Cancer in Dogs and Cats: Staging

Staging of cancer is a process of determining where that cancer exists in the patient.  It is anCancer in Dogs and Cats: Staging essential part of pretreatment evaluation of animals with cancer to make informed and practical decisions regarding the best type of treatment for the patient.  Often times, the term tumor stage is confused with the term tumor grade.  Tumor grade is not taken into consideration when staging cancer.  Tumor grade is a histological determination of the degree of malignancy of the tumor microscopically.  Tumor stage, on the other hand, is a description of the extent of cancer involvement in the patient.  For many tumors, both the grade and stage of the cancer have prognostic implications.

The World Health Organization has developed internationally accepted rules used in the staging of cancer.  Generally, a number and letter is assigned indicating the extent of the disease in the patient where stage I (one) disease is less advanced than stage IV (four) disease.  Different staging systems exist that are specific for the wide range of tumor types that present in people and animals.  Most involve and evaluation of the primary tumor site (T), the presence of absence of lymph node metastasis (N), and the presence or absence of distant metastasis to other organs, such as the lungs (M).  This system is referred to as the TNM staging system.

The TNM system largely applies to staging of solitary tumors.  Cancers such as lymphoma present as a systemic disease (throughout the body) and use of the TNM system of staging does not apply easily.  For cancers such as lymphoma, the disease is staged according to the organ systems that are involved.  For example, dogs with stage III (three) lymphoma have involvement of multiple peripheral lymph nodes which are enlarged with the disease, where as dogs with lymphoma involving the liver and/or spleen have stage IV (four) lymphoma.  A substage disease category exists in which substage-a reflects that the animal is not showing signs of illness from the cancer, whereas substage-b reflects that the animal is showing overt signs of illness from the cancer.

Evaluation of general health in the staging of cancer

Staging of cancer in animals should also involve evaluation of the general health of the patient.  cancer most commonly affects older animals, which are more likely to have concurrent health issues.  If other health issues exist, they may impact an owner’s decision of whether or not to treat the cancer.  Some cancer treatments involve complicated surgery or radiation therapy that involves long or multiple anesthetic procedures where adequate cardiac (heart), renal (kidney) and liver function are necessary.  Certain chemotherapy drugs require dose adjustments if adequate cardiac, renal or liver function is not present. Therefore, if other health issues exist, they may impact the treatment advised by the veterinarian.

The importance of staging of cancer

Staging of cancer is important in determining what the best treatment is for the individual patient with cancer.  If cancer is localized to one area, surgery and/or radiation therapy are the most commonly recommended form of treatment for the cancer as they are both local forms of treatment.  If the cancer has spread to the lymph node or other organs, systemic treatment in the form of chemotherapy is indicated.  Information obtained from staging of cancer allows informed and rational decisions regarding what the best treatment is for the patient that will maximize the effectiveness and minimize the toxicity of treatment.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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