Canine Leukemia | Leukemia in Dogs
In dog leukemias constitute fewer than 10% of all hemolymphatic neoplasms and are therefore considered rare. However, the leukemia to lymphoma ratio is approximately 1:7 to 1:10. This ratio is artificially high, because most dogs with lymphoma are treated by their local clinicians, whereas most dogs with leukemia are referred for treatment. Although most leukemias in dogs are considered to be spontaneous in origin, radiation and viral particles have been identified as possible etiologic factors in dogs with cancer.

Acute myeloid leukemias in dogs are more common than acute lymphoid leukemias, constituting approximately three fourths of the cases of acute leukemia. It should be remembered, however, that morphologically, most acute leukemias are initially classified as lymphoid. After cytochemical staining of the smears or immunophenotyping is performed, approximately one third to one half of them are then reclassified as myeloid. Approximately half of the dogs with myeloid leukemia are found to have myelomonocytic differentiation when cytochemical staining or immunophenotyping is performed.

Leukemias in dogs are malignant neoplasms that originate from hematopoietic precursor cells in the bone marrow. These cells are unable to undergo terminal differentiation, therefore, they self-replicate as a clone of usually immature (and non-functional) cells. The neoplastic cells may or may not appear in peripheral circulation, thus the confusing term aleukemic and subleukemic are used to refer to leukemias in which neoplastic cells proliferate within the bone marrow but are absent or scarce in the circulation.
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Canine osteosarcoma treatment

A novel therapeutic approach for canine osteosarcoma treatment consists of sparing the limb in affected dogs. Instead of amputation, the affected bone is resected and an allograft from a cadavar is used to replace the neoplastic bone; novel biomaterials are also currently being investigated for this purpose. This canine osteosarcoma treatment means that dogs are also treated with intravenous cisplatin, carboplatin, or doxorubicin and, in general, have almost limb function. The main complication is the development of osteomyelitis in the allograft; if that occurs, the limb frequently needs to be amputated. Survival times in animals undergoing this dog osteosarcoma treatment are comparable to those in dogs that undergo amputation plus chemotherapy, the added benefit to the owners of having a four-legged pet.

If owners are reluctant to allow the clinician to amputate te limb, local radiotherapy plus cisplatin, carboplatin, or doxorubicin may be of some benefit. However, in our limited experience, most dogs are eventually euthanized within 3 to 4 months of the initial diagnosis because of the development of pathologic fractures (i.e., after radiotherapy, the tumor is not as painful, therefore the dog regains normal use of the limb and fractures the area), osteomyelitis, or metastasic lesions.

A more conventional canine osteosarcoma treatment is chemotherapy. Chemotherapy apparently modifies the biologic behavior of the tumor, resulting in a higher prevalence of bone metastases and a lower prevalence of pulmonary metastases. Moreover, the doubling time (growth rate) of metastatic lesions appears to be longer than in untreated dogs. Therefore, surgical removal of the metastatic nodules (metastasectomy) followed by additional cisplatin or carboplatin therapy may be recommended for a dog that has been treated with chemotherapy after amputation of the limb and in which one to three pulmonary metastatic lesions are detected.

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Primary bone neoplasms are relatively common in dogs. Most primary bone tumors in dogs are malignant in that they usually cause death as a result of local infiltration (e.g., pathologic fractures, or extreme pain leading to euthanasia) or metastasis (e.g., pulmonary metastases in osteosarcoma). Neoplasms that metastasize to the bone are rare in dogs; some that occasionally metastasize to bones in dogs are transitional cell carcinoma of the urinary tract, osteosarcoma of the appendicular skeleton, hemangiosarcoma, mammary adenosarcoma, and prostatic adenocarcicoma.

Osteosarcomas (OSA) are the most common primary bone neoplasm in dogs. They can affect either the appendicular or axial skeletons, and they occur primarily in large-breed (and giant-breed) middle age to older dogs. Their biologic behavior is characterized by aggressive local infiltration of the surrounding tissues and rapid hematogenous dissemination (usually to the lungs). Although historically it was believed that osteosarcomas of the axial skeleton had a low metastatic potential, it now appears that their metastatic rate is similar to that of the appendicular osteosarcomas in dogs.

Appendicular osteosarcomas in dogs occur predominantly in the metaphyses of the distal radius, distal femur, and proximal humerus, although other metaphyses can also be affected. As just mentioned, they typically affect male dogs of large and giant breeds, and owners seek veterinary care because of lameness or swelling of the affected limbs. Physical examination usually reveals a painful swelling in the affected area, with or without soft tissue involvement. The pain and swelling can be acute in onset, leading to the presumptive diagnosis of a non neoplastic orthopedic problem and thus considerably delaying diagnosis and definitive therapy for the neoplasm.

The treatment of choice for dogs with osteosarcoma is amputation with adjuvant single-agent or combination chemotherapy. The median survival time in dogs with appendicular osteosarcoma treated with amputation alone is approximately 4 months, whereas in dogs treated with amputation and cisplatin, amputation and carboplatin, or amputation and doxorubicin it is approximately 1 year. The dosages and the recommended ways of administring chemotherapy for dogs with osteosarcoma are ruled by cancer Therapy Protocols. However, the cost of carboplatin therapy is quite high (approximately $3.00 per milligram of drug, or roughly $30.00/kg of body weight).

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Lymphoma (malignant lymphoma, lymphosarcoma) is a lymphoid malignancy that originates from solid organs (e.g., lymph nodes, liver, spleen); this distinguishes lymphomas from lymphoid leukemias, which originate in the bone marrow.

It has been reported that approximately 70% of cats with lymphoma have feline leukemia virus (FeLV) infection. Although the prevalence of viremia in cats with lymphoma varies with the anatomic form of presentation, in general, young cats with lymphoma are FeLV positive, whereas older cats are FeLV negative. However, over the past few years, the prevalence of FeLV infection in cats with lymphoma has been decreasing.

Feline immunodefiency virus (FIV) infection increases the risk of developing lymphoma in cats; cats infected with FIV are almost six times more likely to develop lymphoma than non-infected cats, whereas cats coinfected with FeLV and FIV are more than 75 times more likely to develop lymphoma than noninfected cats.

The age of cats with lymphoma at the time of presentation is bimodal, with the first peak occurring in cats that are approximately 2 years of age and the second one occurring in cats that are approximately 10 to 12 years of age. The cats that make up the first peak are mainly FeLV positive, whereas those that make up the second peak are predominantly FeLV negative.

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Cancer in dogs - Hemangiosarcomas

Hemangiosarcomas are malignant neoplasms that originate from the vascular endothelium. They occur predominantly in older dogs (8 to 10 years of age) and in males; German Shepherds and Golden Retrievers are at high risk for this type of dog cancer. The spleen, right atrium, and subcutis are common sites of involvement at the time of presentation.

In general, the biologic behavior of cancer in dogs is highly aggressive, with most anatomic forms of the tumor infiltrating and metastasing early in the disease. The only exception is that of primary hemangiosarcomas, which have a lower metastatic potential than the tumors that originate in subcutaneous tissues.

The nature of owner's complaints and the clinical signs at presentation are usually related to the site of the primary tumor; to the presence or absence of metastatic lesions; and to the development of spontaneous tumor rupture, coagulopathies or cardiac arrhythmias. Dogs with cardiac hemangiosarcomas, regardless of the primary location or stage, are anemia and spontaneous bleeding. The anemia is usually the result of intracavitary bleeding or microangiopathic hemolysis .

Hematologic abnormalities in dogs with cancer have been well-characterized and include anemia, thrombocytopenia; the presence of nucleated red blood cells, and acanthocytes in the blood smear, and leukocytosis with neutrophilia, a left shift, and monocytosis. In addition, hemostatic abnormalities are also common in dogs with cancer.

In summary, hemangiosarcomas are usually diagnosed on the basis of historical, physical examination, and clinicopathologic findings, in conjunction with ultrasonographic and radiographic changes. A morphologic diagnosis can usually be made on the basis of cytologic or histopathologic findings. Although surgery is the preferred cancer treatment in dogs, survival times in such animals is extremely short. Postoperative adjuvant chemotherapy using doxorubicin-containing protocols prolongs survival in regarding cancer in dogs.

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There are several important treatment-related factors to be considered when planning cancer treatment in dogs. First, the specific indication should be considered. Surgery, radiotherapy, and hyperthermia are dog cancer treatments aimed at eradicating a locally invasive tumor with a low metastatic potential (and potentially curing the animal), although they can be used palliatively in dogs or cats with extensive disease or in those with metastatic disease.

On the other hand, chemotherapy usually does not constitute a curative treatment, although palliation of advanced disease can easily be accomplished for several tumor types. Immunotherapy (the use of biologic response modifiers) also constitutes an adjuvant or palliative approache (i.e. tumors are not cured by immunotherapy alone). In general it is best to use aggressive dogs and cats cancer treatments when the tumor is first detected (because this is when the chances of eradicating every single tumor cell are the highest) rather than to wait until the tumor is in an advanced stage - that is, to "treat big when the disease is small".

In most cases, the highest success rates are obtained by combining two or more dog cancer treatment modalities. For example, the combination of surgery and chemotherapy (with or without immunotherapy) has resulted in a significant prolongation of disease-free survival in dogs with osteosarcoma of the appendicular skeleton and in dogs with splenic hemangiosarcoma. Similarly, the combination of radiotherapy and hyperthermia has resulted in a prolongation of disease-free survival in dogs with fibrosarcoma of the oropharynx.

Dogs and cats cancer treatment can be either palliative or curative. Given the current paucity of information regarding specific tumor types and treatments, it is also possible that these two approaches will sometimes overlap (i.e, a treatment initially thought to be palliative may result in cure or vive versa). Every effort should be made to eradicate every single cancer cell in the body shortly after diagnosis. This means taking immediate action rather than wait-and-see attitude.

With very few exceptions, malignancies do not regress spontaneously. therefore, by delaying treatment, in dogs and cats with confirmed malignancy, one is only increasing the probability that the tumor will disseminate locally or systemically, thereby decreasing the likelihood of a cure.

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