Musculoskeletal System
April 2019
M-N07 (NP)

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 with attached muscle and tertiary lymphoid structure:  Arising from the joint capsule, elevating the predominantly intact synovial lining, forming multifocal nodular extensions into the joint space and the adjacent tertiary lymphoid structure, and compressing the multifocally mildly atrophic skeletal muscle is an unencapsulated, paucicellular, multilobulated, well-demarcated, expansile neoplasm composed of poorly defined bundles of spindle to stellate cells on an abundant myxomatous matrix.  Thin bands and bundles of neoplastic cells surround lakes of paucicellular myxomatous matrix.   Neoplastic cells have distinct cell borders, a small amount of occasionally vacuolated eosinophilic cytoplasm, and irregularly round, occasionally compressed and peripheralized nuclei with finely stippled chromatin and one variably prominent nucleolus. Mitotic figures average 0-1 per 10 40x high power field (2.37mm^2).  The synovial lining cells are often either attenuated layer of synovium or occasionally are hyperplastic and hypertrophic, forming multiple layers of plump cuboidal synoviocytes.  Synoviocytes and occasionally subjacent macrophages contain intracytoplasmic hemosiderin.  There are moderate to high numbers of hemosiderin laden macrophages within the adjacent tertiary lymphoid structure.

MORPHOLOGIC DIAGNOSIS:  Joint: Synovial myxoma, Rottweiler, canine






Synovial tumors in the dog:

Non-neoplastic synovial lesions:


Synovial myxomas in other species:

Non-synovial myxomas/myxosarcomas:


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