Cancer is one of the most difficult diagnoses a pet owner can face. In dogs, cancer can take many forms and vary widely in behaviour, severity, and prognosis. Early detection and accurate staging — determining how far cancer has spread — are critical when deciding on the most appropriate treatment plan.
One of the most advanced tools available for detecting and evaluating cancer spread in dogs is the computed tomography (CT) scan. At Noah.ie, pets have been undergoing advanced CT imaging for over 10 years. This article explains how cancer spreads in dogs, what CT scans can reveal, and how veterinarians use this technology to guide treatment decisions.
1. What Is Cancer in Dogs?
Cancer occurs when cells grow uncontrollably, forming abnormal masses known as tumours. These tumours may be:
- Benign: Non-cancerous and unlikely to spread
- Malignant: Cancerous, with the ability to invade nearby tissues and spread to distant organs
Malignant tumours can metastasise by travelling through the bloodstream or lymphatic system. Common cancers seen in dogs include:
- Lymphoma – cancer of the lymphatic system
- Osteosarcoma – bone cancer
- Hemangiosarcoma – cancer of blood vessel walls
- Mast cell tumours – skin or soft tissue cancer
- Melanoma – often affecting the mouth or skin
- Carcinomas – cancers of epithelial tissues such as lungs, liver, or mammary glands
Each cancer type behaves differently, making accurate staging essential for selecting the most effective treatment.
2. How Cancer Spreads (Metastasis) in Dogs
Cancer spread usually follows predictable biological pathways:
a. Lymphatic Spread
Cancer cells can enter lymphatic vessels and travel to nearby lymph nodes. From there, they may spread further to other nodes or organs. Assessing lymph node involvement is a key part of cancer staging.
b. Hematogenous (Blood-Borne) Spread
Some tumours, such as osteosarcoma and hemangiosarcoma, spread through the bloodstream to organs like the lungs, liver, spleen, or kidneys. Detecting small metastatic lesions in these organs is critical for prognosis.
c. Local Invasion
Malignant tumours can directly invade surrounding tissues, muscles, or bone. For example, nasal tumours may extend into the sinuses or cranial cavity. Identifying the full extent of local invasion is vital before surgery or radiation therapy.
Because metastatic spread can be microscopic, it is not always visible on standard imaging such as X-rays — this is where CT scans play a crucial role.
3. Diagnostic Imaging in Canine Cancer
Veterinarians use several imaging modalities when assessing cancer in dogs:
- X-rays: Useful for identifying large tumours and bone changes but limited for small lesions
- Ultrasound: Helpful for evaluating soft tissues such as the liver or spleen
- MRI: Excellent for brain and spinal cord tumours
- CT scans: Provide cross-sectional, high-detail images ideal for cancer staging
CT scanning has become a cornerstone of modern veterinary oncology, particularly for evaluating local invasion and distant metastasis.
4. What Is a CT Scan?
A CT scan uses X-rays and computer processing to create detailed, three-dimensional images of a dog’s body. It allows veterinarians to view internal structures layer by layer, revealing subtle abnormalities invisible on standard radiographs.
How a CT Scan Works
- The dog lies on a motorised table that moves through a donut-shaped scanner
- The scanner rotates, taking multiple X-ray images from different angles
- A computer reconstructs these images into a detailed 3D model
- An intravenous contrast dye is often used to highlight blood vessels, tumours, and organs
CT scans are usually performed under general anaesthesia or heavy sedation to prevent movement and ensure image clarity.
5. The Role of CT Scans in Detecting Cancer Spread
a. Detecting Lung Metastases
The lungs are one of the most common sites of metastatic spread in dogs. While X-rays may miss nodules smaller than 3–5 mm, CT scans can detect lesions as small as 1 mm. This dramatically improves staging accuracy, especially for cancers such as osteosarcoma, melanoma, and mammary carcinoma.
b. Assessing Lymph Nodes
CT imaging allows clear visualisation of lymph nodes, identifying enlargement or abnormal inter
