Lymph Nodes – When, Why and How To Remove Them

Henry L’Eplattenier, Dr.med.vet., PhD, Dipl. ECVS, MRCVS
EBVS^® Specialist in Small Animal Surgery
RCVS recognised Specialist in Small Animal Surgery
Consultant surgeon and Clinical Director, Southfields Veterinary Specialists, Laindon, Essex

Introduction

Haematogenous and lymphatic dissemination are the two major routes of metastatic spread in malignant neoplasia. The majority of sarcomas spread by invading blood vessels, whereas carcinomas and round cell tumour primarily spread through the lymphatic system, initially metastasising to the regional lymph nodes. Metastatic disease is one of the major causes of death for certain types of neoplasia, therefore tumour staging (determining the extent of the spread) is very important to give an accurate prognosis and guide the clinician’s decisions with regards to treatment.

For accurate tumour staging, palpation and sampling of the regional LNs by fine needle aspiration (FNA) has traditionally been recommended in veterinary medicine. However, in many cases, the LN cannot be assessed either by palpation or by ultrasound, which means that a diagnosis of metastatic disease can be missed in a large number of cases. Palpation of the LN alone has a low sensitivity (60%) and specificity (72%) (Langenbach et al. J Am Vet Med Assoc. 2001) Cytological examination of fine needle aspirates of lymph nodes was found to have a sensitivity of 67% and specificity of 91% (Ku et al. Vet Comp Oncology. 2017)

Traditionally, removal of regional lymph nodes was indicated if they were enlarged on palpation or shown to be positive for metastasis on cytological examination. The purpose is to more accurately stage the tumour and to reduce tumour burden for dogs undergoing adjuvant chemotherapy. A recent study has shown the mast cell tumour (MCT) metastasis can be missed if normal sized, cytologically negative lymph nodes are not removed at the time of the excision of the primary tumour (Marconato et al. Vet Comp Oncology. 2018). The removal of metastatic lymph nodes in cutaneous MCTs has been shown to have a beneficial effect on tumour progression and tumour-related death, compared to dogs treated with tumour removal and chemotherapy alone. This is also the case for anal sac adenocarcinoma and removal of enlarged medial iliac and/or hypogastric lymph nodes is recommended. Another indication for removal of lymph nodes is endocrine production, for example in the case of insulinoma, since failure to remove metastatic lymph nodes can cause persistent hypoglycaemia, and in the case of hypocalcaemia associated with anal sac adenocarcinoma.

SENTINEL LYMPH NODES

More recently, veterinary oncologists have developed techniques for sentinel lymph node (SLN) mapping. The SLN is defined as the first LN within the lymphatic basin that drains the primary tumour. The SLN may be the regional LN anticipated to be responsible for tumour drainage, but LNs at unexpected anatomical locations can sometimes also function as the SLN. In human medicine, sampling SLN offers a less invasive approach than radical regional lymphadenectomy, minimises anaesthesia time and reduces tissue trauma and postoperative morbidity. It has therefore been successfully established in human oncology as an integral part of treatment decision-making and prognosis of certain malignancies.

There are multiple ways of mapping the sentinel lymph node. In human medicine, the most common technique used is scintigraphy (lymphoscintigraphy) involving the injection of a radioactive tracer (usually Technetium 99) around the tumour and detection of radioactivity in the lymph node using a gamma camera. Lack of availability of scintigraphy in veterinary medicine makes this technique difficult to use in most veterinary clinics.

Aqueous radiographic contrast can be injected into the tissues surrounding the tumour and is rapidly transported via the lymph vessels to the sentinel lymph node. It is not recommended to inject the contrast directly into the tumour for fear of seeding tumour cells. Radiographs or a CT scan of the regional lymph nodes can be performed within minutes of the injection, revealing concentration of contrast in the sentinel lymph nodes. This technique has been used successfully for detection of SLN in tumour of the head, mammary tumours and anal sac tumours.

Although not yet in clinical use in veterinary medicine, contrast-enhanced MRI and ultrasound are possible ways of determining which lymph node is the SLN.

In order to find the SLN for surgical excision, the surgeon can either be guided by the pre-operative images (e.g. CT scan), or a dye can be injected in the peritumoral tissue, which will then become visible in the SLN. Traditionally, Methylene Blue is used for this purpose. Problems associated with Methylene Blue use is the difficulty of obtaining sterile dye for injection, and in addition, Methylene Blue should be avoided in cats as it can cause Heinz body formation due to the oxidation of haemoglobin.

Recently, the use of fluorescent dyes visible with near infra-red (NIR) light has become a growing field of research in veterinary medicine, and such dyes may make the use of Methylene Blue redundant in the future.

LYMPH NODE BIOPSY

Lymph nodes are mostly removed entirely if possible, rather than biopsied. There are however situations, where histology of the lymph nodes is desired but removal of an entire node is too risky or not beneficial. Such examples include mesenteric lymph nodes, which can be very closely associated with the mesenteric blood vessels such that dissection of the entire node could cause necrosis of large sections of intestine, if the mesenteric blood vessels are damaged during dissection. A biopsy of the lymph node can be obtained by incising a wedge of tissue using a scalpel blade. The capsule of the lymph node can be sutured with 2-3 simple interrupted sutures with 4-0 resorbable suture material.

LYMPH NODE REMOVAL

Most superficial lymph nodes can be readily palpated and excision is straightforward. After incising the skin over the lymph nodes, the subcutis is bluntly dissected until the surface of the lymph node can be seen. The node is usually recognised by its slightly grey-ish or brown-ish colour. The node can then be grasped with an instrument and blunt dissection is continues all around it. In order to avoid haemorrhage from the blood vessels of the lymph node, dissection using an electrocautery device is preferred.

For lymph nodes located deeper and not readily palpable such as retropharyngeal or sublumbar lymph nodes, the approach is guided by advanced imaging (CT or MRI).

Current research in human and canine patients has shown that removal of both the ipsilateral and contralateral lymph nodes is necessary, especially in tumours of the head and neck. Contralateral and bilateral lymph nodes metastases have been also described in dogs (Skinner OT et al., Vet Comp Oncology, 2016). Therefore for tumours of the head in dogs, diagnostic removal of lymph nodes should ideally be performed on both side in order to avoid missing any metastases. Both the submandibular and retropharyngeal lymph nodes can be removed bilaterally through a single skin incision in the ventral midline (Green K and Boston SE, Vet Comp Oncology, 2015). The patient is positioned in dorsal recumbency. The head is positioned to provide exposure to the ventral neck, the area of the ventral mandible to the thoracic inlet and laterally to the level of the external jugular vein, and then surgically prepared in sterile fashion. An incision is made through the skin and platysma muscle on the ventral midline from the caudal third of the mandible to the level of the larynx. The mandibular lymph node is palpated at the caudal extent of the horizontal ramus and isolated using blunt dissection. Care is taken to avoid the facial vein, which passes over the ventrolateral aspect of the mandibular lymph node. Electrocautery and blunt dissection are used to excise the mandibular lymph node once it is isolated. The mandibular

salivary gland is then located and retracted laterally to expose the medial retropharyngeal lymph node. Blunt dissection on the medial aspect of the mandibular salivary gland will allow for palpation of the medial retropharyngeal lymph node, which is at the caudomedial aspect of the mandibular salivary gland. It will be possible to palpate the lymph node before it is visible. Exploration should be limited to palpation and blunt dissection until the medial retropharyngeal lymph node is definitively located to avoid damage to adjacent structures.

Electrocautery and blunt dissection are used to excise the lymph node once it has been isolated. This procedure is repeated on the opposite side to complete removal of both mandibular and both medial retropharyngeal lymph nodes