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Osteosarcoma
(OSA) is also known as osteogenic sarcoma and
is a member of the family of tumors known as “primary bone tumors”.
“Primary”
means that the tumor started
primarily the bone and does not represent spread (metastasis)
of cancer from some other site
in the body. Many tumors in other parts of the body will
spread to bone, and therefore it is crucial to differentiate primary bone
tumors from metastatic disease. Osteosarcoma is the most common primary
bone tumor diagnosed in dogs. In fact, when a tumor is found in the
bone, it is ultimately diagnosed as OSA more than 90% of the time.
Osteosarcoma tends to occur in large and giant breeds of dogs and also
tends to occur in major weight bearing bones. Typically, we diagnose
OSA in older patients, but we also see a fairly high incidence in
younger dogs as well. This cancer is characterized by a high rate of
spread to other parts of the body, especially the lungs and other bones.
In contrast, this disease is rare in cats and doesn’t appear to be
metastatic.
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Radiograph of a patient with osteosarcoma. The
lesion is seen in the distal radius and ulna of the forelimb and is
characterized by lysis and
production of bone .
What are the symptoms of OSA?
Lameness and swelling in
the affected limb are the most common symptoms seen. Oftentimes, the
lameness is initially believed to be due to some traumatic episode such
as jumping out of a car or off of a bed. When a lameness doesn’t
resolve as would be expected with trauma, radiographs are recommended.
Two ACIC patients that had undergone forelimb
amputation for OSA. Both dogs tolerated the surgery beautifully and
were simultaneously receiving chemotherapy
How is the diagnosis made?
If a radiograph is
suspicious of a primary bone tumor, a biopsy is necessary to make a
definitive diagnosis. Most primary bone tumors have a characteristic
appearance on radiographs. The bone often appears to have been “eaten
away” by the tumor. Sometimes the radiographic lesion may not have a
classic appearance. A bone biopsy is typically done under a general
anesthesia for the comfort of the patient. A tiny incision is made in
the skin above the suspected tumor. A small core of bone is taken using
a “Jamshidi” bone biopsy instrument. The sample taken is then sent to
the pathology laboratory for analysis. In cases where patients are
extremely painful or have suffered pathologic fractures, it may not be
in the best interest of the patient to wait for biopsy results. At
ACIC, we take a pre-operative bone aspirate while the patient is being
prepped for surgery. We read the cytology on site and if the sample is
consistent with cancer, we proceed with amputation. Cytology can rule
out fungal disease or bacterial infection but cannot always distinguish
between the type of sarcoma present (i.e. osteosarcoma vs.
chondrosarcoma
Radiograph of a dog’s leg after undergoing limb salvage
surgery. The tumor is replaced with a bone allograft, and then
stabilized with a bone plate. This procedure was performed at Colorado
State University.
Staging the disease:
It is extremely
important to determine if the cancer has spread (metastasized). OSA can
go to any tissue in the body, but tends to spread to lungs and other
bones. Up to 10% of patients will have visible cancer in the lungs at
the time of diagnosis. A complete work-up includes:
ˇ Complete
blood count (CBC), serum chemistry panel, urinalysis
ˇ Thoracic
radiographs (3 views)
ˇ Radiographic
survey of appendicular skeleton
ˇ Bone
biopsy (or pre-operative bone aspirate)
ˇ Nuclear
scan if available.
Can my pet still be treated if the cancer has spread? It is
generally not recommended to pursue definitive treatment once the cancer
has spread beyond the primary site. Treatment at this stage would be
directed towards palliation (relieving pain) and could include radiation
therapy to the tumor site (2-4 dosages of radiation), chemotherapy, or
bisphosphonate therapy.
Treatment of osteosarcoma:
In cats, amputation
alone is often curative. In dogs, surgery and chemotherapy are the
treatments of choice. The primary tumor is removed by amputation or
limb-salvage procedures. We know that greater than 90% of tumors have
spread microscopically to the lungs (i.e. cannot be detected on initial
radiographs) at the time of the initial diagnosis. Therefore, if
amputation or limb-salvage is performed but not followed with
chemotherapy, patients will not gain a survival advantage. The reason
for this is that the microscopic disease already present in the lungs
grows. The purpose of chemotherapy is to prevent or delay this growth.
How is chemotherapy administered? Several drugs have been shown to be effective in treating OSA in dogs.
We recommend either cis-platin or carboplatin alone or in combination
with doxorubicin. The first chemotherapy is ideally administered
immediately following surgery, and then further treatments are given at
21 day intervals for a total of 5-6 treatments. Treatments are
administered intravenously. In generally, chemotherapy is exceptionally
well tolerated in pets. In cats, OSA tends to remain localized,
therefore chemotherapy is not advised.
What is the prognosis for OSA?
With OSA, a cure is
possible, but in dogs actually occurs less than 20% of the time.
Prolonged remissions of 1-2+ years are not uncommon. Median survival
for patients treated with surgery and chemotherapy is approximately 13
months, with 35% of patients still alive and disease free at 2 years.
Many patients survive much longer periods of time, but it is important
for pet owners to understand the statistics for this disease so that
they enter treatment protocols completely informed. The quality of life
during and after treatment is usually excellent.
Palliative therapy:
ˇ Radiation
therapy can be used in patients who are poor candidates for limb-salvage or
amputation. A recent study showed that >90% of dogs treated with 4
dosages of radiation therapy experienced significant pain relief that
lasted a median of 7 months. Chemotherapy combined with palliative
radiation can actually improve survival times.
ˇ Bisphosphonates: are a group of drugs that help decrease bone pain in primary or
metastatic bone cancer and may help delay the progression of the
disease. These drugs are believed to work through several mechanisms,
including inhibiting bone resorption, impeding osteoclast (cells that
break down bone) activity, inducing apoptosis (programmed cell death) in
osteoclasts (cells of malignancy in OSA), and also may exert an anti-angiogenic
effect (inhibiting blood vessel formation that helps tumors grow and
spread). A commonly used bisphosphonate is a drug called Pamidronate.
It is given as a 2 hour IV infusion once monthly. Pamidronate can be
used alone for palliation, or in combination with radiation,
chemotherapy, or definitive therapy for OSA.
ˇ Pain
medications:
OSA as described by people with this disease, is very painful. Removing
the source of pain through amputation or limb-salvage provides the most
complete pain relief. When this is not possible, a variety of analgesics
can be used. Non-steroidal anti-inflammatory drugs (NSAIDS) are usually
the first line medications used. When NSAIDS alone do not provide
adequate analgesia, they are usually combined with other medications
such as Tramadol (UltramŽ), amantadine, gabapentin, transdermal fentanyl
patches, to name a few.
 Cells obtained from a bone aspirate of a dog
with OSA. The black arrow points to a multi-nucleated giant cell,
commonly seen in OSA.
A "pathologic” fracture
(a fracture that occurs because there is underlying pathology in the
bone) in the radius and ulna of a seemingly normal dog that jumped out
of a truck and yelped in pain. This radiograph also shows a classic
bone tumor lesion. Pathologic
fractures are difficult to repair and therefore amputation is usually
necessary on an emergency basis.
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