34th Annual Scientific Meeting proceedings

Lymph node evaluation is important in tumour staging, guiding clinicians in treatment decisions and providing accurate prognostic information. To detect loco-regional nodal metastasis, it is crucial to identify the sentinel lymph node (SLN), which is the first node in the lymphatic basin draining from the primary tumour. While the SLN is often the regional lymph node, nodes at unpredictable anatomical locations may also serve as SLN. The size of lymph nodes, assessed by palpation or advanced diagnostic imaging, does not correlate well with the presence or absence of tumour metastasis. Cytological examination of cells aspirated from a lymph node offers good sensitivity and specificity for detecting neoplastic cell infiltration in some tumour types, but the sample may not be fully representative of the whole lymph node due to the small volume of aspirated material (<1% of the cells within a lymph node) and the inability to evaluate tissue structure and organization. Histological examination remains the most accurate method, especially for detecting micro-metastasis. Excising non-palpable or normal-sized regional lymph nodes can be challenging, potentially increasing surgery time and patient morbidity, leading to extensive dissection and sometimes unsuccessful lymph node localization. Furthermore, not all peripheral lymph nodes are easily accessible due to their anatomical location, size, or patient’s conformation. SLN mapping has been established as a fundamental part of treatment decision-making and prognosis in human oncology for certain malignancies. Techniques reported in dogs include lymphography (radiographic, CT, MRI), contrast-enhanced ultrasound (CEUS), pre- and intraoperative lymphoscintigraphy and the use of blue dyes or fluorescent dyes for near-infrared imaging (NIR). Other SNL mapping techniques such as lymphoscintigraphy, blue dye, NIR and more recently anchor wires (AW) can aid surgeons intraoperatively during SLN excision.

Colorimetric SLN mapping relies on direct visualisation of the SLN and does not require specialised equipment. Methylene blue, the most commonly used dye, is safe, readily available, and inexpensive. It is injected peritoumorally and within 5 to 10 minutes, a blue coloration stains the ascending lymphatic vessels and their SLN blue, facilitating its localisation during surgery. In people, when injected intravenously, it can cause complications ranging from skin rashes to life-threatening anaphylaxis (0.4% cases), or, when injected intradermally, blue discoloration of the operating field, a decrease in oxygen saturation of haemoglobin, permanent skin staining and intense tissue reaction that may result in skin necrosis. Reported complications in dogs and cats following intravenous administration includes Heinz body anaemia, pseudo cyanosis, increased serum alkaline phosphatase activity, and kidney failure. Although no adverse effects have been reported after peritumoral injection or injection directly into the LN, occasional leakage of dye into the subcutaneous tissue, interfering with the surgeon’s ability to identify the lymph node has been reported.

Near infrared fluorescence for SLN mapping is a rapidly developing, safe, and feasible technique in both human and veterinary oncology. It allows real-time visualisation of lymph vessels and SLN using a NIR camera device, with light from the near-infrared spectrum that penetrate tissue up to a few centimetres, ideal for transcutaneous identification of SLN. This system enables a transcutaneous visualisation of fluorescent dyes, with the opportunity to specifically identify the SLN. No major complications have been reported in canine studies. Disadvantages include low depth penetration and short tracer retention time in the lymphatic system.

An intraoperative technique to detect non-palpable LNs involves the use of AW localization needles, which improves the accuracy of detecting LNs, facilitating resection, reduce surgery time, and limit damage to nearby structures. These needles contain a hooked wire and are placed with ultrasound guidance to help to localize nonpalpable lesions or deeply located lymph nodes. The AWs may be deployed within or near to the lesion, depending on the local anatomy and pathologic condition. Complications such as wire transection and migration during surgery and damage to vital structures near lesions are known risks.

The use of radioactive tracers for SLN mapping is standard in human cancer therapy, particularly for breast cancer and melanoma. This includes preoperative lymphoscintigraphy for surgical planning and intraoperative use of a gamma probe. The typical radiotracers, like technetium-99m labelled colloids, remain at the injection site for extended periods, accumulating in the SLNs during the preoperative scan. Despite high detection rates, the use of these tracers is limited by the need of radionucleotide-capable facilities, trained staff, and safety concerns related to radionucleotide exposure.

Overall, mapping and localisation techniques have proven more successful for regional lymph node localisation (84-98%) compared with unassisted methods (72-74%) significantly reducing the duration of lymphadenectomy and postoperative mortality by enhancing intraoperative guidance towards SLN.

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