What is metastasis?
Metastasis is the presence of cancer tissue away from the original (primary) site of the tumour. It occurs when cancer cells break away from the primary tumour, enter the bloodstream or the lymphatic system and form new tumours in other organs. It’s a bit like sowing seeds – some may take and grow others may not.
The new tumours are called secondaries and they will have the same type of abnormal cells and the same name as the primary tumour. For example, if bone cancer spreads to the lungs, the cancer cells in the lungs are actually bone cancer cells. The disease is called metastatic bone cancer and not lung cancer.
The new or secondary tumour is called ‘distant’ or metastatic disease.
What is a secondary tumour?
A secondary tumour is a mass of cancer cells separate from the primary tumour that is a result of spread called metastasis.
The type of primary tumour helps to predict the location and risk of a patient developing secondary tumours. Imaging studies such as X-rays, ultrasound scans and CT or MRI scans can be used to look for secondaries.
However, even if no secondaries are initially seen, this is no guarantee that they will not develop at a later date. A secondary tumour starts as a small ball of cells less than 100th of a millimetre (<0.01 mm) and it must grow to a size where it can be detected.
- A mass as small as 1-2 mm is visible on a CT scan
- A mass needs to be about 5-10 mm or larger to be visible on a radiograph (x-ray)
Secondaries can occur when the original primary tumour is still small (Figure 1). Sometimes the presence of secondaries are the only indication that the patient has cancer and it can be hard to find the primary tumour which may be very small.
Secondaries that appear after a primary tumour has been surgically removed will have been present at the time of surgery but were just not detected at that time as they were too small to find. For tumours with a high likelihood of secondary spread, anticancer drugs (chemotherapy) or radiation therapy may be recommended after surgery to delay or even prevent secondaries. However, not all tumours are sensitive to these treatments.
Why is metastasis important?
Metastasis occurs with malignant tumours; however, not all malignant tumours will undergo secondary spread. Some cancers, often referred to as low grade, have a low metastatic rate and this means that only a small percentage actually spread.
For example, malignant tumours arising from fibrous tissue in the skin of dogs (known as fibrosarcomas) are generally low grade and <15% will metastasise. At the other extreme there are cancers with a high metastatic rate where most will spread. Bone cancers in dogs (osteosarcoma) are ‘high grade’ and at least 95% will metastasise.
However, it is not always possible to predict which patients will experience spread of their disease. Metastatic spread is the most difficult type of cancer to treat and, as the metastases grow, they take over more of the patient’s normal organs.
For example, secondary tumours growing in the lungs will reduce the function of the lungs such that the patient cannot get enough oxygen and tires easily. As the tumours grow further, coughing and general illness may also become apparent. The clinical effects of the spread will also depend on where they are growing. Patients with spread close to the main airways may cough even when the tumours are relatively small, whilst others with spread to the lung tissue may have no clinical signs until the lungs are almost completely taken over by tumour tissue.