A 15-year-old female spayed domestic shorthair cat was presented for evaluation of a 7 day history of hyporexia and inappropriate elimination outside of the litter box. No straining to defecate or urinate was reported. Historically, the cats had cystoliths successfully managed with dietary modification and, at the time of presentation, was being treated with itraconazole for dermatophytosis. On physical examination, the cat showed signs of discomfort upon abdominal palpation. The bladder was approximately 4 cm × 3 cm and the cat leaked urine when lifted. A focal region of erythema lateral to the right anal gland was identified. There was no palpable mass, and a rectal examination was not performed. On neurological examination, the cat was ambulatory with mild paraparesis and a crouched pelvic limb posture. Proprioceptive positioning was normal in the thoracic limbs and absent in the pelvic limbs. There was reduced tone in both pelvic limbs with bilateral reduction of the patellar reflex and pelvic limb withdrawal reflex. Anal tone was absent, the perineal reflex was reduced and the tail was flaccid with no voluntary movement. There was marked hyperesthesia on lumbosacral palpation. These neurological deficits were consistent with a L4–Cd myelopathy. Based on these findings, the following differential diagnoses were considered: neoplasia (eg, lymphoma, meningioma, metastatic disease); trauma (eg, intervertebral disc disease); infectious/inflammatory disease (eg, viral [feline infectious peritonitis, feline leukemia virus], protozoal [toxoplasmosis] or immune-mediated); an ischemic myelopathy (secondary to occult heart disease or systemic hypertension) or multifactorial. The following diagnostic tests were performed for systemic evaluation: venous blood gas analysis, complete blood count, chemistry panel, total thyroxine, urinalysis with cytospin cytology, urine culture and an abdominal ultrasound. Significant blood work findings included an elevated total calcium (13.6 mg/dl), normal ionized calcium and mild elevations in phosphorus, magnesium, sodium, albumin and aspartate transaminase (AST). Urinalysis and cytospin cytology revealed increased epithelial cells but no overt evidence of infection or neoplasia. Abdominal ultrasound showed several hyperechoic splenic nodules, a moderately distended urinary bladder, an enlarged pancreas with heterogeneous cyst-like nodules and regional lymphadenopathy as follows: hypogastric (6.9 mm), medial iliac (left: 3.9 mm; right: 5.4 mm) and ileocolic (2.8 mm) (). Lymphadenopathy was ultrasonographically defined as changes in either lymph node diameter or echotexture. The diagnostic findings, clinical progression and differential diagnoses were reviewed with the owner, and radiographs of the lumbosacral spine, pelvis and urinary system were recommended. Fine-needle aspirates to further characterize the ultrasonographic changes to the spleen and pancreas were discussed as ancillary diagnostic tests. Given the cat’s clinical deterioration and concerns for systemic disease, the owners elected humane euthanasia with necropsy. On gross necropsy, the right anal sac was effaced and expanded by a 1.5 cm diameter firm, white, multilobulated mass. Both medial iliac lymph nodes were diffusely enlarged, firm and tan-colored, with loss of corticomedullary distinction on cut surface. No masses were found in other organs, and the remaining lymph nodes were unremarkable on gross examination. Dissection of the pelvis and sectioning of the spinal canal revealed no grossly evident lesions. Histopathological examination of hematoxylin and eosin-stained slides of the right anal sac mass revealed an infiltrative, unencapsulated, multilobulated neoplasm of polygonal cells arranged in cords and islands with rare tubule formation (). Neoplastic cells had a moderate amount of eosinophilic cytoplasm and a single round-to-ovoid nucleus with stippled chromatin. Anisocytosis and anisokaryosis were moderate to marked. Mitoses were frequent and occasionally bizarre (). A similarly appearing population of neoplastic cells effaced approximately 60% of each medial iliac lymph node. Similar pleomorphic cells infiltrated into the soft tissues and between the epineurium and perineurium of a nerve adjacent to the right medial iliac lymph node (). These cells also surrounded and compressed individual nerve bundles and nerve roots located within the S3–Cd4 vertebrae (). Affected nerve fibers frequently had dilated myelin sheaths. Standard immunohistochemistry (IHC) for epithelial (wide-spectrum cytokeratin [WSCK], Z0622, Rb polyclonal antibody 1:1000 [Agilent; Dako]), T-cell (CD3ε, MCA1477T, Rt monoclonal antibody [mAb] 1:600 [Bio-Rad]) and B-cell (CD79a, CST, 96024, Rb mAb 1:300) markers was performed on the anal sac mass, metastatic lymph nodes and perineural tissues to further characterize the neoplastic cells according to the IHC protocol described by Painter et al. All neoplastic cells were positive for WSCK and negative for CD3 and CD79a, confirming the diagnosis of multifocal metastatic carcinoma of the anal sac ().