REVIEW ARTICLE

Imaging Sentinel Lymph Nodes Roger F. Uren, MD, FRACP, DDU,*† Robert Howman-Giles, MD, FRACP, DDU,*† David Chung, MBBS, FRACP,*† and John F. Thompson, MD, FRACS, FACS†‡ Abstract: As the technique of sentinel lymph node (SLN) biopsy has evolved over the last 22 years, it has become increasingly evident that accurate SLN imaging is vital to allow surgical removal of only the true SLN (s) and not other nodes. Identifying the lymphatic collectors draining a tumor site and following them to the draining SLNs defines which nodes need to be removed for careful histologic examination. Current technology allows the exact location of each SLN to be defined. This allows the full benefits of SLN biopsy to be achieved, that is, highly accurate lymph node staging with minimal morbidity. In melanoma and breast cancer, the current practice of preoperative lymphoscintigraphy (LS) using peritumoral injections of tracer or injection adjacent to an excision biopsy site with dynamic imaging to visualize the lymphatic collectors and delayed imaging including single-photon emission computed tomography/computed tomography gives the best results. This information informs the surgical approach and allows rapid excision of the SLNs at surgery. In patients with visceral tumors where the primary cancer site is difficult to access, it appears that using fluorophores that are fluorescent under nearinfrared light, injected during surgery, is evolving as the preferred technique. Key Words: sentinel lymph node, fluorophores, melanoma, breast cancer

The Principles of SLN Imaging The 2 principles underlying the SLN concept can be restated as the requisites for SLN imaging. The first principle is that in every patient there is a specific pattern of lymphatic drainage from a primary tumor to the SLN or SLNs. Therefore, the imaging should trace all the lymphatic vessels from the tumor site to each SLN. The second principle is that the SLN to which the draining lymphatic collecting vessel drains acts to trap the metastasizing tumor cells. Therefore, the imaging should be able to identify this SLN separately from subsequent second- or third-tier nodes. This localizing information should also be transferrable to the surgeon to guide the SLN biopsy procedure in the operating room. The major advantage of SLN biopsy is its unprecedented accuracy in staging lymph nodes with minimal morbidity. This can only be achieved if surgery removes only the true SLN (or SLNs), leaving all non-SLNs in situ. Imaging SLNs is vital in achieving this goal.

METHODS USED TO IMAGE THE SLN Lymphoscintigraphy With Radiocolloids

(Cancer J 2015;21: 25–32)

A

cquiring an image of the sentinel lymph node (SLN) is important in current surgical oncology practice because it has become a key element of the SLN biopsy technique. First described by Donald Morton and colleagues in patients with melanoma,1 the initial study used blue dye only, injected at the melanoma excision biopsy site, and the SLN was identified visually in the regional node field by following a blue-stained lymphatic vessel until it reached a lymph node. Sentinel lymph node biopsy has revolutionized cancer staging by allowing targeted histologic examination of the actual node receiving direct lymphatic drainage from the tumor. An SLN is best defined as “any lymph node receiving direct lymphatic drainage from a primary tumor site.”2 Most patients have an orderly spread of metastasis first to the draining lymph nodes, and if the SLN is normal, all the regional nodes can be assumed to be normal, and no further nodal surgery is required. Basic principles of the tracer technique require that the tracer be injected at the site of the tumor to accurately reflect the lymphatic drainage of the tumor. The SLN biopsy procedure has now also been applied in clinical practice to patients with many different solid cancers that may involve regional lymph nodes, as discussed in preceding chapters.

From *Alfred Nuclear Medicine and Ultrasound, Newtown; †Sydney Medical School, The University of Sydney, Sydney; and ‡Melanoma Institute Australia, North Sydney, New South Wales, Australia. The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Reprints: Roger F. Uren, MD, FRACP, DDU, Suite 206, RPAH Medical Center, 100 Carillon Ave, Newtown, 2042, New South Wales, Australia. E-mail: [email protected]. Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. ISSN: 1528-9117

After initial studies using the blue dye technique had demonstrated that blue-stained afferent lymphatic collectors drained directly into SLNs (Fig. 1), the first images of the sentinel node were obtained using radiocolloids in patients with melanoma. Radiocolloids used for LS are trapped in the SLN by the phagocytic activity of the macrophages and tissue histiocytes that line the subcapsular sinus of lymph nodes. Lymphoscintigraphy using radiocolloids has the advantage of high contrast, which means that it has high sensitivity for detecting all lymphatic collectors and nodes, including those deep within body cavities because the only radiotracer in the patient away from the injection site is located within the lymphatic system. Another advantage is its wide field of view, which allows rapid imaging of lymphatic drainage to widely separated node fields and to unexpected sites. When the tracer is injected intradermally at the primary melanoma site, the lymphatic collectors should be seen on dynamic imaging. Visualization of such collectors is enhanced when small particle radiocolloids are used. The displayed lymphatic collecting vessels can then be followed to the lymph nodes that directly receive such collectors(Figs. 1 and 2). These nodes are by definition SLNs. The SLNs may be in a single node field (Fig. 3) or 2 or more(Fig. 4), and sometimes the sentinel nodes are interval nodes lying outside standard node fields (Fig. 5). Sometimes the only node to be radiolabeled by the tracer is the SLN, and only this node will be seen on delayed imaging at 1 to 2 hours after injection of tracer (Fig. 3), but often second-tier nodes are also seen. These should not be removed as part of an SLN biopsy procedure. The actual radiocolloid used in clinical practice has usually been determined by what local regulators have approved, but the most commonly used are in the United States filtered 99m Tc sulfur colloid (

Imaging sentinel lymph nodes.

As the technique of sentinel lymph node (SLN) biopsy has evolved over the last 22 years, it has become increasingly evident that accurate SLN imaging ...
12MB Sizes 0 Downloads 17 Views