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 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
- Primary synovial tumors are uncommon in dogs and very rare in other species
- Synovial tumors of canines:
- Tumors arising from type A synoviocytes: Histiocytic origin, immunoreactive for CD18 (other leukocyte markers, i.e. CD1, CD11c, MHC II, require frozen tissue specimens); may alternatively arise from dendritic cells within the synovial subintima; histiocytic sarcoma is the most common joint tumor in dogs
- Tumors arising from type B synoviocytes: although type B synoviocytes are termed “fibroblastic” synoviocytes, these cells produce the viscous glycosaminoglycan component of joint fluid not collagen; tumors of these potentially produce benign synovial myxomas (not proven); there is currently to immunohistochemical marker to label type B synoviocytes
- “Synovial cell sarcoma” should not be diagnosed according to one primary reference; neoplasms previously diagnosed as “synovial cell sarcomas” are likely typical soft tissue sarcomas such as peripheral nerve sheath tumors, perivascular wall tumors, and fibrosarcomas
- Synovial myxoma is the second most common joint tumor of dogs, and may arise from type B synoviocytes; previously often diagnosed as myxosarcoma or nodular synovial hyperplasia
TYPICAL CLINICAL FINDINGS:
- Large-breed, middle-aged dogs are most commonly affected
- Breed predilection: Doberman pinscher, Labrador retriever
- Slow-growing tumors with long (months to years) history of clinical signs preceding diagnosis
- Prognosis is good with surgical excision even if neoplastic cells extend to surgical margins of amputation sites; metastasis has not been reported, although the neoplasms can be locally invasive and therefore are considered by some to be a low-grade malignancy
TYPICAL GROSS FINDINGS:
- The neoplasm affects a single joint; stifle and digit > tarsus, elbow, carpus, vertebral facets
- Soft, white, translucent, gelatinous nodules and pockets of viscous fluid that often line the entire inner joint capsule and can fill the joint cavity; exudes abundant clear viscous fluid on cut section
- May show bone lysis on both sides of the affected joint
- May have grossly invasive growth along fascial planes outside of the joint capsule into adjacent bones and muscle
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Characteristic paucicellular round nodules composed of long spindle to stellate-shaped cells with small hyperchromatic nuclei and rare mitoses suspended in abundant poorly vascular myxoid matrix
- Large amount of Alcian blue positive mucopolysaccharide matrix (due to hyaluronic acid)
Synovial tumors in the dog:
- Histiocytic sarcoma: The most common joint tumor of dogs; most commonly affects the stifle joint; multilobulated, infiltrative, less distinctly nodular than synovial myxoma; cells often have vacuolated cytoplasm and are round to polygonal to spindloid, often admixed with inflammation; CD18 immunoreactive; breed predilection in Bernese mountain dogs, Rottweilers, Bullmastiffs, Golden retrievers, Labrador retrievers, and Flat coated retrievers especially with a pre-existing joint issue (e.g. ruptured cranial cruciate ligament); joint histiocytic sarcoma has a more favorable outcome than histiocytic sarcoma of other locations
- Other sarcomas (fibrosarcoma, hemangiopericytoma, liposarcoma, peripheral nerve sheath tumors, and other soft tissue sarcomas) can arise within joints and can have myxoid variants; similar methods of differentiating soft tissue sarcomas should be used
- Rhabdomyosarcoma including embryonal rhabdomyosarcoma with myxoid stroma
- Multicentric lymphosarcoma
- Chondrosarcoma: arising from cartilaginous metaplasia or rests of cartilage embedded within the synovium rather than mitotically inactive articular cartilage
- Periarticular fibroma: Uncommon, discrete firm nodular white mass attached to the lateral or medial joint capsule or tendon sheath, most often affects the carpal joint, may represent focal fibrous scar tissue but are typically more spherical; some can have dissecting bands of myxomatous stroma
Non-neoplastic synovial lesions:
- Synovial chondromatosis/osteochondromatosis: firmer than synovial myxoma
Synovial myxomas in other species:
- Cats can also be affected by synovial myxoma
- Rabbits (see I-V10): Myxomatosis, caused by the myxoma virus (Poxviridae family, Leporipoxvirus genus), results in exophytic gelatinous skin tumors (myxomas) typically on the face and perineum in wild rabbits and hares with systemic disease in domestic rabbits
- Dog: A report (Choi 2017) of a laryngeal myxosarcoma that histologically contained eosinophilic crystals identified most likely as hemoglobin crystals and not Charcot-Leyden crystals; this report suggests that Charcot-Leyden crystals are human-specific
- Koalas: A report (Gonzalez Astudillo 2015) of two unrelated koalas with multiple intra-abdominal serosal myosarcomas; most koala neoplasia has been associated with compromised immunity, likely secondary to koala retrovirus
- Horses: Myxosarcoma is rare in horses, with metastasis only reported to lymph nodes except for a report (Samuelson 2018) of metastatic myxosarcoma in a Quarter Horse with large retroperitoneal gelatinous mass and similar nodules in the lungs, liver, mesentery, cecum, and caudal mesenteric artery
- Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016: 261-263.
- Choi E, Miller AD, Devenish E, Asakawa M, McConkey M, Peters-Kennedy J. Charcot-Leyden crystals: do they exist in veterinary species? A case report and literature review. Jour Vet Diag Invest. 2017;29(6):904-909.
- Craig LE, Dittmer KE, Thompson KG. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:159-163.
- Delaney MA, Treuting PM, Rothenburger JL. In: Terio KA, McAloose D, St. Leger J. Pathology of Wildlife and Zoo Animals. London, England: Elsevier; 2018:487-489.
- Gonzalez Astudillo V, Schaffer-White A, Allavena R, Palmieri C. Multiple intra-abdominal serosal myxosarcomas in two koalas (Phascolarctos cinereus). J Comp Pathol. 2015; 152(2-3):283-6.
- Olson EJ, Carlson CS. Bones, joints, tendons and ligaments. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 1004-1005.
- Samuelson JP, Echeverria KO, Foreman JH, Frederickson RL, Sauberli D, Whiteley HE. Metastatic myxosarcoma in a Quarter Horse gelding. Jour Vet Diagn Invest. 2018;30(1):121-125.