Tumour Staging In Pets
When your vet makes a diagnosis of cancer, it can be a scary time. You are likely to have a lot of questions about what can be done, and what to expect in the future. To enable us to give you all the options and appropriate advice, we need three key pieces of information:
- What is the tumour type?
- How is the tumour likely to behave? (known as the grade)
- How extensive is the tumour? (known as staging)
The first step in treating a cancer is to know exactly what type of tumour we are dealing with. Sometimes this can be found using a fine needle aspirate, but more often a tissue biopsy is needed. When we submit a piece of the tumour to the pathologists (experts in tissue analysis), they can look at the type of cells present and how those cells are interacting with each other. This allows them to determine not only the type of tumour present, but also valuable information such as how the tumour is likely to behave. This is often referred to as the tumour grade; less aggressive tumours are referred to as low grade, while more aggressive tumours are referred to as high grade.
Commonly, your vet will recommend further testing before deciding on a treatment plan; this is known as staging. Different tumour types behave in different, but often predictable, ways. Once we know the tumour type and grade, we can predict if the tumour is likely to spread and, if so, where it is most likely to spread to. Tumour spread is known as metastatic disease (or “mets”). Knowing whether the tumour has spread helps us decide on treatment, and estimate prognosis. We want to know the likelihood of achieving a cure with surgery, and whether we may need additional treatment, such as chemotherapy or radiotherapy. Some tumours do not spread, in which case it may be appropriate to proceed straight to surgical removal.
Tumours metastasise (spread) when cancer cells leak into the blood, or into the lymphatic system. Although cells can theoretically end up anywhere in the body, these two routes tend to transfer cells to predictable locations. For the majority of tumours, we see mets develop in the lymph nodes, lungs, liver and spleen, so these are the areas we routinely check.
Metastases in the lymph nodes can be detected by sampling the tissue, either with a fine needle aspirate (FNA) or a surgical biopsy. Tumours will spread to the nearest lymph node, so this is where we sample. Occasionally, the nearest lymph node is deep inside the chest or abdomen, so it is not possible to sample it directly.
Metastases in the lungs are screened for using imaging. X-rays are simple to perform, and moderately sensitive for identifying tumour spread. Usually, three views are taken to examine all parts of the lungs. Alternatively, CT imaging can be used to examine the lungs. This is more expensive, and not available in every clinic, but is significantly more sensitive at identifying early spread of disease by detecting much smaller nodules.
Metastases in the liver and spleen can be seen on ultrasound or CT imaging. Some cancers do not form an isolated mass in the liver or spleen, so even in normal looking organs it is recommended to take a small sample of cells by FNA. This carries an extremely small risk of bleeding as the organs are very vascular (have a large blood supply). Most pets will require sedation to allow organ imaging and sampling.
Occasionally, it is recommended to check other sites in the body for metastasis. This is specific to the tumour type diagnosed, but could include blood tests, sampling of bone marrow, sampling of other organs such as the kidneys, or other imaging modalities such as MRI.
