Bone Cancer in Dogs
Obtaining Bone Specimens (Bone Biopsy)
There are several methods used to obtain biopsy specimens: fine needle aspiration and bone biopsy. Several specimens should be obtained to increase the yield of a positive result. The biopsies should be taken from the center of the lesion, since bone cancer grows from the inside of the bone (medullary space) to the outside (cortex).
Fine needle aspiration obtains cells from the tumor using a needle and syringe. It is a fast, reliable and efficient way of obtaining bone specimens for diagnosis. It is minimally invasive compared to other methods. The success of this procedure largely depends on the amount of bone destruction. A recent study in the Journal of Veterinary Internal Medicine found that fine-needle aspiration cytology was as accurate as histologic biopsy for diagnosing destructive bone lesions in dogs.
Bone biopsy obtains a core sample of bone and can be performed either using a trephine or removing a wedge of bone (known as “open technique”) or a specific type of needle called Jamshidi (known as “closed technique”).
The open method allows for removal of a good-sized piece of tissue, which increases the chances of an accurate diagnosis to approximately 90 percent. Bone biopsy requires general anesthesia and can be associated with bruising on the bone, wound infection, seeding of the surgical site with tumor cells and pathologic fractures. If an open incisional biopsy is planned, the incision should be positioned so that the entire area can be completely removed if amputation is later scheduled.
The closed needle (Jamshidi needle) technique has lower incidence of post-operative complications. Even though it provides a smaller specimen, a quick, accurate diagnosis can be made in up to 90 percent of cases. There might be a slightly lower incidence of pathologic fractures with the Jamshidi needle.
Regardless of technique, the biopsy can be “negative” or “non-conclusive.”
Reasons for this include:
- Sample was obtained from reactive bone.
- Not enough samples were submitted.
- The tumor is poorly differentiated and cannot be diagnosed on a small biopsy specimen.
- There is necrosis of the bone.
- The sample was crushed or squeezed during handling, or there was excessive decalcification of the bone specimen.
Because the bone biopsy will not diagnose 100 percent of the cases, and many cases can be managed without a biopsy, this procedure is not always necessary. If the diagnosis is clear, and the dog is a candidate for amputation, the information needed from the specimen can be obtained after the amputation surgery.
If the diagnosis is obvious based on x-rays, and the dog is not a candidate for amputation because of other health problems, or because of the location of the tumor, then biopsies are usually not needed to determine an alternative cause of treatment.
However, in cases where the diagnosis is not clear based on the appearance of the bone lesion, or when more than one lesion is present (which is uncommon in primary bone cancer), then a biopsy or additional testing may be recommended to actually help clarify the diagnosis. These decisions should be discussed with the primary veterinarian or with the consulting oncologist.
Understanding the Biopsy Report
Mitotic index – The higher the number of mitoses (cell reproduction), the faster the tumor is growing at the cellular level (low index is good).
Well-differentiated malignant cells – A lot of “parts.” The cells are more complicated and have more stuff in them. The more differentiated a cell is, the longer it takes to reproduce, so tumors with well differentiated cells don’t grow as fast.
Poorly-differentiated malignant cells – Not a lot of “parts.” The cells are less complicated and do not have a lot of stuff in them so they can reproduce faster, which makes the tumor grow faster. If you choose not to have a biopsy, and you do proceed with amputation, make sure the limb is sent for post-amputation pathology examination and biopsy.