JPC SYSTEMIC PATHOLOGY
SIGNALMENT (UNAM case 2): 10-year-old male rottweiler, canine (Canis familiaris)
HISTORY: This dog presented with a 3-month history of painless swelling of the left pelvic limb that was treated empirically with corticosteroids but showed no improvement. The swelling involved the thigh, stifle and crural region, but it was not associated with lameness and there was no history of trauma.
HISTOPATHOLOGIC DESCRIPTION: Joint capsule and attached muscle: Arising from and focally traversing the joint capsule, there is an unencapsulated, paucicellular, multilobulated, well-demarcated expansile neoplasm. The neoplasm is composed of poorly defined bundles separated by an abundant myxomatous matrix which gives a botyroid appearance; individual nodules are bounded by a condensation of neoplastic cells and thin bands. Neoplastic cells are spindled and elongate with well-defined cell borders and a small amount of vacuolated eosinophilic cytoplasm. Nuclei are irregularly round, occasionally compressed to the periphery, and have finely stippled chromatin with 1 small nucleolus. Mitotic figures are rare. Neoplastic nodules are also present within the associated lymphoid tissue (TLS – tertiary lymphoid structure). The mass is partially lined by a single attenuated layer of synovium or in some areas, multiple layers of plump cuboidal synoviocytes. The joint space contains papillary invagination of synovium which often contains cytoplasmic hemosiderin, and small numbers of hemosiderin laden macrophages infiltrate the periarticular tissue. There are moderate to large numbers of hemosiderin laden macrophages within the adjacent tertiary lymphoid structure. Compressed skeletal muscle adjacent to the neoplasm exhibits mild atrophy.
MORPHOLOGIC DIAGNOSIS: Joint: Synovial myxoma/myxosarcoma, rottweiler, canine
· The 3 most common tumors of the canine synovium are histiocytic sarcoma, synovial cell sarcoma, and synovial myxoma
· It is possible that synovial myxomas arise from the recently described transitional or stem cell type C synoviocyte
· Myxosarcomas are rare malignant tumors of fibroblastic or multipotent mesenchymal cell origin that produce an abundant myxoid matrix of mucopolysaccharides (glycosaminoglycans)
· Occur mostly in middle aged or older dogs and cats, and have previously been reported to affect the skin (mostly trunk and limbs), heart, brain, eyes, vertebral column, and jaw
· The main variants in dogs are benign myxoma, myxosarcoma and myxoid liposarcoma
TYPICAL GROSS FINDINGS:
· Gelatinous nodules that often fill the joint cavity and exude viscous fluid on cut section.
· May show bone lysis or grossly invasive growth.
TYPICAL LIGHT MICROSCOPIC FINDINGS:
· Long spindle to stellate-shaped cells with homogeneously basophilic cytoplasm and oval to elongate nuclei
· Large amount of hyaluronic acid within the mucopolysaccharide matrix > Alcian blue positive
· May recur and rarely metastasize
· Positive for vimentin, heat shock protein 25, and cadherin 11
· Negative for cytokeratin and S100 protein
· Variably positive for CD18
· Cadherin 11 and HSP25 cannot be used to differentiate synovial myxomas from other myxomatous tumors or other synovial tumors
· Synovial sarcoma with myxoid change
· Embryonal rhabdomyosarcoma with myxoid stroma
· Humans: canine synovial myxoma shares some features with the juxta-articular myxoma, a poorly circumscribed myxoid mass that most commonly affects the knee
· Dogs: Labrador Retrievers are overrepresented, Doberman Pinschers and Basset Hounds may be predisposed as well
· Chickens: Myxosarcomas associated with avian leukosis virus subgroup A
· Intra-abdominal serosal myxosarcomas described in two koalas
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