Cancer in Dogs and Cats: TNM Staging

Staging of cancer involves the process of determining where in the patient the disease exists.  TheCancer in Dogs and Cats: TNM Staging system was developed to describe how advanced the cancer is in dogs and cats.  This system, often referred to as the TNM system, takes into account the nature of the tumor at the primary site of involvement (T), whether or not the cancer has spread to the regional lymph nodes (N), and whether or not there is spread of the cancer to other organs (M).  Based on the status of the patient’s disease at these sites, the patient is placed into different stages of the cancer where stage I (one) is less advanced disease compared to higher stages II, III, and IV (two, three, and four).  Different staging systems exist for various tumor types that vary in ways that best describe the particular tumor type, but are typically based on a TNM system of categorization.  Before staging can occur for cancer in dogs and cats, a diagnosis must be made.

Staging the primary tumor (T)

Staging of cancer with respect to the primary tumor involves an assessment of the tumor based largely on size of the tumor, number of lesions and invasiveness of the tumor.  It also takes into consideration the presence or absence of ulceration, inflammation, and its relationship to other body structures.  Sometimes, the tumor is not easily palpated and additional diagnostic tests are necessary to evaluate the tumor.  These additional tests are often in the form of radiographs or ultrasound evaluation.  Other tests that may be necessary include computer tomography (CT), magnetic resonance imaging (MRI) or other techniques such as endoscopy.  A number is applied to the tumor based mostly on the size of the tumor.

Staging the lymph node (N)

For many cancers, the first site of metastasis of a tumor is the regional (draining) lymph node.  Staging of cancer in the lymph node involves evaluation of the size of the lymph node and its mobility.  Most commonly, lymph nodes that have spread of cancer to them are increased in size.  However, some cancers have such a high rate of metastasis that the lymph node should be evaluated even if it is not enlarged.  Sometimes, the lymph node being evaluated is not able to be palpated on physical exam, and additional tests such as abdominal ultrasound or chest radiographs are necessary to evaluate the size of the lymph node.  Less commonly in veterinary medicine, CT or MRI is used to evaluate the size of the lymph nodes.  Most importantly, a key test in determining if the lymph node has cancer is cytological or histological evaluation of the lymph node.  In general terms, if the lymph node is negative for cancer, it is designated N0, where as if the lymph node is positive, it is designated N1.

Staging for distant metastasis of the cancer (M)

Staging of cancer for distant metastasis is the final step of determining where the cancer is in the body.  It screens for spread of cancer to other organ systems in the body, most commonly, the lungs, liver and spleen.  Other less common sites include the bone and brain.  Some tumor types are more or less likely to spread to other organs systems than others, and may increase or decrease, respectively, the importance of screening for distant metastasis.  For example, most soft tissue sarcomas have a low potential for distant metastasis (<20%), where as oral melanoma has a very high potential for distant metastasis.  Therefore, performing staging procedures for distant metastasis is more crucial for a dog with oral melanoma than a dog with a soft tissue sarcoma.  Staging for distant metastasis most commonly involves chest radiographs to screen the lungs and abdominal ultrasound to screen organs in the abdomen for evidence of metastatic disease.

Click here to learn about what types of cancer dogs and cats can get.

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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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