The lymphatic system is a major conduit for the progression of many malignant and infectious diseases. Its investigation has long been undertaken in an attempt to effectively diagnose the extent of disease and to help plan treatment. Imaging methods developed to achieve this purpose include cutaneous lymphoscintigraphy, immuno- lymphoscintigraphy, radiographic lymphography, and recently, MR lymphography.

Of these procedures, lymphoscintigraphy currently provides many advantages over the other imaging methods: minimal invasiveness, short procedure time, low cost, and minimal patient preparation. This article will address many important aspects of clinical cutaneous lymphoscintigraphy, including agents, technique, and indications.

Lymphatic system

The lymphatic vessels are distributed throughout most parts of the body as a counterpart to the venous system. Its main components are lymph capillaries, vessels, and ducts, extending from the periphery to the central venous circulation. Lymphatic capillaries are thinner than blood capillaries and lack basement membranes, allowing increased permeability; they are blind-ending and are most numerous below the epithelium of the skin and mucous membranes.1

Lymph is transported centripetally to regain access to the venous system through the thoracic and right lymphatic ducts. The sites of lymphocyte aggregation and lymph filtration, the lymph nodes are located along the lymph channels, predominantly in the neck, axillae, mediastinum, periaortic, hypogastric, and inguinal regions. The lymphatic drainage pathways in the extremities have fairly defined routes, but variability persists.

Many investigators have demonstrated the lymphatic drainage in the truncal region to be highly variable and unpredictable.2-4 Most recently, lymphatic drainage investigation has further defined the variable pathways, which appear predictable in the extremities and individually unpredictable throughout the rest of the body;4 they are displayed in figure 1. Lymphoscintigraphic evaluation allows effective depiction of lymphatic drainage and increasingly has been used in the evolving treatment options for some cancers, especially malignant melanoma. Investigation has shown this nuclear medicine procedure to be reproducible and effective.

Radiopharmaceutical agents

Many radionuclides have been investigated and can be grouped into three classes: radiolabeled macromolecules, radiocolloids, and radiolabeled antibodies (table 1). Radiolabeled macromolecules and colloids are the most commonly used agents; labeled antibodies are investigational. Radiocolloid particle diameter of less than 50 nm appears to be optimal for lymphoscintigraphy.5 These agents have varied pharmacodynamics and imaging requirements. Although many colloids have been investigated, only 99mTc sulfur colloid (SC), 99mTc antimony trisulfide colloid (ATC), 99mTc dextran (Tc Dx), 198 gold (Au-198) colloid, and radiolabeled antibodies will be mentioned.

Au-198 colloid was the first agent to be documented to tract in the lymphatics in 1953, although its investigation was focused upon therapeutic treatment of lymphatic metastasis.6 Although the 2 nm to 20 nm gold particle diameter was ideal,7 the long half-life of 2.7 days, 0.9 MeV beta particle, and energetic 412 keV gamma ray were not desirable for dosimetry and imaging purposes.8,9 Also, injection site radionecrosis has been documented.10

Dextran (99mTc Dx) has many desirable features for cutaneous lymphoscintigraphy, such as particle size and controllable biokinetics. Investigation has revealed glomerular filtration and urinary excretion with dextran of weights less than 40 kilodaltons (kd) after interstitial injection.11 Dextran is not phagositized, enters the lymphatic system rapidly, and, due to the impermeable characteristics of blood capillaries at molecular weights greater than 40 kd, is a very suitable, specific lymphoscintigraphic agent. Upon interdigital injection, knee/elbow and inguinal/axillary activity were noted in approximately 12 and 98 seconds, respectively, on dynamic imaging, and half-time activity at the injection site was 31.5 minutes.12 These favorable features may allow dynamic image acquisition when compared to other agents, but it is not currently approved for use in the United States.

Sulfur colloid (99mTc SC) has been and continues to be used for lymphoscintigraphy; this agent is the only approved agent for cutaneous lymphoscintigraphy in the United States. Due to the large colloidal particle size, ranging between 100 nm and 2000 nm, variable lymphatic system visualization has been observed. Injection site clearance of 30% is typical at three hours postinjection, and dynamic imaging has been less satisfactory.9

Filtration of Tc SC has been performed to more optimally sized particles, improving its lymphatic absorption.13 Also, recent filtration attempts have been successful in producing reproducible particle sizes ranging up to 50 nm (i.e., the size range identified to be optimal for lymphoscintigraphy), and regional (inguinal) lymph node identification in reasonable postinjection times (30 minutes) can be performed.14 The filtration procedure was simple and performed with commercially available supplies, thus improving scintigraphic features and potentially allowing for dynamic imaging.

Antimony trisulfide colloid (99mTc ATC) has also been investigated. After being described in 1965,15 its physical size is favorable and consistently uniform, ranging from 3 nm-15 nm to 40 nm.5,16 Like unfiltered Tc SC, moderate retention at the injection site is present, approximating 40% at four hours postinjection.9

Also, Tc HSA has been employed. This agent demonstrates more rapid migration from the injection site than observed with the colloids. At 69 kd, its molecular weight is similar to that of dextran. This allows almost immediate visualization of tracer movement, allowing dynamic images and higher injection doses, which improve image quality and potentially lower patient dose.9

When compared to Tc SC, Tc HSA has demonstrated improved image quality, visualization of lymph channels, anatomic detail, and decreased study time.17 As such, Tc HSA is the predominant cutaneous lymphoscintigraphic agent employed at our institution, although retention of activity in the sentinel node(s) may be reduced compared to Tc SC.19 This could limit the usefulness of Tc HSA with the use of intraoperative gamma probe, but it is proven effective in static imaging.

Lastly, in an attempt to increase the sensitivity of detection of tumor metastasis and improve tumor/background ratio, immunolymphoscintigraphy has been studied with tumor-specific monoclonal antibodies and kinetically with nonspecific antibodies. It has been found that Ig classes (IgG and IgM), antibody fragments [F(ab)2 and F(ab)], and colloids have similar lymphatic migration rates (i.e., hours), which can be statistically increased with motion (i.e., ambulation). Also, antibody migration rates differ depending upon injection site, moving faster with limb injection than body wall injection.18 Tc SC and Tc HSA are valuable lymphoscintigraphic agents, providing high-quality images during acceptable imaging times. Radiolabeled antibodies will require further investigation to better define their future clinical role.

Technique

The equipment and materials required to perform cutaneous lymphoscintigraphy are standard in any conventional nuclear medicine department. The radiotracer used primarily at the University of Alabama at Birmingham (UAB) is 99mTc HSA. The dose ranges from 0.5 mCi to 1.5 mCi and should be diluted in 1.0 ml sterile saline; larger doses in this range are utilized to adequately encompass larger or more extensive lesions. Doses ranging from 0.5 mCi to 2.0 mCi have been used with Tc SC and Tc ATC. No specific patient preparation is required. Patients are preferably preoperative, although postoperative studies are commonly performed, especially due to our wide referral patient population. Rubber gloves and reasonable semi-sterile technique should be followed.

The injection site and surrounding area should be cleansed with povidone-iodine or 70% alcohol. In the evaluation of melanoma, a 1.0-ml syringe and 26 ga., one-half-inch needle at 45° or less is used to make several intradermal injections completely surrounding the lesion, each producing a small wheal, ensuring intradermal injection. If excisional biopsy has been performed, at least a 1.0-cm margin should be given about a lesion.

If lesions involve the head and neck, intradermal injections should be performed at the superior aspect (distal from the trunk) of the lesion. This has been proposed due to the inferior lymph drainage of head and neck lesions and the fact that injections at the inferior aspect of the lesion may potentially obscure sentinel nodes due to close proximity of the sentinel node to the lesion.19 Also, attention to detail and accurate node identification is particularly important because lymphoscintigraphy in the head and neck has been noted to be more difficult than that performed in other body regions.20

Intradermal injection is mandatory, as subcutaneous injection may result in tracer entering into distinct, deeper lymphatic channels, obviating the dermal plexus and not identifying the sentinel node(s).21 Evaluations for lymphedema require pedal, interdigital injection with immediate, one-hour, and three-hour images. For breast cancer lymphatic definition, either deep perilesional injection with ultrasound guidance and localization22 or paramidline, subxiphoid rectus sheath injections23 have been previously described.

The injection site needs thorough cleaning to remove any potential contamination postinjection. The patient should immediately assume a supine position under the gamma camera as HSA migration usually begins rapidly (i.e., minutes). We continuously image and supervise the examination to identify the initial lymph node, although we archive only static images, usually requiring 30 to 60 minutes, for later intraoperative review. In our experience, delayed images (i.e., several hours) are rarely needed; Tc SC studies routinely require 2-to-4-hour delayed imaging.

One study revealed that all sentinel nodes were observed within 20 minutes postinjection using a colloidal agent.24 Images of the regional nodal group(s) are obtained after sentinel node identification. Also, if body wall lesions are being investigated, images including the ipsilateral and contralateral axillary/inguinal drainage basin should be performed to ensure coverage, especially due to the variable body wall lymphatics (figure 2).

In our experience, imaging of the ipsilateral axillary or inguinal nodal group for upper and lower extremity lesions, respectively, has been sufficient (figure 3). Lesions of the head and neck should include the upper thorax routinely (figure 4). If migration is stagnant or slow, the injection site can be gently massaged, promoting movement. Although evidence has been reported with respect to truncal lesions, failed tracer movement from the postoperative injection site has been documented (figure 5).25 Surgical division of the local lymphatics has been suggested as a cause. The nature of the wide primary local surgical excision has also been proposed. Although the cause has yet to be defined, preoperative and postoperative lymphatic drainage basins can be markedly different and completely altered and reconnection of these transected lymphatic channels can occur by two years postoperatively.25

Cumulatively, these data suggest that lymphatic drainage has a dynamic feature, utilizing collateral pathways and a flexibility similar to that frequently observed in the venous system.25 A body wall outline is necessary for reference and can be performed with a Tc point source. After identifying the sentinel node, orthogonal images are obtained to help further define sentinel node depth; point source localization on the skin surface is determined, indelibly marked, and measured from a body landmark (i.e., umbilicus, iliac crest).

An alternative combined transmission-emission method has also been described, which potentially may improve image quality and eliminate errors in marker placement.26 This modification has also been performed at our institution with very satisfying results and minimal additional effort.

Most recently, postlymphoscintigraphic, intraoperative gamma probe sentinel node localization has been investigated with very favorable results.19,27 This has proved to be helpful, especially in the head and neck region, to locate the correct nodal incision site and to confirm that the radioactive node(s) has been removed.28

Also, the intraoperative gamma probe may help reduce unnecessary nonsentinel node removal. As has been described, the intraoperative gamma probe has been documented to be very helpful in localizing foci of disease for surgical removal.29 This has been accomplished as the probe assists the surgeon in locating intra- and extraabdominal lesions, helps identify lesions too small for scintigraphic detection and lesions not observed at surgery, and provides evidence of the adequacy of surgical cancer resection.29 Cutaneous lymphoscintigraphy is a technically easy procedure, and attention to detail allows reproducible sentinel node identification for surgical excision with high accuracy.

Indications

Lymphoscintigraphy has many clinical applications. These indications are listed in table 2. Recently, cutaneous lymphoscintigraphy has received renewed emphasis as an adjunctive, diagnostic modality in patients with malignant melanoma. The surgical management of patients with malignant melanoma is varied. Many investigators advocate elective lymph node dissection (ELND) in patients presenting with stage I or II disease. Evidence has been provided of decreased mortality from metastatic disease and improved prognosis.30,31

Conversely, others purport the benefit of local resection with postoperative, short-interval evaluation, in patients with no clinical evidence of regional lymph node metastasis.32,33 Relatively recently, a new paradigm of the progression of metastases of malignant melanoma has been investigated, introducing the sentinel node concept.21,34 This concept suggests that micrometastases of malignant melanoma extend from the primary lesion, gain access to the lymphatic system and progress to the nearest node directly in the lymphatic channels, producing nodal metastasis. Subsequently, malignant cells can migrate centrally as the scope of the disease expands, initially from the sentinel node to the regional group (echelon nodes), and then distantly.21

Other investigation35 supports this concept. Morton and Reintgen discovered that the sentinel node was the site of metastasis in 18% and the sole metastatic site in 87.5%; the presence of metastases exclusive of the sentinel node was very remote, at 2 of 3079 nonsentinel nodes (less than 1%) and no skip metastases were observed. No nodal metastases are presumed in patients with disease-free sentinel nodes. If this surgical paradigm were routinely adopted, with lymphoscintigraphic assistance, the potential for decreased patient morbidity and cost, especially with increasing cost-containment pressures, could be substantial, possibly excluding prophylactic lymph node dissection in almost 80% of patients.21

In addition, cutaneous lymphoscintigraphy affords additional benefits. A more thorough and accurate depiction of lymphatic drainage can be provided by this procedure over the historically defined lymphatic drainage patterns. One investigator revealed 84% discordance between lymphoscintigraphic drainage pathways and established pathways, producing an alteration in patient management by 62%.36

Conversely, lymphoscintigraphy is imperfect. A recent study specifically addressed this question.24 Repeat lymphoscintigraphy demonstrated only 88% sentinel node reproducibility. In a small series, Mudun has reported 50% discordance.37 Other researchers report similar, marked discordances38 and identification of different and new, additional sentinel nodes on sequential and comparative scintigrams.19,24

The nature of this variability is uncertain, although causes have been suggested, including variable injection dose and volume, tracer particle size, prior surgical intervention, site of lesion, type (depth) of injection, timing of imaging, and image and display parameters.19,39 Altogether, cutaneous lymphoscintigraphy can provide an addition option in the surgical evaluation of appropriately selected patients with malignant melanoma.

Breast and prostate carcinomas have been lymphoscintigraphically investigated. Lymphoscintigraphy, via paramidline, subxiphoid rectus sheath injections, has been evaluated and is a simple method to define internal mammary nodal drainage.40,41 As elsewhere, the parasternal lymphoscintigram revealed variability. This method has been investigated in the radiation therapy planning of breast cancer patients.32

In a recent study, an attempt to simulate the lymphoscintigraphic methods applied to melanoma, via deep, circumlesional breast injection, demonstrated 29 of 34 patients with single, sentinel nodes ipsilateral to the breast lesion, no contralateral breast sentinel nodes, and individually variable breast lymphatic drainage.22 This investigation demonstrated that lymphoscintigraphy can map breast lymphatic drainage in patients with breast tumors, that this drainage is variable, and, as seen in cutaneous lymphoscintigraphy, does not reliably follow classically defined routes.22 This method requires further investigation and could possibly influence the surgical approach to breast cancer if favorable results develop.

Iliopelvic lymphoscintigraphy had been performed to evaluate its efficacy regarding many pelvic malignancies, especially prostate carcinoma. Two methods, pedal or perianal interstitial injections, have been developed. The pedal method is technically easier, although it fails to evaluate many nodal groups commonly involved by metastatic prostate cancer (i.e., obturator, internal, and presacral nodes).43

Conversely, the perianal method identified these groups and has shown disease-free predictive value of 90%.44 Due to the questionable significance of nodal involvement45 and the possible contribution of other metastatic routes (i.e., hematogenous) unrelated to nodal disease,46 the current role of iliopelvic lymphoscintigraphy is uncertain and further investigation is required.44

Benign conditions have also been evaluated. Lymphoscintigraphy has been used to evaluate cystic hygromas. The definition of drainage in the pretreatment evaluation has been performed,47 and the treatment options may include surgical excision, ablation with sclerosing agent, or ligation. The proposed advantages of this procedure allow determination of lymph source of the hygroma, lymphatic morphology, extent of the hygroma, and rate of lymph flow.

Furthermore, it has been used to evaluate and classify lymphedema into primary forms (i.e., Milroy's disease, associated with Turner's syndrome, neurofibromatosis) or secondary forms (i.e., postoperative, post radiation treatment, or filarial lymphatic involvement) and venous thrombosis.48,49 The reported sensitivity and specificity of qualitative image interpretation have been 92% and 100%, respectively, in diagnosing lymphedema. With 99mTc ASC injection in the lower extremity, multiple varied findings have been identified as abnormal.50

Conclusion

Although cutaneous lymphoscintigraphy is requested infrequently, it is technically easy, minimally invasive, and potentially may improve management, especially if performed with intraoperative gamma probe techniques in patients with malignant melanoma. AR

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