April 2019

Signalment (JPC #2626516): 10-year-old male Belgian malinois dog

HISTORY:  This dog presented with swelling of the distal radius.

HISTOPATHOLOGIC DESCRIPTION:  Radius, metaphysis:  Multifocally filling greater than 50% of the marrow spaces, replacing medullary components, surrounding and replacing trabeculae, multifocally effacing and replacing the cortex, and extending into the periosteum is an unencapsulated, infiltrative, densely cellular neoplasm composed of spindle cells that often surround foci of eosinophilic homogeneous to fibrillar matrix (osteoid), and are arranged in short interlacing streams, bundles, and whorls on a moderate fibrovascular matrix. Neoplastic cells have indistinct cell borders, a small to moderate amount of eosinophilic fibrillar cytoplasm, oval to elongate nuclei, and one distinct magenta nucleolus. Mitotic figures average two per HPF with rare bizarre mitoses. There is mild anisokaryosis and anisocytosis. Multifocally within the neoplasm are multinucleated giant cells that resemble osteoclasts. Scattered throughout and adjacent to the neoplasm there are trabeculae of immature woven bone lined by osteoblasts with numerous resting and reversal lines and osteoclasts within Howship’s lacunae (remodeling), irregular spicules of brightly eosinophilic bone with lacunae empty of osteocytes (necrosis), trabecular microfractures surrounded by mild hemorrhage, and rare islands of cartilage. Within the adjacent, less affected metaphysis and diaphysis, remaining trabeculae of cancellous bone are separated by increased numbers of fibroblasts and loose fibrous connective tissue (medullary fibrosis).  Multifocally, there is focally extensive periosteal proliferation of reactive woven bone with trabeculae oriented perpendicular to the cortex (exostosis), periosteal fibrosis, fibrocartilage formation, ectatic lymphatics (edema), and few perivascular infiltrates of lymphocytes, plasma cells, and fewer macrophages that extend into the adjacent adipose tissue.

MORPHOLOGIC DIAGNOSIS:  Bone, radius, metaphysis:  Osteosarcoma, Belgian malinois, canine.






Classification of central/medullary osteosarcomas:

Peripheral osteosarcoma



Neoplasm forming osteoid:

Other neoplasms: Differentiate with histopathological evaluation



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