Extra-osseous osteosarcoma in a dog

Kristiina Ruotsalo, Christopher Pinard, Heindrich Snyman

Animal Health Laboratory, University of Guelph, Guelph, ON (Ruotsalo and Snyman), Toronto Veterinary Emergency and Referral Hospital (Pinard).

AHL Newsletter 2022;26(2):21. 

A five-year-old, female spayed Labrador dog was presented for evaluation of a 3 cm firm, subcutaneous mass on the caudal aspect of the left shoulder.  The mass was first noted two months previously, and fine needle aspiration at that time provided a diagnosis of soft tissue sarcoma.  The mass had undergone recent, rapid growth to its current size, but did not appear to be affixed to deep tissues. The dog was otherwise clinically well.

The mass was re-aspirated and slides submitted for cytological evaluation.  Cellular composition varied from slide to slide.  Occasional slides contained a predominance of individual round cells consistent with an osteoblast origin; these cells exhibited round to oval, centrally to eccentrically placed nuclei with variable numbers of small nucleoli and moderately abundant and basophilic cytoplasm.  Intermixed with this round cell population were occasional large fusiform multinucleated giant cells consistent with an osteoblast origin.  The remaining slides contained clusters of pleomorphic fusiform spindle cells embedded in variable amounts of brightly eosinophilic extracellular matrix (interpreted as osteoid), and intermixed with both osteoblasts and osteoclasts (Fig 1).  A cytological diagnosis of sarcoma, most consistent with an extra-osseous osteosarcoma (ExOSA), was made.  Prior to surgical excision, a CT scan was performed and a subcutaneous location to this mass was confirmed, with no underlying bone involvement identified (Fig 2).  Whole-body CT showed no evidence of other lesions.

Histological evaluation of the excised mass and surgical margins revealed a well-demarcated, encapsulated expansile mass in the deep subcutis.  The neoplasm was composed of spindle cells arranged in streams, bundles, and whorls on a fibrovascular matrix which occasionally contained embedded foci of eosinophilic extracellular matrix, consistent with osteoid (Fig 3).  Occasional multinucleated giant cells consistent with osteoclasts were noted.  The inked surgical margins did not contain any neoplastic tissue.  A histological diagnosis of extra-osseous osteosarcoma, fibroblastic, productive, was made.  ExOSA is a malignant, osteoid-producing stromal neoplasm without primary periosteal or bone involvement.  Although less common than its skeletal counterpart, ExOSA are occasionally reported in dogs and cats, with the mammary gland representing the most common location.  Various other visceral and soft tissue sites have been reported (e.g., muscle, eye, thyroid gland), with neoplasms also noted secondary to injection site reactions, trauma, granulomas (e.g., Spriocerca lupi infection), and foreign bodies.  ExOSA are often grouped along with visceral soft tissue and mammary osteosarcomas, both of which tend to have a poor prognosis.  Information on the specific prognosis of skin/subcutaneous osteosarcoma in dogs is limited.  One relatively large study included 13 dogs with ExOSA at these sites, and reported median survival times from 240 to 486 days.  Although the mass described in this report appears to have been completely excised with clean surgical margins, the prognosis for this patient is unknown.   AHL

Figure 1. Cytological features of the subcutaneous mass including stromal cells (slender arrow), osteoblasts (thick arrow), and stromal cells embedded in osteoid (yellow arrow).  Wright’s stain.

Figure 2. CT image showing a well delineated, soft tissue and mineral density mass (red rectangle) that is mildly heterogeneously contrast enhancing on the lateral aspect of the left thorax in the subcutaneous fat, caudal to the left scapula.'

Figure 3. Histological features of the excised mass illustrating plump spindle-shaped cells arranged in interlacing streams, bundles and whorls with foci of eosinophilic matrix (osteoid). H&E stain.

Reference

1. Langenbach A, Anderson MA, Dambach DM. Extraskeletal osteosarcomas in dogs: A retrospective study of 169 cases (1986-1996). J Am Anim Hosp Assoc 1998;34(2):113-120.