Conference 11 - 2009 Case: 03 20100106

Signalment:

9-year-old, male neutered Golden Retriever dog (Canis familiaris).The right rear leg fractured and was pinned. One year later the leg was amputated.

Histopathologic Description:

The mass has a complex histology with tumor trabecular bone, necrotic tumor and tumor bone, foci of chondro-osseous matrix, chondroid matrix, undifferentiated sarcoma, fibrosarcoma and reactive trabecular bone. Undifferentiated tumor cells, sometimes in bundles, form either densely cellular or moderately cellular sheets; pleomorphic nuclei have coarse chromatin and some are giant. Mitotic figures are common in some fields. Separating the nuclei in various regions is either an osseous, chondro-osseous or chondroid matrix as well as dense collagen. There is extensive necrosis of tumor trabecular bone and the undifferentiated cells between these trabeculae. Reactive trabecular cortical bone is at the periphery and is bordered by atrophied and fibrotic skeletal muscle.

Morphologic Diagnosis:

Bone, rear leg, telangiectatic osteogenic sarcoma.

Condition:

Telangiectatic osteogenic sarcoma

Contributor Comment:

Medullary osteogenic sarcomas have been reported to occur at fracture sites with intramedullary pin fixation.

JPC Diagnosis:

Bone: Osteosarcoma.

Conference Comment:

For many conference participants, the differential diagnosis included chondrosarcoma and reactive new bone (e.g. callous associated with fracture repair). Those favoring chrondrosarcoma based the diagnosis on the striking degree of chondroid differentiation present throughout many sections; however, careful evaluation reveals the presence of malignant osteoblasts surrounding and investing osteoid in a disorderly orientation, most consistent with osteosarcoma (OSA). Those favoring a reactive process based the diagnosis on the proliferation of irregular trabeculae of new woven bone, a feature found not only in some cases of OSA, but also in a number of inflammatory bone lesions and in less aggressive neoplasms. Histologic differentiation can be difficult, but in reactive bone the trabeculae are interconnected and lined by a single layer of well-differentiated osteoblasts, with the intervening spaces occupied by non-neoplastic connective tissue; in the present case, spicules of neoplastic bone are lined and surrounded by haphazardly arranged pleomorphic osteoblasts.(3) Ancillary changes present in many of the slides include focally extensive skeletal muscle atrophy and fibrosis.

While the conference moderator and vast majority of conference participants concurred with the contributors diagnosis of OSA, none of the slides evaluated by the attendees contained blood-filled spaces lined by neoplastic osteoblasts, the histologic features diagnostic for the telangiectatic subtype. The gross appearance of telangiectatic osteosarcoma is distinctive, and reflects multiple blood-filled spaces reminiscent of hemangiosarcoma; the contributor did not provide a gross description of the bone tumor in this case.

Osteosarcoma represents the single most common primary neoplasm of the appendicular skeleton in dogs and cats, where it accounts for 80% and 70% of primary bone neoplasms, respectively. In dogs, OSA is characterized by rapid progression and early metastasis to the lungs, resulting in an early and high mortality rate. The appendicular skeleton is affected far more commonly than the axial skeleton, and the forelimbs are affected more commonly than the hind limbs. The neoplasm exhibits a strong predilection for the metaphyses of the distal radius, proximal humerus, distal femur, and proximal tibia.(2,3)

Osteosarcoma can be categorized based on its site of origin, with the majority of tumors being central, i.e. arising within bones, and fewer being categorized as one of two types of peripheral OSA, i.e. arising within the periosteum. Peripheral osteosarcoma includes the periosteal subtype, which clinically behaves similar to central OSA; and parosteal OSA, which is slower-growing and more well-differentiated, with a more favorable overall prognosis. The conference moderator cautioned participants that, because of the marked intratumoral histologic variability, OSAs can be difficult to accurately classify; nevertheless, subclassification into one of six categories based on the predominant histologic pattern is well-described in veterinary pathology, and summarized below:(2,3,4)

Poorly differentiated: undifferentiated neoplastic cells range from primitive mesenchymal to large and pleomorphic; identification as OSA depends on identifying small quantities of tumor osteoid; highly aggressive, osteolytic tumor frequently associated with pathological fractures
Osteoblastic: anaplastic osteoblasts with angular cell borders, variable amounts of basophilic cytoplasm, and hyperchromatic, often eccentric nuclei predominate; further subclassified as nonproductive or productive based on presence or absence of tumor bone production
Chondroblastic: neoplastic cells produce both prominent chondroid and osteoid matrices
Fibroblastic: interlacing fascicles of spindle shaped cells, resembling those of fibrosarcoma, that produce osteoid or tumor bone
Telangiectatic: subtype associated with the least favorable prognosis; consists of solid areas and large blood-filled spaces lined by malignant osteoblasts that occasionally form spicules of osteoid
Giant cell: tumor giant cells predominate in large areas of the neoplasm, which otherwise resembles nonproductive osteoblastic OSA

Based on the classification scheme above, and assuming the examined sections are representative of the entire neoplasm, the histologic features of the present case are most consistent with the chondroblastic subtype.

References:

1. Gleiser CA, Raulston JH, Jardine RH, Carpenter RH, Gray KN: Telangiectatic osteosarcoma in the dog. Vet Pathol 18:396-398, 1981
2. Thompson K: Bones and joints. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 1, pp. 112-118. Elsevier Saunders, Philadelphia, PA, 2007
Thompson KG, Pool RR: Tumors of bones. In: Tumors in Domestic Animals, ed. Meuten DJ, 4th ed., pp. 263-290, Iowa State Univ. Press, Ames, IA, 2002
Unni KK, Inwards CY, Bridge JA, Kindblom LG, Wold LE: Tumors of the Bones and Joints, AFIP Atlas of Tumor Pathology, 4th series, vol. 2, ed. Silver Spring, MD, pp. 119-192. American Registry of Pathology, Washington, DC, 2005