Canine
Osteosarcoma
Case
of the month
Diesel, a 9 year
old male French Mastiff presented for a 3 day history of right forelimb
lameness. His owners reported that Diesel was running around chasing squirrels
in the backyard and when they let him back in the house noticed he was favoring
his forelimb. Diesel was otherwise doing well but on the morning of
presentation did not want to eat his breakfast and seemed to be more lethargic.
Due to the persistent lameness his owners brought him in to be examined. On presentation Diesel was bright and alert
but preferred to sit rather than walk around. On ambulatory exam Diesel
exhibited a weight bearing right forelimb lameness. No swelling was palpated
but Diesel attempted to bite when his right forelimb was extended forward. The rest of his general physical examination
was unremarkable. Diesel had a history
of a torn cruciate ligament in his right stifle that was surgically repaired 4
years prior. Otherwise he had no significant medical history.
Radiographs were
performed and revealed a swelling at the proximal aspect of his right humerus
with lysis of the bone and a moth eaten appearance. Chest radiographs were
performed and did not reveal any abnormalities. Bloodwork was performed and was
within normal limits. A suspicion of
osteosarcoma was discussed with Diesel’s owner and he was referred to an
orthopedic surgeon for consultation. Three days later, Diesel underwent surgery
to amputate his right forelimb. Chemotherapy was offered but his owners decided
not to pursue any further treatment, as they were interested in just keeping
Diesel as comfortable as possible. Diesel recovered wonderfully from surgery and
learned to get around on three limbs with ease. Five months later, Diesel
returned for an exam as he was not doing well. He was very lethargic, had no
appetite, and seemed to have increase respiratory effort at home according to
his owners. Chest radiographs revealed a large number of metastatic lesions in
his lungs. At this time due to his poor
quality of life and underlying cancer diagnosis, Diesel’s owners elected
euthanasia.
Canine
Osteosarcoma
Osteosarcoma
(OSA) is the most common primary bone tumor and accounts for more than 85% of
malignant bone tumors in dogs. Tumors arise from the central region of bones
and expand outwards disrupting the normal bone structure. OSA can affect the appendicular skeleton (the
limbs) or axial skeleton (head and trunk), less commonly. The most common
location for appendicular OSA is the metaphysis (the long narrow portion of the
long bones) and it more commonly occurs in the forelimb than the hind
limb. The most common location for axial
OSA is the mandible and maxilla (bones of the mouth) and less commonly affects
the vertebrae, skull, and ribs. Large or giant breed dogs are at a higher risk
of developing OSA; these include Saint Bernards, Great Danes, Irish Setters,
Dobermans, Rottweilers, German Shepherds, Golden Retrievers, and others.
The most common
clinical signs are progressive lameness and/or swelling at the primary
site. Sometimes there is association
with a recent mild trauma so owners often believe their dog may have suffered a
sprain or fracture. Dogs typically
exhibit some degree of pain on physical examination.
Diagnosis
Diagnosis of OSA
involves radiographs of the affected limb. Radiographs will show abnormalities
(lysis, destruction of bone, abnormal bony proliferation) that are consistent
with OSA. Although radiographs do not provide a definitive diagnosis, they
usually provide very high suspicion. If OSA is suspected, radiographs of the
chest are often performed at the same time to look for lung metastasis (as this
is the most common place for the tumor to spread). Less than 10% of dogs with OSA will have
metastasis at the time of diagnosis, but 90% will die of metastatic disease.
Bloodwork is often recommended to screen for concurrent illness, which may
affect treatment plans. Often times physical
examination, history, and radiographs are enough to make a diagnosis, but if
there are doubts, a cytology or bone biopsy can be performed. Bone biopsy and
cytology are typically not needed.
Treatment
& Prognosis
If staging tests
do not reveal metastatic disease, treatment includes resection of the primary
tumor followed by systemic chemotherapy. Due to location and aggression of these
tumors, resection usually involves amputation of the affected limb. Limb
sparing surgery is a possibility, but a patient needs to fit certain criteria
to be a candidate and it has its own complications. Even if owners are not interested in pursuing
chemotherapy, amputation is often recommended to provide comfort to the
patient, as the tumor can be extremely painful. Median survival time for animals that undergo amputation only is
approximately 6 months; those who undergo chemotherapy is approximately 1 year.
Surgical approaches to axial OSA can be more difficult due to location and
prognosis varies pending location as well. Radiation therapy is a consideration
in these cases as well.
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