The Dog Cancer Survival Guide (100 page)

When you need to look up the definition of a word or phrase, find it listed in
Chapter 5
, which begins on
page 46
.

Depending upon where they are located in the skeleton, bones are called axial (bones in the head and trunk) or appendicular (bones in the limbs). Osteosarcoma tumors, which can occur in the cells of any bone, typically arise in appendicular bones (the long bones of the limbs).

OSA tumors are usually found at the end of the bone (called the metaphysis) and this location differentiates them from bone tumors that have metastasized from other cancer types, which most often occur in the middle of the bone (called the diaphysis). The most common locations for OSA are “towards the knee and away from the elbow”: the top of the shoulder (top of the humerus bone), the wrist (bottom of the radius bone) and the knee (bottom of the femur bone or the top of the tibia bone). Another common site is the bottom of the tibia bone, at the ankle or hock joint. There is usually only one tumor, and it typically does not invade other bones through joints. OSA can sometimes be found in the middle of bones and, less commonly, in torso bones.

OSA tumors can generate excessive new bone growth (production), bone lysis (destruction) or both. Both types of activity can usually be seen on X-rays, although the appearance can vary widely from tumor to tumor, which can make misidentification possible. Fungal and bacterial infections can look like OSA, for example, so, in areas where fungal infections are prevalent, it is wise to get a bone biopsy evaluated both for cancer and for a fungal infection. Veterinarians in areas near rivers or other waterways with sandy, acidic soil are particularly vigilant about the fungus blastomycosis, which can involve the bones of about 30% of dogs who incur the infection. While OSA does not typically make a dog feel sick, blastomycosis infections usually do. If your dog is not eating well, has a fever or suffers from weight loss, coughing, depression, eye problems, lameness or skin problems, make sure you rule out blastomycosis as a cause for any bone changes you see on X-rays. Blastomycosis is a special concern for dogs exposed to water in the late summer and the fall in endemic areas such as Ohio, Mississippi, and the St. Lawrence River Valley.

OSA bone tumors can cause weakness and even severe bone fractures at the primary site. The risk of a pathological fracture – a sudden and painful bone break – is real, which is why amputation is the most prudent immediate course. OSA most commonly metastasizes to the lungs and 10-15% of dogs have detectable spread at the time of diagnosis. Less often, tumors can metastasize to other bones. The regional lymph nodes can be involved, also, although this occurs in only 5% of dogs.

Micrometastasis is not only possible with OSA, but has probably already happened by the time of diagnosis. Ninety percent of dogs with OSA have microscopic cancerous lesions that cannot be detected by chest X-rays (the most common and widely available test used to look for lung spread). If available and used, advanced imaging tools, such as CT scans, can detect smaller masses otherwise missed on chest X-rays. Even when the primary tumor is successfully removed, these tiny metastases will grow and eventually be the cause of death for dogs with OSA. This makes systemic treatments aimed at controlling micrometastasis a critical part of conventional care.

OSA is usually seen in middle-aged and older dogs, age seven to nine, and we also see a smaller peak of incidence in dogs between the age of eighteen months and two years. Axial OSA can be seen in any breed at any time; appendicular OSA is usually seen in the front limbs of large and giant breeds, including Great Danes, Saint Bernards, Irish Setters, Rottweilers, German Shepherds, Golden Retrievers and Doberman Pinschers.

Breed is not as important as height and weight; heavier and taller dogs are more likely to develop OSA. For example, dogs that weigh more than sixty-six pounds are sixty times more likely to develop OSA than dogs weighing less than twenty-two pounds. For dogs weighing more than eighty-eight pounds, 95% of primary bone cancer is OSA; for those weighing less than thirty-three pounds, only 40-50% of primary bone cancer is OSA.

The exact cause of OSA is unknown. One theory is that small “micro-fractures” occur in the long bones as they bear the weight of the dog’s body. The multiple minor traumas and injuries, the thinking goes, stimulate excessive inflammation and bone growth; this may increase the chances that genetic mutations and malignancies will develop.

Radiation therapy (for other types of cancer) can cause OSA later in life (rarely, 3-5%). In these cases, OSA develops in a bone that was included in the original radiation therapy field, and usually happens a full three to five years after radiation treatments.

Dogs that have experienced a bone fracture, bone trauma or bone infection are at increased risk for OSA, as are those who have metallic implants in their bones (used to repair a fracture). Taller dogs also tend to be at increased risk.

Sex hormones may protect against OSA development; in one study, Rottweilers who were spayed or neutered before one year of age were four times more likely to develop OSA later in life.

A number of genes have been implicated in OSA development, including p53, retinoblastoma, PTEN, and possibly c-Kit. There are also several chemical factors involved in angiogenesis – new blood vessel formation – that seem to play a role in OSA development and progression.

The first sign of OSA is usually limping or refusing to put weight on the involved leg. Bone tumors hurt, especially when the bone bears weight; they are painful enough to make even the most stoic of dogs flinch or limp.

Swelling may be visible in the area of the tumor, although if the tumor is high on the leg, beneath the shoulder or thigh muscles, or hidden under a lot of fur, this may be less obvious.

Many dogs are so unwilling to show pain that they hide these symptoms until very late; be sure to get X-rays early when your dog does not respond to pain medications and rest (especially important for large breeds). If there is significant bone destruction (lysis), the bone can weaken enough to cause a pathologic fracture. This happens rarely – in fewer than 3% of cases; when it does it causes sudden, severe pain.

Most dogs will feel well, other than the pain associated with the tumor, unless lung metastasis has occurred, when they may have a few other general signs, such as decreased appetite and low energy. It is uncommon for the dog with lung metastasis to cough or have difficulty breathing until it is advanced.

While OSA is not usually caught in routine blood work, sometimes elevated alkaline phosphatase (ALP) levels are detected. If this is the case, the prognosis is worsened. (Increased ALP is not specific for bone cancer; other conditions are also associated with it).

Fluoridated water may also be a risk factor for male dogs. I write about this at length on
page 77
.

When OSA is suspected, X-rays can show classic bone changes associated with these tumors. The location of lesions and their typical “sunburst” appearance will warn the vet that the tumors may be OSA. When the dog is older, of a larger breed, or if pain cannot be controlled easily, suspicions rise even higher.

Normally, I recommend confirming a cancer diagnosis before performing a cancer surgery; I make an exception when the lesions are suspected to be OSA. Whatever their cause – OSA or an infection – bone lesions cannot be repaired, and they put the dog at risk for a serious and painful fracture. Aggressive surgery, usually amputation, is the immediate next step for severe bone lesions, because this removes the destructive process (whatever it may be) immediately, and the dog’s bone-related pain ceases. A biopsy on the removed limb can confirm what caused the lesions, and appropriate post-surgery treatments can be planned after the results are in.

In other words, if OSA is suspected, the usual course of action is to skip an aspirate or biopsy and proceed immediately to staging the cancer, followed by an amputation.

Some owners and some vets don’t want skip the aspirate or the biopsy, however. These owners may not want to treat if the lesion is confirmed to be OSA, or they may want to try a “limb spare” procedure (see below). Not every limb can be saved; either way, these owners want a confirmed diagnosis before deciding how and whether to treat the cancer, and certainly before they make a decision that cannot be reversed. To confirm OSA, a bone aspirate can be taken or a biopsy can be performed.

The advantage of a bone aspirate is that it is done under sedation and is less likely to cause a fracture in the bone. Unfortunately, it is not always conclusive and can only confirm the presence of a sarcoma, not what type it is. One test that may help in this case is a special stain, recently developed, which can be applied to the aspirate to show if alkaline phosphatase is present. If it is, OSA is confirmed. However, alkaline phosphatase is not always present in the bones of dogs with OSA, so negative staining for alkaline phosphatase does not rule out OSA.

A bone biopsy can conclusively confirm OSA, but can also be problematic. Biopsies remove a small portion of bone for examination and this can increase the risk of fracture. The need to “know for sure” should be carefully weighed against the possibility of a serious break.

If a biopsy is done, an experienced surgeon should perform the procedure, because it is common to see biopsies taken from the edge, rather than the middle, of the lesion. This mistake leads to a misdiagnosis of OSA in 50% of cases. It is also important to make sure that the tract the bone aspirate created and/or the site of the bone biopsy are either removed in a later surgery or included in the radiation field, during later treatments.

Whether the limb is removed pre- or post-biopsy, it should also be submitted for analysis. The more tissue the pathologist has to look at, the more thorough his evaluation can be.

Whether you forgo an aspirate and biopsy or not, definitely run staging tests before starting chemotherapy, radiation treatments or even, an amputation. Three-view X-rays of the chest cavity should be taken, to look for lung metastasis. Ten to fifteen percent of dogs with OSA have detectable spread to the lungs when they are diagnosed and it is important to accurately assess the situation. The prognosis worsens significantly with detectable metastasis on the chest X-rays; then, you may reconsider putting your dog through an amputation.