performing the ultrasound (B.K.) positioned the probe perpendicular, controlling the suspicious SLN in real-time mode while an assistant (J.W.) marked the skin. The ultrasound planes of the maximal longitudinal and cross-sectional dimensions of the conspicuous lymph nodes were marked on the skin (Fig. 1). The intersection of these axes was designated M1.
For lymphoscintigraphy, 40–50 megabecquerels of Tc-99m colloid (Nanocoll®; Sorin Biomedica, Saluggia, Italy) in a volume of 0.5 mL was injected intradermally in 4 depots of 0.15 mL each around the melanoma or the biopsy site. The patient then was positioned as described earlier (investigational conditions). Dynamic images were acquired for the first 10 minutes. Serial static views were acquired until 1 h p.i. in a matrix of 256 × 256 with a double-headed gamma camera with a large field of view (ECAM; Siemens, Berlin, Germany) fitted with a high resolution collimator. Care was taken to image not only the greater lymphatic basins in the inguinal area and the axilla but also the lesser lymphatic basins of the cubital and popliteal fossae and possible lymph nodes along the drainage paths on the trunk. In cases of multiple drainage (especially in cases of melanoma of the trunk), multiple projections were acquired to follow all drainage pathways. On the serial images the SLN was defined as the first focus of activity in the respective drainage pathway. As part of the standard procedure, the SLN or SLNs were localized roughly by scintigraphy; the exact location of the respective lymph nodes in relation to the skin surface was determined by using a handheld gamma probe (CXS; Crystal, Berlin, Germany) with a lower threshold (120 kiloelectron volt) window fixed at 20%. An identical probe was used intraoperatively. The nuclear medicine physician (J.H.), who was unaware of the true meaning of the skin indications, marked the point on the skin (M2) (Fig. 2) at which the gamma probe recorded the highest count rate when the instrument was held vertically to the skin surface. The nuclear medicine physician (J.H.) and the 2 surgeons (B.K. and W.H.) who took part in the current study had established the SLN biopsy as a routine procedure 2 years before preoperative ultrasound examination of the SLN was initiated. An essential part of the introduction of the gamma probe was the effort to always keep the probe perpendicular to the skin surface because this positioning of the probe was believed to provide the most consistent and comparable interindividual results with regard to localization of hot spots.
Before surgery, ultrasonography of the basin was repeated; the size, shape, and position of the SLN were registered and the markers M1 and M2 were compared (Fig. 2) and documented. Local anesthesia (tumescence solution containing 50 mL of prilocain in 500 mL of sodium chloride) then was infused around the melanoma/site of previous melanoma removal and in the regional lymph basin (axilla or inguinal region). Approximately 5–10 minutes before surgery was initiated, 1–2 mL of Patent Blue V (Patent Blue 2.5%; Guerwe GmbH, Sulzbach, Germany) was injected strictly intracutaneously around the tumor or the site of the primary excision. After application of the anesthesia and Patent Blue V, the patients were positioned on the surgery table as described previously for SLN biopsy.
Intraoperatively, the SLN was dissected using the handheld gamma probe. If after removal of the SLN only background activity was recorded in the surgery site, the excision of the SLN was considered complete. The removed SLN then was (ex vivo) cut in two along the longitudinal axes for histologic (hematoxylin and eosin [H & E] staining) and immunohistologic examination (HMB 45 and S-100 staining) of multiple sections per half. Before being sent to the laboratory, the two halves were measured and photographed for documentation (Fig. 3).
Careful sonographic measurements of the entirety of each respective lymphatic basin revealed a distinctive asymmetry between the cortex and medulla in only one or two lymph nodes compared with all the other lymph nodes of that basin (Fig. 4). These lymph nodes had a localized thickening of the cortex reminiscent of a cap (Figs. 4, 5). To quantitate this visual impression of asymmetry, the ratio between the cortex and medulla was calculated on either side of a vertical line through the hilus (Fig. 5).
Lymph nodes with ratio of > 1.2 between the cortex and medulla as described earlier were regarded as the SLN. Inspection of the SLNs after extirpation revealed an increase of the cortex in a peripheral direction in the longitudinal dimension, corresponding to the location of the sonographic “cap,” and also corresponding to the point of maximum Patent Blue V staining in 58 cases (Fig. 3). Ex vivo count rates measured with the gamma probe always were found to be highest in the region of the “cap.” Longitudinal and cross-sectional dimensions of the lymph nodes measured by preoperative sonography corresponded to those found macroscopically after lymph node removal.
In 67 melanomas, 70 conspicuous lymph nodes were detected sonographically and 82 SLNs identified by lymphoscintigraphy. All marks made during sonography (M1) corresponded directly to those made during scintigraphy (M2). The points of intersection of the green axes used to locate the lymph nodes detected by ultrasound corresponded to the scintigraphic markers (Fig. 2).
After lymphoscintigraphy but before local anesthesia was administered, the ultrasound scan was repeated. Only in those lymph nodes considered SLN by sonomorphologic criteria had the cortex become less hypoechoic in comparison with the initial examination. The demarcation from the surrounding tissue was no longer as clear as in the first investigation.
The number of SLNs per patient diagnosed by scintigraphy demonstrated no dependence on respective tumor thickness or regression zones present in the melanoma or the subsequent detection of distant metastases.
Melanoma of the legs
In 29 of 30 patients with melanoma located on the legs, the lymphatic pathway drained in the inguinal area. In all these cases the SLNs were detected sonographically. In six patients, two SLNs were found both by ultrasound and by lymphoscintigraphy. In 29 patients, the number of SLNs diagnosed by sonography was the same as that diagnosed by scintigraphy (n = 36). In one patient with melanoma of the sole in the calcaneal region the SLN was diagnosed scintigraphically in the popliteal fossa. No suspicious lymph nodes were found in the inguinal area of this patient; the popliteal region was not screened routinely with ultrasound. The mean dimension of the SLN in the inguinal area was 7.5 mm ± 3 mm and the SLNs were found to be situated 16 mm ± 4 mm below the skin surface.
Melanoma of the arms
All 14 melanomas located on the arms drained into the ipsilateral axilla. In nine patients one SLN and in two patients two SLNs were detected by both ultrasound and scintigraphy. The cutaneous markers M1 and M2 corresponded in these cases. In three patients, accurate sonographic detection of the SLN was not possible because of significant obesity but lymphoscintigraphy detected four SLNs. In two of the three obese patients the SLN could not be found intraoperatively because of excessive adiposity of the axilla. The cubital fossa was not assessed routinely by sonography. The mean dimension of the SLN in the axilla was 8.9 mm ± 3 mm and the SLNs were situated 20.5 mm ± 4.7 mm below the skin surface.
Melanoma of the trunk
In 23 patients with melanoma of the trunk, 21 SLNs were identified on ultrasound and 27 were identified by lymphoscintigraphy. The cutaneous markers M1 and M2 were found to correspond in these 21 cases. Sixteen of the 23 cases of melanoma of the trunk drained into 1 SLN in 1 axilla. Only 11 of these cases were identified by ultrasound. Two melanomas drained into both axillae. Three of the four SLNs were detected by both ultrasound and scintigraphy, and one SLN was identified by scintigraphy only. Two patients with melanomas of the trunk had SLNs in one axilla and the ipsilateral inguinal area. Four of five SLNs were found by both ultrasound and scintigraphy, and one SLN was found by scintigraphy only.
Two tumors drained only into the inguinal area, in which all SLNs were identified sonographically.
In one patient, the lymphatic drainage was into the neck. This region was not routinely examined by ultrasound; the axillae demonstrated no suspicious lymph nodes.
In 11 patients, there was histologic and immunohistologic evidence of metastasis of the SLN. Intraoperatively, these lymph nodes had demonstrated no abnormalities in size or shape; however, one of these lymph nodes was firm on palpation. The capsule had not been penetrated in either case. In one case the metastasis was localized in the “cap.”
Patent blue v staining
Preoperative peritumoral intracutaneous injection of Patent Blue V stained 58 SLNs. In these cases, Patent Blue V was found only in the periphery (i.e., the pole of the lymph node pointing toward the tumor). In the ex vivo longitudinal section, the Patent Blue V accumulated mainly in that part of the SLN with the highest radioactive count rate, which corresponded to the hypoechoic cap noted on ultrasound imaging (Fig. 3).
SLN biopsy, which is reportedly less invasive than an elective lymphadenectomy,11 is recommended for patients with malignant melanoma with a tumor thickness > 1.0 mm.12, 13
In our institution, sonography of the lymphatic drainage is part of the staging of malignant melanoma. During these routine sonographies, we observed a particular morphologic pattern that was present in only one or two lymph nodes in the entire basin. In a first step we attempted to characterize the special sonomorphology, and in a second step we tried to establish whether these lymph nodes corresponded to SLN as characterized by lymphoscintigraphy.
The hallmark of the lymph nodes presumed to be SLNs in the current study was their conspicuous asymmetry. This asymmetry, which can best be expressed by variations in the ratio between the cortex and medulla as described earlier, is due to a localized thickening of the cortex at the peripheral pole that reminds one of a “cap” (Figs. 4, 5). Corresponding to the cortical thickening, a localized staining of the lymph node cortex was found during surgery by prior intracutaneous injection of Patent Blue V in 58 cases (Fig. 3). One explanation could be that the cap-like cortical thickening of the SLN diagnosed by sonography corresponds to the region in which the melanoma-specific afferent lymph vessel joins the lymph node.
The time interval between excision of the melanoma and SLN dissection most likely is unimportant in terms of the extent of the asymmetrical cortical thickening. In 35 of 67 patients in the current study, the primary tumor had not been removed at the time of sonography and scintigraphy. The SLN of these patients demonstrated the “cap” phenomenon to the same extent as those of the patients who had undergone melanoma removal 6–8 days earlier. Different tumor locations and distances between the SLN and the primary tumor likewise appear to have no influence on the shape and size of the SLN.
Sonography performed as part of melanoma follow-up often detects so-called reactive lymph nodes that are, however, not necessarily suspicious. These lymph nodes with reactive changes demonstrate, in addition to a marked enlargement, a symmetrically thickened cortex that, through the abundance of cells, appears nearly anechoic and therefore contrasts well with the echo-rich medulla. This structure, described as resembling a cockade, differs markedly from the “cap” phenomenon and is typical of reactive lymph nodes. Moreover, reactive lymph nodes are symmetrical.10
In contrast, a considerable asymmetric thickening of the cortex and additional morphologic patterns such as round, anechoic areas in the lymph nodes; a distinct vascularization pattern; and occasionally a penetrated capsule are interpreted as evidence of metastasis.1, 4, 5 Apart from the typical “cap” phenomenon, the SLN we identified with sonography demonstrated no further abnormal morphology. The central vascular architecture was retained. Histology and immunohistochemistry of the SLNs in the current study demonstrated metastatic disease (micrometastasis) in 11 of 82 lymph nodes (13.4%).
To our knowledge, there is no common marker that reliably labels a lymphatic node for both sonography and radio-guided detection. Therefore, it was not possible to prove directly the identity of sonographic and scintigraphic findings. To ensure as much as possible that the lymph node identified by sonography was indeed the SLN as identified by scintigraphy and the gamma probe we adhered to the following principles:
1The nuclear medicine physician was unaware of the meaning of the skin marks when he saw the patient for lymphoscintigraphy. Moreover, the M1 ultrasound marks were applied several centimeters away from the probe-guided mark M2.
2Because the gamma probe provides the best signal when placed vertically on the skin surface and directly above the SLN, the ultrasound examination also sought to position the ultrasound probe likewise vertically to the skin when the SLN was marked.
3The location of a lymph node with regard to the neighboring anatomic landmarks varies with the position the patient assumes during a procedure. For the four procedures involved (sonography, scintigraphy, probe-guided detection, and surgery), we always positioned the patient in the same way. This was particularly important for surgery of the axilla, during which the supine patient was asked to elevate the arm and place the hand below the head.
4To identify the conspicuous lymph node in a manner independent of the indications on the skin, the position of a particular lymph node in relation to the surrounding surgical anatomy and in relation to other surrounding lymph nodes was recorded meticulously.
The anatomic situation recorded preoperatively for the SLN identified by sonography coincided in each case with the anatomic findings when the SLN identified by scintigraphy and the gamma probe was harvested. However, in a proportion of our patients we cannot exclude a possible bias of the surgeon who performed the SLN biopsies. In approximately 33% of patients, ultrasound was performed by the same surgeon (B.K.) who later performed the SLN biopsy. In the remaining 67% of the patients, the surgeon who harvested the SLN (W.H.) was unaware of the ultrasound findings.
At biopsy, the distance of the SLN from the skin surface was roughly equal (because of tumescence anesthesia) to that determined preoperatively. No other lymph node was detected by ultrasound and during surgery between the skin surface and the presumed SLN. Vertical stacking of lymph nodes therefore can be excluded as a source of error.
The patients in the current study were included in the regular follow-up ultrasound examinations. The lymph nodes remaining after SLN biopsy were scanned at intervals of s6 months. The remaining lymph nodes did not demonstrate any conspicuous asymmetry between the cortex and medulla during follow-up.
Our initial observations regarding lymph node morphology were made in the inguinal area. Here, the tissue covering the lymph nodes was between 10–20 mm wide in the current study patients, and the presumed SLN with a mean dimension of 7 mm and the anatomic landmarks were readily discernible. Given that M1 and M2 coincided with a disagreement between both probes is only possible if the probes are tilted. The probes have to be tilted by at least 12 degrees to shift the target volume 3 mm sideways in a depth of 10–20 mm from the skin. This means that the probe has to be tilted visibly to miss a SLN that measures an average of 7 mm and usually is separated from neighboring lymph nodes by 1–2 mm.
An additional hint are the changes in lymph node appearance after injection of the radioactive colloid that is transported by the lymphatics. When after lymphoscintigraphy the lymph node that appeared as the SLN by ultrasound was reexamined, the cortex of only this lymph node but none of the neighboring lymph nodes had become less echogenic.
In each patient in the current study, lymphoscintigraphy was followed by resection of the primary tumor and/or wide local excision in addition to SLN biopsy. Further changes in the ultrasound morphology of the presumed SLN because of injection of a local anaesthetic around the primary tumor/site of wide local excision or into the lymphatic basin cannot be excluded. Moreover, it is important to note that the ultrasound observations reported in the current study all were made preoperatively.
In addition, all SLNs identified by ultrasound were found by scintigraphy but not vice versa. Therefore, we currently do not recommend ultrasound as the means of localizing the SLN intraoperatively. However, taking the anatomic and functional information together, we believethat the asymmetric lymph node demonstrating the “cap” phenomenon indeed corresponds with the SLN found by standard lymphoscintigraphy and a gamma probe.
The best correspondence between SLNs identified ultrasonographically and scintigraphically was observed in the inguinal area (Table 1). In all melanomas draining into the inguinal area, including those of the trunk, the SLN was detected by both ultrasound and lymphoscintigraphy (Table 2). In 1 of the 37 melanomas of the lower limb, the tumor was located in the drainage region of the small saphenous vein, and the SLN was found in the popliteal fossa by scintigraphy. Because SLNs are very rarely found in the popliteal or cubital fossae, both regions were not examined routinely by ultrasound, and thus the one SLN in the popliteal fossa was not detected. In the remaining 36 of 37 melanomas of the lower extremity, the SLNs were situated in the inguinal area below the inguinal ligament and were detected by ultrasound.
Table 1. Detected SLNs per Melanoma
Melanomas and position
Detected by sonography M1
Detected by scintigraphy M2
M1 = M2
M1: intersection of the axes of the ultrasound planes of maximal longitude and cross-sectional dimensions of the conspicuous lymph nodes; M2: the point on the skin at which the gamma probe recorded the highest count rate when the instrument was held vertically to the skin surface; SLN: sentinel lymph nodes; US: ultrasound; Sono: sonography; scinti: scintigraphy.
30 melanomas of the lower limbs
29 × inguinal 1 × popliteal
36 of 37, 97%
Popliteal SLN not identified by US
14 melanomas of the upper limbs
14 × axillar
13 of 17, 76%
21 of 28, 75%
23 melanomas of the trunk
16 × 1 axilla
11 of 16, 69%
2 × both axilla
3 of 4, 75%
2 × axilla plus inguinal
3 of 5, 60%
1 × inguinal
1 of 1, 100%
1 × both inguinal
2 of 2, 100%
1 × nuchal
Total of 67 melanomas
70 of 82, 85.4%
Table 2. Percentage of Identified SLNs by Ultrasound per Basin
Identified SLN by ultrasound, M1
Identified SLN by scintigraphy, M2
M1 = M2
SLN: sentinel lymph node; M1: intersection of the axes of the ultrasound planes of maximal longitude and cross-sectional dimensions of the conspicuous lymph nodes; M2: the point on the skin at which the gamma probe recorded the highest count rate when the instrument was held vertically to the skin surface.
41 of 41, 100%
29 of 40, 72.5%
In the axilla, the agreement between ultrasound and lymphoscintigraphy was 72.5%, which was lower than that noted in the inguinal area. One reason for the less favorable results in the axilla was obesity; the obesity noted in three patients resulted in SLN biopsy in the axilla being unsuccessful and having to be terminated prematurely.
Our initial experience in the axilla was comprised of not only melanomas of the arms but also four melanomas located over the inferior margin of the scapula. From the position of the melanomas we had expected lymphatic drainage only to the ipsilateral axilla. Unexpectedly, in each of these patients lymphoscintigraphy found a SLN in both the contralateral and ipsilateral axillae. Drainage pathways on the trunk are known to be complex and unpredictable.14–17 In the patients that followed, ultrasound in truncal melanoma was extended to both axillae as well as both inguinal areas. In an additional patient with melanoma of the shoulder, ultrasound correctly identified one SLN in the axilla but lymphoscintigraphy detected a second SLN in the neck.
At the current time, lymph node sonography has an established role in the diagnosis and follow-up of patients with malignant melanoma. In the hands of an experienced investigator, the method achieves a high sensitivity and specificity.18–20 In our opinion, preoperative imaging of the SLN represents an additional, new indication for lymph node ultrasonography in malignant melanoma. The surgeon thereby obtains important information regarding the position of the SLN in relation to its neighboring structures and can prepare for surgery and plan the procedure accordingly. In some cases of SLN extirpation, the surgical approach is anatomically easier by when the skin incision is made away from the approach indicated by the gamma signal (e.g., when the SLN lies under the great pectoral muscle or superior to the inguinal ligament). In the axilla, it is helpful to know the anatomic position of the SLN, especially its depth, to decide whether the SLN can be biopsied with regional anesthesia or whether general anesthesia is needed. In the ideal case, the surgeon is present during the “sentinel ultrasonography” or performs it himself or herself.
In our opinion, the results of the current study demonstrate that the SLN shows a typical sonomorphologic pattern. Preoperative knowledge of the position of the SLN in relation to critical anatomical structures may impact the surgical approach and strategy.