An eight-year-old neutered male retriever mix dog diagnosed with osteosarcoma of the mid-left ulna in February of 2016, had a partial mid-ulnectomy with clean margins and had been doing well. No adjuvant chemotherapy or radiation therapy were administered post-operatively. Four months after the mid-ulnectomy, a small growth on the lateral aspect of left forelimb was noted and had been getting larger. Radiographs (see below) show a mass (approximately 2×2 cm) in the soft tissues where the partial ulnectomy was performed. The mass was nonpainful and firm. A fine needle aspirate of the mass was obtained and submitted for cytologic evaluation.
Submitted slides were moderately to very highly cellular and consist of few to many red blood cells and a nucleated cell population predominated by individual round to spindle shaped cells (osteoblasts) with few scattered leukocytes in proportions consistent with peripheral blood. The osteoblasts had a small to moderate amount of basophilic cytoplasm that rarely contained pink granules and occasionally exhibited a small perinuclear clear area (Golgi). The nuclei were eccentric, round to oval, and had finely stippled chromatin pattern and 1-3 prominent nucleoli. Anisokaryosis was moderate; mitotic figures were frequently noted; binucleated cells were occasionally noted. Cellular aggregates were occasionally associated with a very small to moderate amount of pink extracellular matrix. The cytologic diagnosis was osteosarcoma.
Unlike many types of sarcoma, osteosarcoma can often be diagnosed definitively via cytology. While most of sarcomas consist of numerous of spindle-shaped cells, the osteoblasts that comprise osteosarcomas are more round to irregular in shape with a characteristic large oval, eccentrically-placed nucleus coupled with a perinuclear clear area (a characteristic that osteoblasts share with plasma cells). Many, but not all, osteoblasts also contain many fine intracytoplasmic pink matrix granules. Aside from these characteristics that identify their cell of origin, the malignant osteoblasts of osteosarcomas commonly exhibit many easily discernible criteria of malignancy (anisokaryosis, multiple prominent nucleoli).
Although the biopsy of the tumor was reported to have clean margins, recurrence of osteosarcoma at the amputation site suggests neoplastic osteoblasts were left behind within the soft tissues of the distal forelimb, where achieving wide soft tissue surgical margins are often difficult or impossible. A recent retrospective study of ulnar osteosarcoma (n = 30 cases) suggests that prognosis was no worse for these dogs compared to dogs with osteosarcoma involving other appendicular sites, although a variety of different treatment strategies were employed in this case series.1 Partial ulnar ostectomy was associated with a low complication rate and good to excellent function post-operatively, but may be associated with an increased risk of local tumor recurrence given the challenges of achieving wide surgical margins in the distal forelimb. The role of adjuvant chemotherapy in delaying tumor progression in ulnar osteosarcoma remains undefined, although most veterinary oncologists would likely advocate for its use in these cases given the high likelihood of metastasis.
- Ulnar osteosarcoma in dogs: 30 cases (1992–2008). Ramesh K.Sivacolundhu, Jeffrey J. Runge, Taryn A. Donovan, Lisa G. Barber, Corey F. Saba, Craig A. Clifford, Louis-Philippe de Lorimier, Stephen W. Atwater, Lisa DiBernardi, Kim P. Freeman, Philip J. Bergman, JAVMA 2013; Jul 1; 243(1):96-101).
Image 1: Lateral radiograph of the left forelimb. Note the mid-forelimb radiopacity in line of where the ulna had previously resided.
Image 2: 50x objective. Cytology of the mid-forelimb mass. Several round to irregularly-shaped malignant osteoblasts with the typical eccentric nucleus, basophilic cytoplasm, and a perinuclear clear area within the cytoplasm. Also, note the anisokaryosis, prominent nucleoli, and binucleation. A small amount of extracellular pink matrix is in the center of the photo.
Image 3: 50x objective. Cytology of the mid-forelimb mass. Several round to irregularly-shaped malignant osteoblasts with the typical eccentric nucleus and a perinuclear clear area within the cytoplasm.