Sentinel lymph node

Encyclopedia of Cancer, 2015

Synonyms

First-echelon node; First-tier node

Definition

A sentinel lymph node is a lymph node upon which the primary tumor drains directly.

Characteristics

General anatomy and physiology of the lymphatic system

Lymphatic capillaries are 10–50 mm in diameter, consist of a single endothelial layer with a discontinuous membrane, and are supported by collagen filaments. They are filled with lymph fluid originating from the interstitial space due to an osmotic pressure gradient and fluctuating intraluminal pressures. These intraluminal pressures are caused by lymphatic flow that is generated by lymph formation, contractions of the vessel wall, and external pressure. Lymph fluid absorbed by lymphatic capillaries drains into larger collecting lymphatic vessels. Such lymphatic vessels drain into marginal and medullar sinuses located between germinal centers within a lymph node. These centers contain large numbers of phagocytic cells.

A plexus within the lymph node drains to the efferent lymphatic vessel adjacent to the artery and vein in the hilum. Direct drainage of the marginal sinus into the efferent vessel also exists (Fig. 1).

The sentinel node

The sentinel node is the lymph node upon which the primary tumor drains directly. Lymph fluid moves subsequently to second-tier and third-tier lymph nodes (Fig. 2).

The sentinel node concept implies orderly progression of metastases from a primary lesion through the lymphatic system. The concept is only relevant in cancers with predominant lymphatic dissemina- tion, such as melanoma and cancer of the breast, penis, or colon. If the first node contains a metastasis, there is a chance of tumor spreading downstream. In case of a tumor-negative sentinel node, second-tier and third-tier nodes are generally without metastases.

Lymphoscintigraphy

The lymphatic drainage pattern can be visualized by lymphoscintigraphy after injection of a radio-labeled tracer in or near the site of the tumor. The radio-labeled tracer is cleared from the lymphatic channels and accumulated by the phagocytic cells in the lymph node. Lymphoscintigraphic images depict the lymph channels and the lymph node or nodes that contain the injected tracer. Dynamic scintigraphy demonstrates in the lymphatic drainage pattern, which enables identification of the sentinel node(s). Single-photon emission computed tomography combined with radiographic computed tomography (SPECT/CT) improves the identification of the correct number of sentinel nodes and visualizes the anatomy of their location.

Sentinel lymph node 1

Fig. 1. The different relations between lymphatic vessels and lymph nodes. Afferent lymphatic ducts on the left discharge their contents into the marginal sinus. One lymphatic duct runs through the node on the right and another over its surface, bypassing the germinal centers (Illustration made by Tanis PJ; Reprinted from Tanis et al. (2001), with permission from “The American College of Surgeons”)

Sentinel lymph node 2

Fig. 2. A sentinel node is node upon which lymph fluid from the tumor drains directly

Sentinel node biopsy

A sentinel node biopsy is the minimally invasive technique to identify the sentinel lymph node. A vital blue dye is administered at the lesion site. An incision is made over the lymph node region and the blue lymphatic channel is identified. This channel is dissected and followed to the first draining lymph node. Alternatively, a gamma ray detection probe can be used to track the radioactive sentinel node. Afferent and efferent lymph vessels and blood vessels are ligated and divided, and the node is submitted for pathology evaluation. The pathologist obtains multiple sections of the node and uses sensitive and specific staining techniques in his search for malignant cells.

Sentinel lymph node 3

Fig. 3. A tumor (black area) with lymphatic channels to nodes in the axilla (a) and to lymph nodes below (b) and above (c) the clavicula. A metastasis may be found along this lymphatic pathway

Sentinel lymph node 4

Fig. 4. Patient with a left breast cancer. The radiopharmaceutical was injected in the tumor, and the sentinel node in the left axilla is depicted

The sentinel node procedure provides staging and prognostic information. It identifies patients who may benefit from early regional therapy ( locoregional therapy) and/or adjuvant systemic treatment ( adjuvant therapy). In some cancers, this leads to an improved survival rate.

Breast cancer

The predominant lymphatic drainage pathway from the breast is toward the axilla. Metastases are usually situated in the lower axilla first and then may travel higher up the chain to the subclavicular and the supraclavicular basins. Occasionally, they travel directly to the more cranial glands (Figs. 3 and 4).

In the past, axillary lymph node dissection is used to be performed in almost every breast cancer patient. This operation has several potential side effects, such as lymph edema, pain, and decreased mobility of the arm, while often no metastases were found. With the introduction of sentinel node biopsy, axillary lymph node dissection is more selectively performed. As a result, many patients are spared an unnecessary operation.

Large observational studies revealed excellent results in patients who did not receive axillary node dissection because of a tumor-negative sentinel node. Recurrence rates vary between 0.12% and 0.6% in these patients.

Melanoma

The sentinel node can be identified in almost all patients. Unfortunately, the false-negative rates are often between 10% and 20%. False negative means that the sentinel node is disease-free, while there are metastases in the lymph node basin. A large randomized study showed that early regional node dissection based on a positive sentinel node improves survival in patients with an intermediate-thickness melanoma.

Concluding remarks

The development of the sentinel node concept is a milestone in the understanding of dissemination of solid malignancies. It is now well integrated in the clinical management of a variety of cancers. Many patients are now spared unnecessary surgery without compromising regional control and the accuracy of staging. Important prognostic information is obtained, and survival is improved in patients with mela- noma and penile cancer. The application in patients with other cancer types is being explored.

References

Kroon BK, Horenblas S, Lont AP, Tanis PJ, Gallee MP, Nieweg OE (2005) Patients with penile carcinoma benefit from immediate resection of clinically occult lymph node metastases. J Urol 173:816–819 Morton DL, Wen DR, Wong JH, Economou JS, Cagle LA, Storm FK et al (1992) Technical details of intraoperative lymphatic mapping for early stage melanoma. Arch Surg 127:392–399

Morton DL, Thompson JF, Cochran AJ, Mozzillo N, Nieweg OE, Roses DF et al (2014) Final trial report of sentinel-node biopsy versus nodal observation in melanoma. N Engl J Med 370:599–609

Nieweg OE, Tanis PJ, Kroon BBR (2001) The definition of a sentinel node. Ann Surg Oncol 9:538–541

Tanis PJ, Nieweg OE, Valdés Olmos RA, Kroon BBR (2001) Anatomy and physiology of lymphatic drainage of the breast from the perspective of sentinel node biopsy. J Am Coll Surg 192:399–409

Van der Ploeg IMC, Nieweg OE, Van Rijk MC, Valdés Olmos RA, Kroon BBR (2008) Axillary recurrence after a tumour-negative sentinel node biopsy in breast cancer patients: a systematic review and meta-analysis of the literature. Eur J Surg Oncol 34:1277–1284

 

 

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