Lymphoma in dogs

Overview

The generalised, or multicentric, form accounts for around 80–85% of canine lymphomas. It affects lymph nodes throughout the body. Lymphoma can also start outside lymph nodes, for example in skin or the digestive tract. Mediastinal lymphoma can arise from lymph nodes in the chest or the thymus.

Lymphomas are classified partly by whether the tumour cells originate from B or T lymphocytes. The WHO classification recognises over 30 canine subtypes identifiable from tissue samples. Lymphomas of the same WHO subtype may share features such as progression rate and treatment response. Diffuse large B-cell lymphoma is the most common individual subtype, accounting for around 40% of canine lymphomas.

Causes

How tumour cells develop

In lymphoma, tumour cells derived from the same lymphocyte multiply and may replace normal lymph node cells. B lymphocytes contribute to antibody-mediated defence in healthy dogs. T lymphocytes contribute to cell-mediated immunity. Cell division rate is one feature influencing tumour grade. A tissue sample may show tumour cells extensively replacing normal lymph node architecture. Over 75% of canine lymphomas are intermediate- or high-grade large-cell forms.

Genetics and environment

The precise cause is usually unknown in an individual dog. Clustering in some canine families suggests inherited susceptibility. Various genetic alterations have been identified in canine lymphoma cells. Chromosomal changes also occur, with their implications for disease course still partly unclear. No clear causal link between viruses and canine lymphoma has been established.

Risk factors

Risk is greatest in middle-aged and older dogs. However, young dogs can also develop lymphoma. In a study across eight European countries, breed differences varied by country. The study examined 1,529 dogs with nodal lymphoma, comparing breed proportions with other clinic patients in the same countries. Bernese Mountain Dogs had lymphoma odds ratios above three in four of the eight countries. A higher-than-usual proportion of Boxer lymphomas were T-cell. B-cell was more common than T-cell lymphoma in Rottweilers. Increased susceptibility was found in Dobermanns in several participating countries.

In Finnish Hounds, lymphoma has been described as most commonly developing at four to seven years old. In a US study population of 494 dogs, T-zone lymphoma was diagnosed at a median age of 10 years.

Typical symptoms

Behaviour and appetite may remain normal despite clearly enlarged superficial lymph nodes.

• Larger-than-usual lymph nodes may appear beneath the jaw.
• Enlarged nodes may also be felt in front of the shoulders or behind the knees.
• Dogs may tire more easily.
• Digestive tract lymphoma can cause vomiting or diarrhoea.
• Weight may fall as disease progresses.
• Lymphoma in the chest can cause coughing.
• Drinking can increase if blood calcium rises.

Skin lymphoma can cause crusting, ulcerated or weeping lesions. Generalised lymphoma can cause reduced appetite alongside enlarged lymph nodes. Raised blood calcium can also increase urination.

Complications and consequences

Effects on blood and organs

Lymphoma can cause anaemia, sometimes visible as pale mucous membranes. Low platelet numbers can increase bleeding tendency. Substances released by tumour cells, such as parathyroid hormone-related protein, can raise blood calcium. High calcium can be associated with impaired kidney function. It can also affect heart rhythm. Lymphoma growing in the chest can impair breathing by taking up space needed by the lungs.

Treatment side effects

Chemotherapy can reduce neutrophils, white blood cells involved in fighting bacteria. Severe neutropenia can predispose to infection with fever. Bladder inflammation causing blood in urine is one possible chemotherapy side effect. Doxorubicin, used for lymphoma, can damage heart muscle.

Diagnosis at the vet

Cell sampling and further classification

The vet feels the lymph nodes and assesses whether changes are localised or affect several sites. Infection or immune-mediated disease can also enlarge nodes. A fine needle can collect a small cell sample from an enlarged node for microscopy. Sampling usually needs no sedation. One study estimated 92.6% sensitivity for identifying lymphoma when assessing lymph node fine-needle samples. If a cell sample is insufficient, a biopsy can show tissue architecture.

Precise classification of nodal lymphoma uses tissue sampling and staining of tumour cell markers, called immunohistochemistry. Flow cytometry can identify B- or T-cell lymphoma from a fresh cell sample. PARR testing can investigate whether sampled lymphocytes share a common origin. A population derived from one cell may support suspected lymphoma, but is interpreted alongside cell appearance and other findings. For suspected digestive tract lymphoma, tissue samples can be taken during endoscopy or surgery. A suspicious skin lesion can be biopsied to establish where tumour cells lie within skin layers.

Establishing disease extent

A blood count checks for anaemia or abnormal blood cells. Blood biochemistry provides information about organ function and factors such as calcium. Chest X-rays can look for a mediastinal tumour or enlarged lymph nodes within the chest. Abdominal ultrasound assesses spread to abdominal organs. Bone marrow sampling may be needed if marrow involvement is suspected.

Staging runs from one to five, with higher numbers indicating more extensive disease. Stage one is confined to one lymph node or one lymphoid organ, excluding bone marrow. Stage two involves enlarged nodes on only one side of the diaphragm. Stage three affects nodes on both sides. Stage four involves the liver or spleen. Stage five includes spread to blood or bone marrow. Substage a denotes a dog without symptoms; b denotes disease-related symptoms. Over 80% of canine lymphomas are diagnosed at stages three to five.

Treatment

Chemotherapy and symptom relief

Rapidly progressing generalised lymphoma is usually treated with a combination of chemotherapy medicines. The vet selects treatment according to type, extent and the dog's overall condition. Combination treatment aims to shrink the tumour and prolong good quality of life. Surgery or radiotherapy can sometimes be used for localised lymphoma. Severe tumour-related hypercalcaemia can be treated with fluids and calcium-lowering medication. Corticosteroids can relieve symptoms if chemotherapy is unsuitable. Corticosteroids alone can sometimes produce a short remission lasting around one to two months. Starting them before sampling can make lymphoma harder to diagnose. Their use before combination treatment can reduce subsequent chemotherapy response.

Treatment response and prognosis

In complete remission, lymph nodes have returned to normal size and examinations show no signs of tumour. Partial remission means measured nodes have shrunk by at least 30% from baseline. New tumour lesions during follow-up indicate progression. Lymphoma can recur after complete remission. Following recurrence after a good initial response, the same drug combination can sometimes be used again. Another combination can be tried for progressive B-cell lymphoma resistant to the first. The most effective rescue treatment for CHOP-resistant B-cell lymphoma is unknown because outcomes have not been adequately compared in this specific patient group.

A European study of 502 dogs examined nodal B-cell lymphoma responses to two CHOP combination treatments. Among dogs evaluable for response, 117 of 145 in the first group, or 80.7%, achieved complete remission. In the second group, 276 of 337, or 81.9%, achieved complete remission. Median progression-free times were 196 and 209 days respectively. Median survival was 302 and 321 days. At two years, 13.5% of the first group and 15.4% of the second were alive. There was no statistically significant survival difference between treatments.

Untreated high-grade lymphoma often allows only around four to six weeks' survival. In a study of 20 dogs with slow-growing T-zone lymphoma, median survival was 637 days.

Prevention

Prevention is difficult because no single cause is known. No special diet has been shown to prevent canine lymphoma. Some Golden Retriever breed clubs advise excluding affected dogs from breeding as part of cancer prevention. Their breeding recommendations also take account of cancers in the prospective parents' families. Feeling lymph nodes during health checks helps detect enlargement in otherwise healthy-looking dogs. Some kennel club breeding databases allow relatives' causes of death to be checked if owners have reported them.

Important for dog owners

Tell the vet about all medication, particularly corticosteroids, as these can affect lymphoma test results. Monitor appetite and weight during treatment: nausea or diarrhoea can reduce food intake. Contact the treating clinic immediately if a dog receiving chemotherapy develops fever or sudden weakness. Blood tests can identify falling white cell numbers before severe symptoms appear. Depending on the medicine, the greatest marrow suppression may occur seven to 14 days after an individual treatment, so blood tests are timed for each dog. Chemotherapy medicines or their breakdown products can pass into urine and faeces. Wear protective gloves and follow the clinic's instructions when cleaning up bodily waste after treatment. Children and pregnant people should avoid contact with waste from dogs receiving chemotherapy.

Regular follow-up after treatment can detect renewed lymph node enlargement. Visit frequency depends partly on tumour growth rate, disease extent and treatment given.

Frequently asked questions

Does an enlarged lymph node always mean lymphoma?

Enlargement can also reflect inflammation, for example during infection. The vet can take a fine-needle cell sample from the node. If results are unclear, biopsy also allows tissue architecture to be assessed.

How do B- and T-cell lymphoma differ?

The names indicate whether tumour cells originate from B or T lymphocytes. The precise subtype can help predict progression rate and treatment response. T-zone lymphoma is a slow-growing T-cell subtype whose prognosis differs from rapidly progressing forms.

Does remission mean lymphoma is cured?

In complete remission, examinations reveal no lymphoma signs and enlarged lymph nodes have returned to normal size. Tumour cells may nevertheless remain, allowing later recurrence. The vet therefore monitors lymph nodes after the treatment course ends.

How long can a dog with lymphoma live?

Survival depends on subtype, extent and treatment response. In a European study, median survival for dogs with nodal B-cell lymphoma receiving two different combination treatments was 302 and 321 days. A separate study found median survival of 637 days for slow-growing T-zone lymphoma.

Do dogs get the same chemotherapy side effects as people?

Dogs generally tolerate chemotherapy well. Temporary nausea, diarrhoea or loss of appetite can still occur. Chemotherapy can also affect bone marrow function. The vet monitors blood counts during treatment to detect any reduction in neutrophils.