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Science to Practice |
1 B1D 502, Department of Radiology, University of Michigan Medical Center, 1500 E Medical Center Dr, Ann Arbor, MI 48109-0030 ronbude@umich.edu
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The sentinel lymph node (SLN) is the first node (or nodes) to drain a tumor into a lymphatic drainage basin. For tumors such as malignant melanoma and breast carcinoma (1,2), the histologic status of the SLNs predicts the status of the regional nodes well enough that lymph node dissection may not be needed if the SLNs are free of metastasis.
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The Science
Blue dye injection is performed by injecting the blue dye into or adjacent to the primary tumor. This is followed by surgical exploration to follow the blue lymphatic channel into the first blue node, the SLN. At times, more than one draining channel and/or more than one SLN are found. In many cases, SLNs are removed and evaluated for metastatic disease. If findings are negative, further surgery is prevented; if findings are positive, extensive node dissection may be indicated. Scintigraphy works in much the same way, with SLNs detected as "hot" regions in the expected nodal basins.
In a swine melanoma model, Goldberg et al (3) localized 28 of 31 SLNs with a peritumoral injection of a US microbubble contrast agent and imaged the transit time of the agent from the injection site through draining lymphatic channels to SLNs. In some cases, SLNs were seen within seconds after injection. At times, SLNs were found in distant or unexpected locations. Contrast agent was not observed in nodes or lymph channels distal to the SLNs. Last, and perhaps of considerable importance, the authors observed and detected signal voids in SLNs, which represented intranodal metastases in 19 of 20 SLNs with metastases (95% sensitivity). The specificity was 63% (five of eight SNLs).
The Practice
Clinical use.—Identification of metastasis-free SLN(s) may obviate extensive regional node dissection in the treatment of breast carcinoma and malignant melanoma (1,2,4). If node dissection can be avoided, the patient is spared both morbidity, such as persistent arm lymphedema, and expense.
Although blue dye and scintigraphic techniques work well for SLN detection, each has drawbacks. The blue dye technique is invasive, requiring surgical dissection in the nodal basin to find the lymphatic channel and then the SLN(s). The rate of allergic reactions, including anaphylaxis, is as high as 2.5% (5). It has a technical failure rate of about 20% (6,7). It may be difficult to differentiate the SLN from closely adjacent nonsentinel nodes, which can take up the dye, the removal of which leads to a more extensive nodal dissection than is necessary. Drawbacks of scintigraphy include a technical failure rate of up to 12% for breast cancer (1) and a possible detection of nonsentinel nodes, which may again lead to unnecessarily extensive nodal dissection.
The results of the Goldberg et al (3) study suggest that US may be as good as the currently accepted techniques for SLN detection. Its use could lead to a reduction in the number of extensive node dissections performed, owing to more accurate detection of SLNs. It also shows promise for detection of SLNs in unusual locations that might not be anticipated, and thus not detected, with scintigraphy but that can be detected sonographically with real-time visualization of lymph drainage.
Future opportunities and challenges.—If US proves to be only as good as scintigraphy or blue dye for SLN detection, it may be difficult to persuade clinical colleagues to use it in favor of familiar techniques. It is likely that one or more of the following encouraging findings of this study will need to bear fruit for US to be adopted: (a) The real-time visualization of lymphatic drainage may allow easier and less invasive detection of SLNs in expected locations and detection in unexpected locations where they might have otherwise been missed; (b) SLN metastases may be detected sonographically as contrast-deficient regions, perhaps enabling accurate percutaneous SLN biopsy, which may eliminate the need for any surgery in some cases; and (c) the test can be repeated relatively easily if initial results are inconclusive. Larger studies of statistical significance in humans will be necessary to determine if US improves the outcome or reduces the cost of medical treatment.
Summary
Using a malignant melanoma model in swine, Goldberg et al (3) have shown the potential for a subcutaneously injected US contrast agent to localize SLNs. They have also demonstrated the potential of this agent to depict intranodal metastases, which is not possible with current SLN localization techniques. More studies are needed. If the results are encouraging, adoption of this technique will depend on the Food and Drug Administration approval of these agents, which have long ago been approved for clinical use in Europe.
FOOTNOTES
See also the article by Goldberg et al in this issue.
REFERENCES
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