Metastasis

Metastases are daughter tumours of a primary tumour. They detach from the site of origin and spread via the blood or lymphatic vessels to other body regions. Metastases indicate that cancer is advanced and that the tumour has the ability to metastasise. Their detection worsens the prognosis and increases the complexity of treatment.

What are metastases?

Metastases occur when cancer cells from the original (primary) tumour migrate to other body regions and multiply there. This process is characteristic of malignant tumours (carcinomas, sarcomas) and distinguishes them from benign tumours, which do not metastasise.

The most common organs to which metastases spread in dogs are:

• Lungs
• Liver
• Lymph nodes
• Bones
• Spleen

Whether a tumour metastasises depends on its type and aggressiveness. Some types of tumour spread more frequently than others.

How do metastases develop?

Metastases develop in several steps:

1. Detachment of cancer cells from the primary tumour: Cancer cells separate from the original tumour and enter the blood vessels or lymphatic vessels.
2. Transport through the body: The cancer cells are transported to other parts of the body via the blood or lymphatic vessels.
3. Attachment at a new site: The cancer cells attach themselves at a new site, leave the blood or lymphatic vessels and penetrate the surrounding tissue.
4. Formation of a new tumour: At this new site, the cancer cells begin to grow and form a secondary tumour (metastasis). This tumour is genetically identical to the primary tumour.

Tumour types that frequently metastasise in dogs

Not all tumours metastasise, but some cancers in dogs are more prone to doing so. The tumour types that frequently form metastases include:

1. Mast cell tumour: Mast cell tumours are common skin tumours in dogs that can spread to the lymph nodes, spleen, liver and other organs in advanced stages.
2. Osteosarcoma: A malignant bone tumour that frequently metastasises to the lungs. Osteosarcomas mainly affect large dog breeds.
3. Lymphoma: Lymphomas are cancers of the lymphatic system that can spread to lymph nodes, liver, spleen and other organs.
4. Melanoma: Melanomas frequently occur in the mouth or on the paws and can metastasise to the lungs, liver or lymph nodes.
5. Mammary tumour: Malignant tumours of the mammary glands (breast cancer) in bitches tend to metastasise to the lungs, lymph nodes and liver.

Symptoms of metastases in dogs

The symptoms depend on where the daughter tumours are located in the body. Common signs of metastasis include:

• Lung metastases: Coughing, shortness of breath, rapid breathing, reduced exercise tolerance.
• Liver metastases: Weight loss, loss of appetite, vomiting, jaundice (yellow discolouration of the skin or mucous membranes).
• Bone metastases: Lameness, pain, swelling in the bones.
• Lymph node metastases: Enlarged, palpable lymph nodes, often located under the jaw, in the armpit or in the groin area.
• Spleen metastases: Abdominal swelling, anaemia, weakness.

However, many dogs only show symptoms at a late stage of the disease, which makes diagnosis and treatment more difficult.

Diagnosis of metastases

To diagnose metastases, the vet usually combines several examination methods:

1. X-ray: Chest X-rays are frequently used to look for lung metastases, as the lungs are a common target organ for many tumours.
2. Ultrasound: An abdominal ultrasound can make metastases in the liver, spleen or other abdominal organs visible.
3. CT or MRI: Computed tomography (CT) or magnetic resonance imaging (MRI) provide detailed images of organs or bones and detect metastases that may not be visible on X-rays.
4. Fine-needle aspiration or biopsy: If lymph nodes or tumours are suspicious, a fine-needle aspiration or tissue biopsy can be performed to take a tissue sample and examine it microscopically.
5. Blood tests: Blood tests can provide information about the function of organs such as the liver and kidneys and give indications of cancer spread.

Treatment of Metastases in Dogs

The treatment options depend on the type of cancer, the location of the metastases, the dog’s general condition and the prognosis. Common options include:

1. Surgery: If the metastases are locally limited and surgically accessible, an operation may be considered. This is often possible with individual lung or lymph node metastases.
2. Chemotherapy: Chemotherapy is used to fight cancer cells throughout the body, especially when metastases are present. It can be effective for certain types of cancer such as lymphomas, osteosarcomas or mast cell tumours.
3. Radiation therapy: Radiation therapy aims to destroy tumour cells, particularly in cases of local metastases or tumours that cannot be operated on. It is frequently used for bone tumours or tumours in the mouth area.
4. Palliative therapy: If the metastases cannot be cured, palliative treatment can improve the dog’s quality of life. This includes painkillers, anti-inflammatory drugs and palliative chemotherapy to slow tumour growth and alleviate symptoms.

Prognosis for Metastases

The prognosis depends heavily on the type of primary tumour, the spread of metastases and the dog’s general condition. Metastases often indicate that the cancer is advanced, in which case the prognosis is often unfavourable. Some dogs can live for several months to years with targeted therapy, especially if the tumours respond well to chemotherapy.