A two year old intact male German Shorthair Pointer was presented with a history of a small growth noted several weeks prior on his caudal thorax. The growth seemed to rapidly increase in size over the few days prior to presentation. Thoracic radiographs revealed bony destruction of the ventral aspect of the left 12th rib associated with a firm, fixed mass. The patient was otherwise healthy on physical exam. A CBC, serum chemistry panel, and urinalysis were largely within the reference interval, including serum alkaline phosphatase (ALP) activity. A fine needle aspirate 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 fewer large multinucleated giant cells (osteoclasts). The osteoblasts had a small to moderate amount of basophilic cytoplasm that rarely contained pink granules and occasionally exhibited a small perinuclear clear area. The nuclei were eccentric, round to oval, and had finely stippled chromatin pattern and 1 to >5 prominent nucleoli. Anisokaryosis was moderate; mitotic figures were frequently noted. Cellular aggregates were occasionally associated with a small to moderate amount of pink extracellular matrix. Few macrophages containing red blood cells and/or hemosiderin were also noted. The cytologic diagnosis was osteosarcoma.
Contrary to common misconception, fine-needle aspirates (FNAs) of lytic bony lesions are commonly definitively diagnostic. Veterinarians often obtain ultrasound-guided or blind FNAs which typically result in highly cellular specimens. Unlike many types of sarcoma, osteosarcoma can often be diagnosed definitively via cytology. As in this case, the presence of: 1. osteoblasts (round to oval cells with eccentric nuclei +/- perinuclear clear area) with malignant criteria, 2. large multinucleated osteoclasts, and 3. extracellular pink matrix, allow the for the definitive cytologic diagnosis.
This patient was referred to a multispecialty hospital. The tumor was resected with wide surgical margin, including full thickness of both the thoracic and abdominal walls and removal of the distal portions of the last 5 ribs and part of the diaphragm. Reconstruction included use of a synthetic mesh. The histopathologic diagnosis was chondroblastic osteosarcoma with complete excision, high mitotic activity, marked nuclear atypia, and extensive central necrosis.
In a study of 39 dogs with tumors arising from the chest wall, 25 had osteosarcoma, 12 had chondrosarcoma, and 2 dogs had hemangiosarcoma. Median survival time (MST) for dogs with rib osteosarcoma was 290 days. Increased activity of total ALP significantly decreased survival in dogs with osteosarcoma (210 days versus 675 days).1 In a more recent study evaluating canine osteosarcoma of the extracranial flat and irregular bones, an association between tumor grade and survival was not found. However, increased serum ALP activity and tumor location in the scapula were associated with decreased survival times.2
This patient had a serum ALP activity within normal limits, underwent complete surgical excision of a rib osteosarcoma, and is currently undergoing adjuvant chemotherapy. Taken together, these factors may support a better than average prognosis for this patient. The mainstay of therapy for canine bone tumors, whenever possible, is definitive surgical treatment to provide local tumor control. Adjuvant chemotherapy is warranted for dogs with osteosarcoma of either the axial or appendicular skeleton to address the high likelihood of metastatic disease.
Liptak JM, Kamstock DA, Dernell WS, Monteith GJ, Rizzo SA, Withrow SJ. Oncologic outcome after curative-intent treatment in 39 dogs with primary chest wall tumors (1992-2005). Vet Surg, 37: 488-496, 2008. http://onlinelibrary.wiley.com/doi/10.1111/j.1532-950X.2008.00415.x/full
Kruse MA, Holmes ES, Balko JA, Fernandex S, Brown DC, Goldschmidt MH. Evaluation of clinical and histopathologic prognostic factors for survival in canine osteosarcoma of the extracranial flat and irregular bones. Vet Path, 50 (4): 704-708, 2012. http://vet.sagepub.com/content/50/4/704.short
Image 1: 20x objective. Many round to spindle-shaped malignant osteoblasts with one osteoclasts (the large multinucleated irregularly-shaped cell, top, middle).
Image 2: 50x objective. Several irregularly-shaped malignant osteoblasts with multiple prominent nucleoli surrounded by extracellular pink matrix.
Image 3: 50x objective. Several round to irregularly-shaped malignant osteoblasts with the typical eccentric nucleus, basophilic cytoplasm, and a perinuclear clear area within the cytoplasm.
Image 4: 50x objective. Several round to irregularly-shaped malignant osteoblasts including three mitotic figures (two mid right, one upper left).