Osteosarcoma

In this episode, Dr Lottie talks about osteosarcomas.


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Further Reading

• Ettinger & Feldman (2020). Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, 8th ed., Elsevier — pp. 1325–1328.
• Tilley, L. P., Smith, F. W. K., & Oyama, M. A. (2021). The 5-Minute Veterinary Consult: Canine and Feline, 7th ed., Wiley-Blackwell — section on Osteosarcoma (OSA).
• Makielski, K. M., Mills, L. J., Sarver, A. L., Henson, M. S., Spector, L. G., Naik, S., & Modiano, J. F. (2019).**Risk Factors for Development of Canine and Human Osteosarcoma: A Comparative Review. Veterinary Sciences, 6(2), 48. https://doi.org/10.3390/vetsci6020048
• Boerman, I., Selvarajah, G. T., Nielen, M., et al. (2012). Prognostic factors in canine appendicular osteosarcoma – a meta-analysis. BMC Veterinary Research, 8, 56. https://doi.org/10.1186/1746-6148-8-56

References

• Szewczyk, M., Lechowski, R., & Zabielska, K. (2015). What do we know about canine osteosarcoma treatment? – review.Veterinary Research Communications, 39, pp. 61–67. https://doi.org/10.1007/s11259-014-9623-0
• Frimberger, A. E., Chan, C. M., & Moore, A. S. (2016). Canine Osteosarcoma Treated by Post-Amputation Sequential Accelerated Doxorubicin and Carboplatin Chemotherapy: 38 Cases. Journal of the American Animal Hospital Association, 52(3), pp. 149–156. https://doi.org/10.5326/JAAHA-MS-6315
• Bush, J. M., Fredrickson, R. L., & Ehrhart, E. J. (2007). Equine Osteosarcoma: A Series of 8 Cases. Veterinary Pathology, 44(2), pp. 247–249. https://doi.org/10.1354/vp.44-2-247
• Bloomfield, R. (2015). Stereotactic radiation therapy in veterinary medicine. The Canadian Veterinary Journal, 56(1), pp. 95–97.
• Rohrer Bley, C., Meier, V., Turek, M., Besserer, J., & Unterhirkhers, S. (2024). Stereotactic Radiation Therapy Planning, Dose Prescription and Delivery in Veterinary Medicine: A Systematic Review on Completeness of Reporting and Proposed Reporting Items. Veterinary and Comparative Oncology, 22(4), pp. 457–469. https://doi.org/10.1111/vco.13011
• de Moura, F. B. C., Amorim, R. L., & Fonseca-Alves, C. E. (2025). Tyrosine kinase inhibitors in canine solid tumours: a systematic review of indications, response and safety. Veterinary Oncology, 2, 21. https://doi.org/10.1186/s44356-025-00036-1
• Makielski, K. M., Mills, L. J., Sarver, A. L., Henson, M. S., Spector, L. G., Naik, S., & Modiano, J. F. (2019). Risk Factors for Development of Canine and Human Osteosarcoma: A Comparative Review. Veterinary Sciences, 6(2), 48. https://doi.org/10.3390/vetsci6020048
• Boerman, I., Selvarajah, G. T., Nielen, M., et al. (2012). Prognostic factors in canine appendicular osteosarcoma – a meta-analysis. BMC Veterinary Research, 8, 56. https://doi.org/10.1186/1746-6148-8-56

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Dr. Lottie Wilkinson

Creator and host of the Current Vet podcast

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Hello, hello, and welcome back to Current Vet. This is the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and today we are talking about osteosarcomas. Osteosarcomas are the most commonly occurring malignant bone tumor in dogs. They are aggressive, painful, and metastasise really quickly.
So are a condition that all small animal vets should be really knowledgeable on. We are gonna look at what osteosarcomas actually are, how they form, how they spread the clinical signs that should make you suspicious, the diagnostic steps to confirm it, and then different treatment options available. And like always, we're gonna start with our case.
It is a Friday afternoon and you are halfway through your consults when Merlin. A 7-year-old male neutered great Dane limps into your exam room. His owner, Mark says he's been lamer on his left fore limb for the past few weeks. It started subtly with things like hesitating on walks, slowing down coming down the stairs, but in the last few days, he's been reluctant to bear weight on the leg at all.
He initially thought that Merlin had pulled a muscle chasing a ball, but rest just hasn't helped, and the lameness now seems constant. There's been no known trauma and Merlin's otherwise, eating and drinking. Normally though, he's been a bit quieter than usual on clinical exam. Merlin is bright and alert, but very obviously painful when you palpate his distal radius.
A couple of things you note down about your findings on your clinical exam are that he has a firm non-mobile swelling over the metaphyseal region of his left forelimb. There's no heat or fluctuance, so we don't think it's gonna be an abscess.
There's marked pain on palpation and there's mild muscle atrophy of the affected limb. You check the opposite leg and it seems completely normal. His temperature and general parameters are all within normal limits. So given the progressive lameness firm, bony swelling and signalment of being a large breed, your differentials are going to include osteosarcoma.
Other primary bone tumors like a chondro sarcoma or fibro sarcoma, osteomyelitis, especially if there's been potentially a penetrating injury or hematogenous spread from a previous infection, or potentially metastatic neoplasia to the bone forming a secondary tumor.
You discussed options with Mark and suggest starting with radiographs.
But before we get into what we could do for Merlin, let's talk about what osteosarcomas actually are. As always, we love etymology here because the words tell us exactly what is going on. So let's break down what the word osteosarcoma means. Osteo comes from the Greek word 'osteon', meaning surprise, surprise, bone, and sarcoma actually comes from two Greek words.
So 'sarx' means flesh, and '-oma' means a growth or a tumor. So sarcoma literally means a tumor of the flesh, or more specifically, a malignant tumor of mesenchymal tissue. Mesenchymal tissues are gonna include our soft tissues, our bone, cartilage, muscle, connective tissue. So then when we put the osteo in front of sarcoma, it tells us that it is a malignant tumor arising from bone or bone forming cells, so osteoblasts.
In dogs osteosarcomas most commonly affect the appendicular skeleton, and that is the long bones of the limbs, as opposed to the axial skeleton, which is going to be the skull, the neck, and the rest of the spinal column.
The key phrase that you have to remember for knowing the predilection sites for osteosarcoma growth is away from the elbow and towards the knee. I'll say it again. It's away from the elbow and towards the knee. So if we imagine the limbs of the canine skeleton. That means that in our front limbs, the sites that osteosarcs love to develop in are the distal radius down towards the carpus and the proximal humerus towards the scapula.
So away from the elbow got the distal radius and the proximal humerus. Then for the hind leg, osteosarcs develop towards the knee, so classically in the distal femur and the proximal tibia. This is not to say that Osteosarcomas can't develop in other sites in the limbs or along the axial skeleton, but it just means that these are less common.
So we may prioritize other differentials if different sites are affected,
The exact cause of osteosarcomas isn't fully understood, but there are a couple of known and suspected risk factors like a genetic predisposition, so particularly large and giant breed dogs like Great Danes, rottweilers, greyhounds, and Irish Wolfhounds. So that means Merlin is definitely a suspect as a Great Dane.
Other factors might be rapid growth and bone turnover, possibly linked to trauma and repeated stress on bones, old fracture sites, so osteosarcomas can develop at sites of chronic bone injury, or if an animal has had an implant placed in a bone and occasionally due to radiation exposure.
Osteosarcomas are not only a canine condition, they can theoretically occur in any species, it just varies in likelihood. So it's quite rare in cats, but when it does occur, it tends to be less aggressive and slower to metastasize than in dogs. In horses, it's quite uncommon, but typically affects the mandible or maxilla.
Possibly due to constant chewing and grinding of forage causing those microtraumas in the bone. But this obviously is gonna cause signs like facial deformity rather than lameness, so it'll have quite a different presentation in horses compared to dogs.
Osteosarcomas are locally aggressive and highly metastatic. It destroys normal bone tissue through osteolysis while simultaneously producing abnormal bone. So this is why we see those classic mixed lytic and proliferative lesions on radiographs. As the tumor expands and grows, it weakens the surrounding cortex and stimulates periosteal new bone formation.
And that is why we see the irregular sunburst appearance and codman's triangle on radiographs. But the real concern here is metastasis, because even when osteosarcoma is diagnosed, early micro metastases have almost always already occurred.
So these tumors tend to spread haematogenously most often to the lungs, but it can also spread to other bones, lymph nodes, and soft tissue.
That is a huge load of information. So I'm gonna quickly summarize it before we move on to clinical signs. So in short, osteosarcoma is a malignant tumor of bone or bone forming cells.
They cause aggressive bone lysis as well as new bone formation. The way to remember the predilection sites for osteosarcoma is away from the elbow and towards the knee. Osteosarcomas have high metastatic potential, especially to the lungs, and they progress really rapidly and are exceptionally painful for our patients.
In Merlin's case, this explains why his distal radius is swollen, firm and so, so painful.
Now we understand how the tumor develops. Let's talk about how it actually presents in our patients. In most cases, like with Merlin, the first sign owner's notice is lameness. It might start subtly, maybe after exercise or play, but it quickly becomes progressive, persistent, and non-responsive to rest.
As the tumor grows, it causes local bone pain, a firm swelling over the affected area, reluctance to bear weight, which then leads to muscle atrophy as the dog is no longer using that limb. And sometimes we see pathological fractures because as the tumor causes lysis in the surrounding bone, it becomes weaker and weaker.
So that means that the bone can break under normal activity.
If we have a rarer axial osteosarcoma, we might see facial swelling, dysphasia, halitosis, or tooth loss if it's in the mandible or maxilla , neurological deficits. If there's spinal cord compression from a vertebral tumor or a palpable thoracic mass dyspnea or exercise intolerance, if we have a rib tumor.
There are also very rare extra skeletal forms. That can occur when the tumor metastasizes and neoplastic osteoblasts seed in random places in the body. So signs of those are gonna vary depending on the organ or soft tissue involved. Because osteosarcoma is such a painful and metabolically active tumor, you might also see a couple of non-specific signs like lethargy, weight loss, inappetence, and occasionally a low grade pyrexia caused by inflammation or necrosis from the tumor.
So Merlin's progressive, non-weight-bearing lameness, and firm swelling over the distal radius are exactly what we'd expect in a classic presentation.
Although Merlin's presentation is very indicative of osteosarcoma, we should always aim to confirm our suspicions because other conditions like osteomyelitis, chondro sarcoma, or even metastatic neoplasia can cause very similar presentations.
Or we could actually have a mixed presentation and something else completely might be going on at the same time. So we always have to check. Step one is going to be radiography.
With radiographs, we have to take images of both the lateral and cranio-caudal views so we have a multidimensional view of the lesion.
What you're typically gonna see with an osteosarcoma are aggressive bone changes, so that mixed lytic and proliferative lesion where bone destruction and abnormal formation are occurring at the same time, poorly defined margins around the tumor, suggesting fast infiltrative growth, lysis of the bone cortex, a sharp, irregular, and spiky periosteum, which is what's referred to as the classic sunburst pattern, and sometimes a codman's triangle, which is where the periosteum is lifted by the tumor, creating a triangular shadow at the edge of the lesion.
Radiographs of adjacent joints and the contralateral limb must be taken too to rule out joint disease or other causes of lameness.
In Merlin's case, let's say we took our radiographs and they show that mixed lytic proliferative lesion in the distal radius with cortical disruption and an aggressive periosteal reaction. All of these findings are very suggestive of osteosarcoma, but now we are going to stage it and this is essential before deciding on any treatment plans.
We know that by the time of diagnosis, up to 90% of dogs with osteosarcoma already have metastases, even if they're not visible on radiographs, to be clearly visible on radiographs, nodules have to be at least one centimeter in diameter and this is something we have to explain well to clients because they need to understand that even when we can't see something on an x-ray, that doesn't mean that it isn't there. So even when we don't see anything on x-ray, there is still a high chance that the tumor has already spread, making the prognosis for this condition generally poor and treatment attempts are basically futile. staging workup can include a couple of steps. Firstly, we're gonna have three view thoracic radiographs, so a right view, a left view, and ventral dorsal. And this is to assess for pulmonary metastasis. Abdominal ultrasound can be used to check for abdominal organ involvement, and then sometimes if it is available, we can do CT or bone scintigraphy to look for metastasis in other bones and in soft tissue.
So finally, while radiographs are highly suggestive of osteosarcoma, they can't give a definitive diagnosis as this requires cytology or histopathology. Fine needle aspirate can sometimes give a diagnosis if enough malignant osteoblast is sampled. The hallmark findings for osteosarcoma on cytology are going to be pleomorphic cells, high mitotic rates, and cells producing a pink fibrillar material known as osteoid. And this is basically just unmineralized bone tissue being produced by the neoplastic cells.
The other option is core or incisional biopsy, and this is often needed to confirm the diagnosis. We need to avoid sampling through large areas of healthy tissue as you risk seeding tumor cells along the biopsy tract. So ideally take the sample from the center of the lesion using imaging as a guide if you need.
Histopathology will confirm malignant mesenchymal cells producing again that osteoid matrix.
In Merlin's case, his thoracic radiographs revealed multiple round soft tissue opacities scattered throughout both lung fields, which is consistent with pulmonary metastases. So this confirms that the disease has already spread beyond the primary sites, and based on these findings, his owners decided not to pursue cytology or histopathology and not to pursue treatment,
Whether we decide to do treatment depends on a couple of factors. So firstly, whether there's evidence of metastases, the location of the tumor, financial and emotional considerations for the owner, and most importantly, the patient's quality of life if we go through with treatment.
Because Osteosarcoma is both locally aggressive and highly metastatic, successful management requires both local control of the primary tumor and systemic therapy to target micro metastases.
So let's talk through what options we have if we do pursue treatment. Firstly is curative intent treatment and this approach is most appropriate for patients without visible metastasis at the time of diagnosis. So first we could do limb amputation, which is going to remove the source of pain and obviously achieves excellent local control of the tumor.
Dogs generally adapt remarkably well to amputation as long as there's no other orthopedic or neurological conditions. So this is an example of when taking radiographs of all the other limbs is essential, even if you don't suspect there are metastases in them. Because progressive arthritis or joint disease can become a big problem if one of the limbs has been amputated.
However, amputation alone is not curative, and the median survival time is only around four to six months.
The second option is combining amputation with chemotherapy, and this can extend survival time to 10 to 12 months.
Commonly used drugs are carboplatin and doxorubicin. They're generally very effective and well tolerated. Cisplatin is also available, but less commonly used now due to the side effects associated with it like nephrotoxicity. We can also do a combination therapy, for example, alternating between carboplatin and doxorubicin.
But some studies show that there's actually no difference in mean survival time in patients receiving the combination therapy compared to monotherapy. The key difference in the two protocols was that patients experience fewer side effects when given combination over monotherapy.
You have to know though that chemotherapy doesn't eliminate metastases, but it helps to slow their progression and prolongs remission, so it is not a cure.
The third option is limb sparing surgery. In a few cases, for example, when the lesion involves the distal radius limb sparing surgery can be performed, which is basically when the diseased bone is removed and replaced with a graft or prosthetic to basically remodel the limb and preserve limb function.
But this comes with a few more serious complications like infection, implant failure or recurrence and this would mean that the patient doesn't actually retain limb function and they may need salvage amputation in the future anyway. And as we said, not many patients are candidates based on tumor location and the extent of spread.
Radiation therapy can sometimes be used for local control when surgery isn't possible, especially for axial tumors or in addition to a limb sparing surgery. But combining it with chemotherapy is going to give better results.
So just to recap our curative intent treatment approaches, we have limb amputation surgery and chemotherapy, combined limb sparing surgery and radiation therapy as our options.
Now we're gonna move on and talk about palliative care. So when metastases are already present, like in Merlin's case, the goal of treatment is comfort rather than cure. So our focus is on pain management and maintaining quality of life for as long as possible. Palliative radiation therapy can be very effective at reducing pain for a couple of weeks or months, even if the tumor isn't removed.
It helps to destroy some tumor cells, decrease bone pain and improve mobility. There is a lot of pain associated with osteosarcomas, so multimodal pain management plans are really key. We can use NSAIDs, opioids, amantadine, or gabapentin for neuropathic pain and bisphosphonates can also provide some comfort as they inhibit osteoclast activity so prevent bone lysis and resorption occurring.
Chemotherapy can still be considered for palliative purposes. Just slow tumor progression, even when metastasis are present and eventually regardless of supportive efforts, osteosarcoma pain will become difficult to control, and euthanasia is often the kindest option, especially when mobility and comfort are compromised.
There are a couple of newer treatments being explored, so I'm just gonna touch on those quickly. First, we have immunotherapy, like, I'm gonna butcher this name, but it's liposome-encapsulated muramyl tripeptide phosphatidylethanolamine, which activates macrophages to target tumor cells. We then have tyrosine kinase inhibitors like Toceranib, and stereotactic radiation, and electro chemotherapy, which can help with targeted local tumor control, similar to our standard radiotherapy.
These are definitely not standard of care yet, um, but they may play a role in the future or in referral settings. Unfortunately, osteosarcoma carries a very poor overall prognosis, even with aggressive treatment.
The high rate of metastasis means that by the time of diagnosis, the condition has usually spread microscopically, even if imaging appears clear. There are a couple of factors which indicate a worse prognosis. So those are gonna be one, the presence of metastasis at diagnosis. Two, the location of the tumor. Appendicular tumors are easier to surgically remove or target with radiation in comparison to axial tumors. Three, if clear surgical margins are not possible. Four, if there is a high histological subtype or grade. And five, if there's a high serum, ALP level, this is also associated with worse prognosis.
So for Merlin, given the presence of the lung metastases, the advanced stage of disease, merlin's owners made the decision to choose euthanasia rather than pursue aggressive or palliative treatment.
Before we wrap up, let's summarize the main points to remember . So osteosarcoma is a malignant tumor of bone or osteoblasts, and it's the most common primary bone tumor in dogs. It typically affects large and giant breeds, especially the appendicular skeleton, and follow the pattern of away from the elbow and towards the knee.
The tumor is both locally aggressive and highly metastatic, most often spreading to the lungs. Radiographic signs of osteosarcomas include a mix of lytic and proliferative lesions, poorly defined margins, and the classic sunburst periosteal reaction. Diagnosis can be confirmed via cytology or biopsy.
Though in practice, strong clinical and imaging findings can be sufficient for decision making. Amputation combined with chemotherapy or radiotherapy, offers the best survival outcomes, but it remains incurable in most cases. Palliative care will focus on maintaining comfort and mobility using multimodal analgesia, bisphosphonates or palliative radiation, or a mix of all of them.
And above all treatment should always prioritize quality of life. And we have to feel okay having these open, empathetic discussions with owners so they're fully aware of the poor prognosis that osteosarcoma has.
And that is osteosarcoma, a complex aggressive disease that can be really emotionally challenging to navigate. So it's always important to fully understand why it behaves the way it does, so we can recognize it, manage cases more effectively, and most importantly, feel confident in supporting our patients and their owners through this diagnosis.
Thank you for listening to Current Vet. If you found this episode helpful, make sure to follow or subscribe wherever you get your podcasts. Share it with a friend or colleague who needs to learn more about oncology, and as always, you'll find the references and further reading linked in the show notes. If there's a topic you'd like us to cover next, find us on Instagram or TikTok @veterinaryvista and let us know what you wanna hear about.
See you next time.