JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL

April 2025

M-N03 (NP)

 

Signalment (JPC #2594133): 5-year-old female Chesapeake Bay retriever

 

HISTORY: This dog had an 8-month history of progressively worsening lameness of the right rear leg. Radiographic examination revealed lysis of the right tibial tarsal (tarsus) bone.

 

HISTOPATHOLOGIC DESCRIPTION: Bone, tibial tarsal (per contributor): Filling the marrow cavity, effacing over 75% of normal bone, and infiltrating the surrounding connective tissue is an unencapsulated, well demarcated, infiltrative, moderately cellular neoplasm composed of haphazardly arranged plump neoplastic spindle cells surrounded by irregular islands and thick trabeculae of abundant, variably basophilic, chondroid matrix. Neoplastic cells are individualized or congregated in lacunae within the matrix and have a moderate amount of finely granular eosinophilic to basophilic cytoplasm. Nuclei are oval, have 1-2 magenta nucleoli and finely stippled chromatin. There is one mitotic figure per 2.37 square millimeters. Multifocally there are irregular trabeculae of reactive woven bone and foci of osteolysis characterized by scalloped margins and few multinucleated osteoclasts within Howship's lacunae. Cartilage occasionally undergoes endochondral ossification. Loose, well-vascularized mesenchymal tissue is interspersed between islands of cartilage. There is mild hemorrhage and low numbers of hemosiderophages. 

 

MORPHOLOGIC DIAGNOSIS: Bone, tibial tarsal (per contributor): Chondrosarcoma, Chesapeake Bay retriever, canine.

 

GENERAL DISCUSSION:

neoplastic cells produce variable quantities of cartilaginous or fibrillar matrix, but not osteoid; although bone may be present in chondrosarcomas, it forms by endochondral ossification of tumor cartilage, rather than being produced directly by malignant mesenchymal cells

• Reported most frequently in the dog; accounts for 10% of the primary tumors of the bone and is second to the osteosarcoma in incidence; most commonly affects medium to large breed, middle to older aged dogs, particularly boxers, German shepherds, and golden retrievers; rare in giant and small breeds
• Most commonly affected sites involve the flat bones more often than long bones (ribs, nasal bones, pelvis, and mediastinum)
• Rare tumor in other species 
• Primary chondrosarcomas arise from either within bone (central or medullary) or from the periosteal surface (peripheral); central chondrosarcomas are the most common form in veterinary species 
• Secondary chondrosarcomas develop by malignant transformation of cartilage in an osteochondroma or osteochondromatosis (rare)
• Slower growing and less invasive than osteosarcoma (OSA) but local invasion and recurrence after excision is common; metastasis most commonly to the lungs

 

TYPICAL CLINICAL FINDINGS:

• Sinonasal tumor: Sneezing, epistaxis, nasal swelling
• Rib tumor: Large, nonpainful swelling at costochondral junction
• Pelvic tumor: Tenesmus, constipation, hindlimb lameness
• Long bone tumor: Pain, lameness, pathologic fractures
• Multilobular (“popcorn”) appearance radiographically

 

TYPICAL GROSS FINDINGS:

• Firm or hard, resembling hyaline cartilage
• Multiple small, contiguous nodules of translucent blue, white, or pink tissue 
• Areas of necrosis and hemorrhage
• Some tumors (especially in the nasal cavity) contain soft mucoid areas, chalky white foci, areas of mineralization or ossification

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Lobules of mesenchymal cells with variable amounts of disorganized hyaline cartilage, no osteoid production
• Mitoses are rare: Presence any mitotic figures indicates malignancy 
• Histologic findings typically have to be paired with clinical and radiographic findings to differentiate from osteosarcoma

 

ADDITIONAL DIAGNOSTICS:

• Cytology: Variable amounts of lakes of bright pink-purple chondroid matrix with embedded cells (chondroblasts)
• Individualized chondroblasts can resemble those of osteosarcoma (M-N02)—malignant chondroblasts and osteoblasts cannot be reliably differentiated on cytology alone and require histology; less cellular than OSA, prominent anisokaryosis; large, pleomorphic, round to fusiform cells with basophilic cytoplasm +/- fine pink granules; large hyperchromatic nucleus
• If any tumor is causing bone lysis, osteoclasts may also be found in cytologic specimens. 

 

DIFFERENTIAL DIAGNOSIS:

• Chondroma: Differentiation difficult; lack cellular pleomorphism and lack invasive growth and metastasis
• Osteosarcoma: Osteosarcomas can have cartilaginous dysplasia, and chondrosarcomas with mineralization can produce osteoid and bone; may be difficult but is important to differentiate as this carries a worse prognosis

 

COMPARATIVE PATHOLOGY:

• Dogs: One case report of a clear-cell variant of chondrosarcoma in the nasal cavity of a mixed breed dog (Moloney et. al., J Comp Pathol, 2024); extra skeletal chondrosarcomas can very rarely occur in the tongue (Rahe, J Vet Diagn Invest, 2020), or mammary gland (Yi et. al, J Comp Pathol, 2023)
• Sheep: Chondrosarcomas are the most common bone tumor, affect ribs and sternum 
• Chickens and other avian species, except pelicans: Osteosarcomas more common than chondrosarcomas, and chondrosarcomas are more common than chondromas; pelicans develop chondrosarcomas of the beak. 
• Waterfowl, cranes and storks: Chondrosarcomas (and chondromas) occasionally affect the caudal aspect of feet and hocks.
• Cats: 
  • Primary intraocular chondrosarcomas have been reported (often post-traumatic)
  • 30% of chondrosarcomas arise in subcutis and are frequently associated with vaccination sites
• Rabbit: One report of intraocular sarcoma (Mikiewicz, J Comp Pathol. 2020) in a rabbit with serologic evidence of Encephalitozoon cuniculi infection
• Llama: One report of a sinonasal chondrosarcoma (Cheng et al, J Vet Diagn Invest. 2024)

 

REFERENCES:

1. Barger AM. Chapter 14: Musculoskeletal System. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:507-509.
2. Beckwith-Cohen B, Teixeira LB, Dubielzig RR. Presumed primary intraocular chondrosarcoma in cats. J Vet Diagn Invest. 2014;26(5):664-668).
3. Boes KM. Chapter 5: Respiratory System. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:195-196. 
4. Cheng A, Horzmann K, Yin JH, Titos P, Bayne JE, Neto R. Sinonasal chondrosarcoma in a llama. J Vet Diagn Invest. 2024;36(6):823-826.
5. Fielder SE. The Musculoskeletal System. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:207.
6. Fletcher OJ, Barnes HJ, Abdul-Aziz T. Chapter 3: Skeletal System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016: 78. 
7. Grimes CN, Fry MM, LeBlanc CJ, Hecht S. The Lung and Intrathoracic Structures. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:284.
8. Mikiewicz M, Pazdzior-Czapula K, Gesek M, Dziubinska-Bartylak P, Otrocka-Domagala I. Intraocular Chondrosarcoma in a Rabbit. J Comp Pathol. 2020;179: 41-44.
9. Moloney M, Dobromylskyj M, Sanchez-Jimenez C, Defauw P. Nasal clear cell chondrosarcoma in a dog. J Comp Pathol. 2024;213:37-40.
10. 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 Inc; 2017; 992.
11. Rahe MC, Westegaard T, Yaeger M. Extraskeletal chondrosarcoma in the tongue of a dog: case report and retrospective analysis of 236 tongue masses (2011-2019). J Vet Diagn Invest. 2020;32(3):440-443. 
12. Schmidt R, Reavill DR, Phalen DN. Musculoskeletal System. In: Pathology of Pet and Aviary Birds. 2nd ed. Ames, IA: John Wiley & Sons, Inc.; 2015:214.
13. Stidworthy MF, Denk D. Sphenisciformes, Gaviiformes, Podicipediformes, Procellariiformes, and Pelecaniformes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:663. 
14. Thompson K. Bones and Joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Saunders Elsevier; 2016: 117-120. 
15. Thompson KG, Dittmer KE. Tumors of Bone. In: Meuten, DJ, ed. Tumors in Domestic Animals. 5th ed. Ames, IA; John Wiley & Sons, Inc; 2017: 394-400. 
16. Wellman ML, Radin MJ. Nasal Exudates and Masses. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:127. 
17. Yi SW, Cho AR, Do YJ, et al. Multi-organ metastases of primary chondrosarcoma of the mammary gland of a spayed Toy Poodle. J Comp Pathol. 2023;204:35-38.

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