Cancer in dogs: symptoms, tests and treatment

Overview

Even a benign tumour can grow large and interfere with nearby tissue function. Malignant cells can travel elsewhere through blood or lymphatic vessels. Cancer growing at a new site is called a metastasis. Similar-looking lumps can be cysts, inflammatory lesions or tumours, for example.

Four pathology laboratories recorded 54,986 canine tumours in Switzerland between 2008 and 2020. Malignant tumours occurred at 338 per 100,000 dog-years. This Swiss figure describes laboratory-diagnosed tumours, and true incidence may be higher.

Benign and malignant tumours together occurred at 775 per 100,000 dog-years. Tumours were diagnosed in 42,743 dogs, some with more than one. Of all tumours, 34.64% were in the skin.

Causes

Disrupted control of cell growth

Cancer involves changes in genes regulating cell growth. An affected cell may keep dividing when tissue does not need new cells. Inherited predisposing factors have been identified in some breeds. Prolonged or intense UV exposure can promote some skin tumours.

The precise cause of an individual dog's tumour often remains unclear. Leonberger osteosarcoma has been associated with a chromosome 11 region containing CDKN2A and CDKN2B.

Cancer type depends on its tissue of origin

Carcinoma arises from cells lining tissue surfaces or forming glands. Sarcoma can arise from bone or other connective tissue. Lymphoma is cancer of lymphoid tissue and may cause enlarged lymph nodes. A tumour can also originate in an internal organ, such as a kidney.

Leukaemia is malignant disease of blood-forming cells and may occur without a visible lump.

Melanoma in the mouth or skin can sometimes spread rapidly.

Risk factors

Cancer becomes more likely as dogs age. In the Swiss registry study, malignant tumour incidence in dogs aged eight to 11 was around 18.2 times that in dogs aged zero to three, after accounting for sex and breed group. Prolonged exposure to oestrogens and progestogens is associated with increased mammary tumour risk.

Inherited osteosarcoma susceptibility has been found in Leonbergers. In one breed health register, 197 of 838 Leonbergers reported dead between 2012 and 2021, or 23.5%, had tumour disease or cancer recorded as the cause. These were reported data, with no cause supplied for 87 dogs during the same period. Average lifespan among the Leonbergers dying from tumour disease or cancer was seven years and ten months.

Median age at tumour diagnosis in the Swiss dogs was nine years. The largest number of tumours was recorded in ten-year-olds.

Typical symptoms

Cancer can appear as a lump or cause organ-specific symptoms depending on location.

• A lump on or beneath the skin growing or changing.
• Enlarged lymph nodes.
• A non-healing sore on the skin or in the mouth.
• Weight loss or reduced appetite.
• Difficulty eating or swallowing with an oral lesion.
• Persistent lameness or stiffness associated with tumour pain.
• Reduced stamina on walks.
• Unexplained bleeding from the mouth, nose or anus.

A persistent dry cough can be a cancer symptom. The vet may find a lesion beneath the skin or in an organ by palpation or imaging.

A gastrointestinal tumour may cause reduced appetite. Unusually bad breath is another possible sign of cancer.

Complications and consequences

A splenic tumour can rupture and bleed into the abdomen. An invading tumour can damage neighbouring healthy cells. Bone cancer can cause severe pain. Cancer may spread to a nearby lymph node or a distant organ, for example. Confirmed metastasis often changes the outlook and can affect treatment choice.

Disease and treatment can reduce appetite enough to cause muscle and weight loss. Marrow suppression can predispose to infection if infection-fighting neutrophils decrease. Intensive radiotherapy can irritate skin or mucosa in the treated area.

Chemotherapy may cause adverse effects in an estimated 15–30% of dogs. Common mild effects include vomiting, diarrhoea and tiredness. Some chemotherapy medicines can temporarily suppress marrow function.

Late radiotherapy effects may appear more than a year after treatment. Hair in the treated area may later become paler or remain permanently thinner.

Diagnosis at the vet

Cell and tissue samples

The vet feels the lump's size and assesses its mobility relative to surrounding tissue. Fine-needle sampling collects cells for microscopy. It can often be done without sedation. A cell sample may identify a mast cell tumour or lymphoma, for example.

A pathologist can assess tissue invasion in a biopsy. Grade describes microscopic tumour characteristics. Removed tissue can be checked for tumour cells reaching the surgical surface.

If a fine-needle sample provides insufficient information, a biopsy can follow. Tissue also allows assessment of dividing cell numbers. If routine histology cannot identify cell type, special stains or antibody-based staining can be used.

A biopsy from only part of a mass can help plan definitive removal.

Spread and general health

Staging assesses local extent and looks for metastases. Even a normal-sized lymph node can contain tumour cells. Fine-needle sampling or biopsy can investigate lymph node spread. A blood count, biochemistry and urine testing help assess general health before treatment.

Ultrasound or CT can assess an internal tumour's extent. With limb bone cancer, chest X-rays can look for lung metastases. Imaging and sampling are tailored to tumour type so findings guide prognosis or treatment.

Inflammation can also enlarge lymph nodes, so sampling helps establish the cause. Suspected bone involvement can be assessed with X-rays or CT to establish its extent.

Treatment

Surgery, medication and radiotherapy

A localised tumour can be removed surgically if location and general health allow. A rim of healthy tissue is often removed to reduce local recurrence risk. Chemotherapy can treat canine lymphoma, for example. It may also follow surgery if microscopic spread is suspected.

Radiotherapy targets a defined area, damaging tumour cells with radiation. Palliative radiotherapy can relieve bone tumour pain, for example. Local and medical treatments may be combined according to tumour type.

Tumour cells at a surgical margin often mean greater local recurrence risk. Depending on type, chemotherapy can slow progression and extend symptom-free time. Radiotherapy needs specialist equipment and is usually given at a specialist centre.

When tumour remains at a margin, wider removal around the scar may be considered. Postoperative radiotherapy can help treat residual local cells.

Supportive care and monitoring

Symptomatic treatment can relieve pain and improve daily life during treatment. Treating nausea can support appetite. Persistent poor intake may require tube feeding. Nutritional status is monitored from diagnosis throughout treatment.

After certain chemotherapy medicines, neutrophils are usually lowest around seven days later, when a blood test can be useful. Low-grade tumours are often monitored every four to six months. High-grade tumours may need checks every one to three months.

Severe dehydration from chemotherapy side effects may need hospital fluids. Some cases need 24–48 hours of hospital supportive care. Other diseases, such as kidney disease or pancreatitis, are considered in the diet.

Prevention

Protecting pale-skinned dogs from intense sunlight can reduce UV-associated skin cancer risk. A smoke-free environment reduces tobacco smoke exposure. In follow-up of 120 Scottish Terriers, bladder cancer odds were around six times as high in smoking households.

Spaying before the first season may reduce mammary tumour risk. Timing also considers breed and other health risks. Regular health checks help find tumours as dogs age.

Castration removes the testes and thereby reduces testicular cancer risk.

Important for dog owners

Book a vet visit for a new or changing lump. Bring previous records if another practice has already examined your dog. Agree beforehand how appetite, mobility and other daily functions will be assessed during treatment. Contact emergency care immediately if a dog receiving chemotherapy develops fever and deteriorates. Some kennel club breeding databases allow owners to record a dog's date and cause of death.

Report vomiting or diarrhoea after chemotherapy so supportive care can be provided when needed.

Before treatment, discuss its aims and what to do if adverse effects occur.

Frequently asked questions

Is every lump cancer?

A lump can be a cyst, abscess, benign tumour or malignant tumour. Fine-needle sampling often identifies tumour type. If cytology is inconclusive, a biopsy can establish the tissue of origin more precisely.

What do grade and stage mean?

Grade is assessed from tissue, examining tumour structure and cell division. Stage describes extent of growth and whether metastases are present. These findings guide treatment once tumour type is known.

Can a dog enjoy life during cancer treatment?

Maintaining quality of life is a major treatment goal. Nausea management can improve eating during chemotherapy. If intake remains inadequate, the vet assesses nutritional support such as a feeding tube.

Will chemotherapy cause hair loss?

Chemotherapy can damage rapidly dividing follicular cells and cause hair loss. This is more likely in continuously growing coats, such as Poodles'. Hair usually regrows after treatment, but colour or texture may change.

How is recurrence monitored?

Intervals depend on tumour type, grade and previous treatment. Visits look when needed for local recurrence, new metastases or late treatment effects. Tests may include general examination, blood or urine tests and X-rays.