A 7-year-old, neutered male, canine Mastiff mix presented to the Oncology Service at a specialty hospital for evaluation of a submandibular mass, difficulty swallowing, and lethargy. Physical examination revealed an approximately 4 cm diameter, firm, deep, and adherent mass in the area of the left mandibular salivary gland and a soft, small, left mandibular lymph node. Serum chemistry and complete blood count were unremarkable. Thoracic radiographs and abdominal ultrasound did not reveal evidence of metastasis. Computed tomography confirmed a soft tissue mass originating from the left mandibular salivary gland, measuring 4.9 × 2.8 × 3.3 cm. The mass contained a 3.1 cm tubular extension likely within the sublingual salivary duct, suggesting involvement of the left sublingual salivary gland. In addition, mild-to-moderate left medial retropharyngeal lymphadenomegaly was noted. Fine-needle aspirate biopsy of the left medial retropharyngeal lymph node was performed and submitted for cytologic examination. (Figure 1). The sample was highly cellular, consisting of many neoplastic round cells on a pink background with scattered red blood cells. Round cells were predominantly individualized and occasionally arranged in dense aggregates. Cells were plasmacytoid and contained fairly distinct cell borders. Cells displayed a low to moderate nuclear to cytoplasmic ratio with an eccentric, round nucleus with stippled chromatin and a single indistinct nucleolus. Cells contained a moderate to abundant amount of basophilic cytoplasm that occasionally exhibited cytoplasmic blebbing and a faint eosinophilic margin. Cells displayed moderate anisocytosis and anisokaryosis, and occasional binucleation and multinucleation. Occasional small lymphocytes were present. The neoplastic cells displaying plasmacytoid features were suggestive of extramedullary plasma cell tumor metastasis given the presence of a submandibular mass. However, metastasis of a medullary thyroid carcinoma with round or plasma cell appearance could not be entirely excluded. Biopsy with histopathology was recommended for confirmation. Left-sided mandibular and sublingual sialoadenectomy and surgical excision of the left, medial retropharyngeal lymph node were pursued. Surgical dissection revealed an encapsulated mass containing the entire left mandibular salivary gland. No margins were reported. Excisional biopsy of the left, medial retropharyngeal lymph node and submandibular mass were submitted for histologic examination. (Figures 2 and 3). Histologic evaluation of the retropharyngeal lymph node revealed a focal area with replacement of normal lymph node by sheets of round cells with a moderate amount of cytoplasm. Nuclei were sometimes eccentrically located with a perinuclear clear zone. Nuclei were of intermediate cell size, and mitoses were up to 6 per high-powered field. Cells were contained within the lymph node capsule. The findings were consistent with a malignant plasma cell tumor (Figure 2). Histologic evaluation of the submandibular mass revealed a discrete nodule in salivary tissue that completely effaced the adjacent lymph node. The lesion consisted of sheets of round cells with a moderate amount of cytoplasm. The nuclei were frequently eccentrically located with a perinuclear clear zone. The nuclei were 1.5 times the size of a red blood cell, mildly pleomorphic, and had small, indistinct nucleoli. Mitoses were up to 6 per high-powered field (Figure 2). Immunohistochemistry of the submandibular mass was strongly positive for MUM1 in the nuclei of the neoplastic cells (Figure 3). Based on these findings, an epithelial origin was excluded, and the final diagnosis was malignant plasma cell tumor. Extramedullary plasmacytomas (EMPs) are solitary plasma cell neoplasms that occur outside the bone marrow. EMPs have been reported in humans, dogs, cats, hamsters, ferrets, horses, and sheep 1. Canine EMPs are not common, consisting of approximately 2.4% of all canine neoplasms 2. Breed predispositions include the American Cocker Spaniel, English Cocker Spaniel, and West Highland White Terrier. The skin (86%), mucous membranes, specifically the oral cavity and lips (9%), and gastrointestinal tract (4%) are common locations 3. However, canine EMPs have been reported in the trachea, eye, eyelid, larynx, liver, spleen, uterus, brain, and kidney 2-5. The metastatic potential of EMP appears to be location dependent. With the exception of cutaneous plasmacytosis, canine cutaneous and oral EMP tend to be benign, carrying an excellent prognosis with surgical removal 3, 5. However, metastasis is possible. For example, a recent case report discusses a canine oral EMP metastasizing to both kidneys 5. Case reports of EMP of the trachea, liver, and uterus demonstrate benign behavior. EMPs found in the gastrointestinal tract are more likely to metastasize to local lymph nodes; however, long-term survival is common with surgical excision with or without chemotherapy 3. There is a single case report of EMP of the parotid salivary gland with metastasis to the medial retropharyngeal lymph node in a 12-year-old dog, confirmed with MUM1. This patient was euthanized 685 days after surgical excision of the left parotid salivary gland and left medial retropharyngeal lymph node due to hindlimb paresis of undetermined cause 2. In the current case, as of 3 months post-surgery, there was no evidence of recurrence. To the authors' best knowledge, the present case is the second example of a canine salivary gland EMP with good response to treatment despite metastasis to local lymph nodes. These two case reports suggest that salivary EMPs behave similarly to those of the gastrointestinal tract in that they are more likely to metastasize locally but may carry a good prognosis with treatment. However, more research is needed to assess the metastatic potential of canine salivary gland EMP. W.K.C. is part of a training program at North Carolina State University sponsored by Antech Diagnostics, Mars Petcare Science & Diagnostics. The authors declare no conflicts of interest.
Chandler et al. (Fri,) studied this question.