Introduction

The number of lymphocenters in dogs is relatively small with small groups and number of nodes in each group.¹ Shapes vary with some nodes being oval or round, bean‐shaped, bilobed, elongated, etc. Most nodes have between 1 and 3 afferent lymphatics with upward of 50 efferents exiting a node that then combine to form three to five larger efferent lymphatics.¹ These efferent lymphatics then drain into either venous vasculature or directly into the thoracic duct. Lymphatic vessels transport lymph from organs and tissues, lipids from the intestines and liver, and fluid from local sites back into circulation.² The lymphatic system also functions to mount an immune response when it detects foreign materials and infectious agents from the skin, respiratory, and gastrointestinal systems. These foreign or infectious agents are presented to the nodes via afferent lymphatics,² which divide into branches within the cortex, then drain into the medulla of the node from where the efferent lymphatics exit the node. The nodal cortex is primarily comprised of B‐lymphocytes arranged into follicles; the medulla consists of lymphocytes, macrophages, and plasma cells; and the paracortex between the cortex and medulla consists of T‐lymphocytes.^2,3 The three regions work in tandem to present antigens and elicit a primary immune response.^2,3

Nodal involvement is an integral part of staging a cancer patient. In addition to distant metastasis, locoregional nodal involvement is part of the World Health Organization TNM system.⁴ The primary tumor extent is T, nodal (lymph node) extent is N, and M represents distant metastasis. Stage of the tumor affects prognosis and the treatment plan for the patient. Knowledge of cancer behavior, as well as knowledge of draining lymph nodes relative to site of the primary tumor, sheds light on whether nodal involvement is possible and how this affects prognosis, treatment plan, and patient survival. For example, mast cell tumors, oral melanoma, and anal sac adenocarcinomas are a few commonly encountered tumors that metastasize to regional lymph nodes. Nodal staging is accomplished by palpation, imaging, sentinel lymph node mapping, cytology, and histopathology.¹ Palpation alone has a sensitivity of 60% and specificity of 72% in predicting metastasis; thus, it should be used in conjunction with other modalities.⁵ Cytology has a 66.6–100% sensitivity, 91.5–96% specificity, and 77.2% accuracy.^5,6 The correlation between cytology and histopathology is 90.9%.⁷

Cancer can alter lymphatic drainage, so lymph nodes other than the nearest draining regional lymph node may be affected. In fact, the sentinel lymph node differed from the regional lymph node in 52% of dogs in a recent study.⁸ Identification of the at‐risk lymph node can be accomplished using contrast lymphography followed by imaging, such as radiography or CT, lymphoscintigraphy using a radioactive tracer, or perioperative injection of a dye. This at‐risk lymph node is the sentinel node, and identification of these nodes is called sentinel lymph node mapping. The sentinel node identification in and of itself does not mean the node is metastatic; it simply means that if cancer is going to spread via the lymphatic system, it would spread first to the sentinel lymph node. Extirpation of the lymph node with histopathology identifies whether a node is metastatic, and metastasis in the sentinel node is reported in 42–45% of the lymph nodes.^8,9

Pre‐operative Considerations/Indications

Lymphadenitis, inflammation, and subsequent enlargement of lymph nodes, most commonly due to infection, are indications for removal (i.e., lymphadenectomy or lymph node extirpation). Presurgical culture and antibiotics are indicated if bacterial infection has led to marked cellulitis around the surgery site and to decrease the odds of an incisional infection. A macerated tissue culture of nodal tissue at the time of surgery is also indicated to ensure that the antibiotics used have treated the infection. Another common reason for marked nodal enlargement is malignancy that may be identified on cytology. Removal of lymph nodes that are metastatic may be the part of treatment with improvement in prognosis for some cancers, such as canine mast cell tumors. Removal of regional lymph nodes is also indicated for highly malignant tumors like melanoma, after which the node can be thoroughly evaluated for malignancy to aid in staging, prognosis, and guidance for adjuvant therapy recommendations. Sentinel lymph node mapping as discussed above to help identify at‐risk lymph nodes is becoming increasingly popular in veterinary medicine.

Commonly extirpated peripheral lymph nodes are mandibular, medial retropharyngeal, prescapular (i.e., superficial cervical), popliteal, and superficial inguinal lymph nodes. The axillary lymph node is also accessible for extirpation; however, thorough knowledge of anatomy in and around the brachial plexus is needed before undertaking removal.

Mandibular lymph nodes are readily palpable and are easy to approach surgically. Landmarks include the bifurcation of the external jugular vein into the linguofacial vein and external maxillary vein, but this palpable structure is not to be confused with the normally larger but similarly located mandibular salivary gland. The linguofacial vein divides the mandibular nodes into dorsal and ventral groups. Each group consists of one to two nodes, thus making a total of two to four nodes in the mandibular lymph node basin in most companion animals.¹ These nodes receive afferent drainage from the nose, lips, cheek, intermandibular region, zygomatic arch, gums, hard and soft palate, eyelids, various bones of the head, and muscles of the head and neck.¹

The medial retropharyngeal lymph nodes serve as the lymphatic collection center of the head and receives efferent drainage from the lateral retropharyngeal, parotid, and mandibular lymph nodes. This lymph node tends to yield the most diagnostic information for potential metastatic spread of oral cancers.¹⁰ This node is located caudomedial to the mandibular salivary gland, caudal to the digastricus muscle, ventral to the wing of the atlas and the longus colli muscle, medial to the mastoid aspects of the brachiocephalicus and sternocephalicus muscles, and dorsal to the thyroid cartilage of the larynx, and these structures serve as landmarks for locating this lymph node. Afferent drainage is from similar regions as the mandibular lymph nodes, in addition to drainage from the tonsils, pharynx, esophagus, thyroid, larynx, trachea, and ears.¹ There is sometimes but inconsistently a lateral retropharyngeal lymph node present.

Superficial cervical nodes are relatively deep lymph nodes in the neck located at the junction of the neck and shoulder. Most dogs have one to two nodes on each side. They are sandwiched between the supraspinatus muscles and the brachiocephalicus and omotransversarius muscles superficially, which serve as landmarks for localization in surgery. The bulk of the afferent drainage is received from the skin of the caudal part of the dorsal head region, pinna, parotid and neck areas, caudal half of the cranial neck region, forelimb digits, metacarpus, carpus and forearm, most of the lateral side of the upper shoulder, upper foreleg region, medial side of the humeral region, thoracic inlet, and cranial part of the ventral thorax.¹

The axillary lymph node lies in the brachial plexus region at the level of the shoulder joint. There is usually only one axillary node in dogs, and this lymph node receives afferent drainage from nearly all bones of the forelimb and the cranial mammary glands. It also receives afferent drainage from the skin lying between the shoulder and the last rib on the lateral and ventral thoracic wall, lateral side of the shoulder, lateral brachium, olecranon, and nearly all muscles of the forelimb.¹

Located on the caudal aspect of the stifle joint between the distal end of the biceps femoris and semitendinosus muscles encased in fatty tissue is the popliteal lymph node. Typically, this is only a single node in this location with rare occurrence of two nodes. Most of the afferents are received from the skin on the lateral aspect of the stifle joint and lower leg, metatarsus and digits, tibia, fibula, tarsal and metatarsal bones, phalanges, and muscles distal to the stifle joint.¹

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