Histiocytic sarcoma in dogs – symptoms and treatment

Overview

Overall, histiocytic sarcoma is a rare canine cancer. However, it can affect any breed. The disease is abbreviated to HS.

In the localised form, a tumour is identified in one area, such as a joint or the skin. Other common local sites include bones and lungs. In the disseminated, or widespread, form, several organs or tissues are affected simultaneously.

A distinct haemophagocytic form, in which cells engulf red blood cells, usually starts in the spleen and progresses rapidly. Despite the similar name, an ordinary cutaneous histiocytoma is benign. Inflammatory histiocytosis also belongs to a different disease group.

Causes

The tumour arises from immune-system cells

Histiocytes are tissue immune cells that include dendritic cells and macrophages. Dendritic cells help other immune cells recognise foreign structures. Such cells occur in the skin and lungs, among other sites.

Conventional histiocytic sarcoma originates from tissue dendritic cells. Their precursors come from the bone marrow. The haemophagocytic form instead develops from macrophages, or scavenger cells.

The name haemophagocytic describes red blood cells being engulfed by tumour cells. This form can involve severe anaemia, a shortage of red blood cells. Platelet numbers may decrease at the same time.

Localised disease can spread elsewhere

Most dogs with localised disease later develop more generalised illness. Disseminated disease is found in the spleen, liver, lungs, bone marrow and lymph nodes. The exact progression from a localised form is not understood in every disseminated case.

Risk factors

Inherited predisposition matters particularly in Bernese Mountain Dogs. Flat Coated Retrievers, Rottweilers and Golden Retrievers are also predisposed. Disease is found more often than usual in Labrador Retrievers, Miniature Schnauzers and Pembroke Welsh Corgis too.

Most patients are middle-aged or older, but young dogs can be affected. In Flat Coated Retrievers, disease often begins after 8 years of age. In Bernese Mountain Dogs, it is often found as early as 6 years. A typical age in Miniature Schnauzers is around 10 years, although cases have been described in dogs as young as 3.

Bernese Mountain Dogs often have disseminated disease. In Flat Coated Retrievers, a localised tumour may involve a joint.

Typical symptoms

The clinical picture depends on tumour location and disease extent. A new subcutaneous lump may be found by chance while stroking your dog. A localised lung tumour can also be an incidental finding.

• Your dog is more tired or reluctant to move than usual.

• Appetite declines and weight falls.

• Your dog limps or an area around a limb joint swells.

• Your dog coughs, becomes breathless or has difficulty breathing.

• Your dog vomits or has diarrhoea.

• The gums may be pale or yellowish.

• Unusual bruising or bleeding is visible on the skin.

Disseminated disease can also involve nervous-system dysfunction. Not every dog has marked general signs initially.

Complications and consequences

Spread and organ damage influence the outlook

Untreated disseminated disease progresses rapidly and is fatal. A local tumour can also cause severe pain and reduce quality of life. Severe anaemia and platelet deficiency in the haemophagocytic form can cause weakness and bleeding.

In many cases, survival is only around 3–4 months. Disseminated disease commonly follows a fatal course lasting around 4–6 months. The haemophagocytic form has a particularly poor outlook, with survival often under 2 months.

Some localised cases survive considerably longer

Effective treatment of a localised tumour may extend a dog's life by 12–18 months. Some localised cases receiving surgery and chemotherapy have average survival times of 1.5–3 years. More precise treatment selection and prognosis require information on disease extent.

Diagnosis at the vet

A tissue sample confirms the tumour type

The vet examines the whole dog and looks for changes elsewhere in the body too. A fine-needle sample or tissue biopsy is collected from the tumour. Fine-needle sampling collects cells, while a biopsy takes a small piece of tissue for detailed examination.

Sarcoma cells may resemble other malignant cells, so further testing may be needed before treatment. Tissue immunostaining identifies proteins indicating the tumour's cell lineage. Examples of these markers include Iba-1 and CD204.

Histiocytic sarcoma is distinguished from both benign histiocytoma and reactive histiocytosis. The benign behaviour of histiocytoma therefore does not describe sarcoma behaviour. Taking a tissue sample does not accelerate sarcoma growth.

A benign skin histiocytoma arises from epidermal Langerhans cells, a different dendritic-cell subgroup. Tumour type is confirmed before extensive surgery because removal without a diagnosis may be inadequate.

Spread is investigated before treatment is decided

A blood count, blood biochemistry and urinalysis assess effects on organ function. Results also help judge treatment tolerance. Lymph nodes near the tumour or otherwise abnormal are sampled to look for metastases.

Chest radiographs and abdominal ultrasound look for internal organ changes. Computed tomography can clarify tumour, lung and abdominal findings. Magnetic resonance imaging is also needed in some situations.

Treatment

Treating a localised tumour

A localised sarcoma in the skin, bone or joint is removed surgically where possible with adequate healthy tissue margins. A limb tumour may require amputation, removal of the entire limb. An early, small local tumour may be easier to remove while preserving the limb.

Systemic chemotherapy is often recommended after surgery because of later spread. Radiotherapy may be used if complete removal is impossible. It can also provide symptom relief for an inoperable tumour.

A large tumour may be irradiated before planned surgery. Radiotherapy visits involve general anaesthesia, so suitability also depends on the dog's general condition.

Treating disseminated disease

Widespread histiocytic sarcoma is primarily treated with chemotherapy. Removing an individual tumour is generally not recommended for extensive disease. Spleen removal may benefit some dogs with the haemophagocytic form.

Chemotherapy drugs used include lomustine and doxorubicin. A response rate of around 27–46% has been described for lomustine. The response has lasted around 100 days.

When an earlier chemotherapy drug stops working, an alternative may still produce a short response. In a group of 9 dogs treated with vincristine, the response rate was 33.3%. Median progression-free time was 21 days, with a range of 7–71 days.

Progression-free time runs from starting treatment to tumour progression or death, rather than representing total survival from diagnosis.

The group included 5 localised and 4 disseminated sarcomas. None had the haemophagocytic form. Complete tumour disappearance occurred in 1 dog. Two dogs had a partial response.

Tumours remained stable in 5 dogs without the shrinkage counted as a treatment response. Disease progressed in 1 dog despite treatment. A partial response meant at least a 30% reduction in the combined diameters of measurable tumour lesions.

Median overall survival from diagnosis in the same group was 169 days, ranging from 122–217 days. Some dogs later received other chemotherapy too, so overall survival does not reflect vincristine's effect alone.

A veterinary-prescribed corticosteroid may be combined with chemotherapy.

There is not yet clear evidence for the effectiveness of antibodies studied as immunotherapy.

Symptom relief and adverse-effect monitoring

Chemotherapy can cause temporary vomiting or diarrhoea. White blood cell counts may fall temporarily. Reduced appetite and tiredness are also possible.

Radiotherapy can cause local skin or tissue irritation. Surgical adverse effects include wound-healing problems and infection. The vet may prescribe supportive care for nausea and diarrhoea at home.

Prednisone may also be used in palliative care. However, its effect on survival in histiocytic sarcoma has not been adequately studied. Treatment choice may change according to the dog's condition and response.

One vincristine-treated dog developed severe thrombocytopenia, a reduction in platelets. Blood counts and biochemistry are monitored during treatment to assess adverse effects and general condition.

Radiotherapy can also interfere with surgical wound healing, affecting coordination of different treatments.

Prevention

An HS risk-variant test can support breeding decisions aimed at reducing disease in Bernese Mountain Dogs. The result is a statistical index based on genetic markers. It does not reliably predict whether the individual tested dog will develop this cancer.

The index is considered alongside temperament, conformation and other health traits. Some breed-club guidance advises against excluding a dog with a C result solely because of the index. It recommends an A- or B-result mate and avoiding C-to-C matings.

Important for dog owners

New lameness, a lump or respiratory signs are investigated even in a dog younger than the usual patient age group. Disease extent and organ function determine treatment feasibility.

Record appetite, ease of breathing, movement and interest in ordinary activities daily. Include nausea, diarrhoea, bruising and visible mucous membrane changes. Ask specifically about treatment aims: removing a local tumour, slowing spread or easing symptoms. Agree beforehand when to contact the vet if your dog cannot eat, move or rest comfortably.

Prognosis becomes clearer through investigations and treatment response. Palliative care remains an option even when the tumour cannot be cured.

Signs of nausea may include reduced appetite and excessive salivation or drooling.

Frequently asked questions

Is histiocytic sarcoma the same as histiocytoma?

No: an ordinary skin histiocytoma is a benign histiocytic tumour. Histiocytic sarcoma, meanwhile, is malignant. Similar names therefore do not mean the same outlook, and diagnosis is confirmed with a cell or tissue sample. Reactive histiocytosis is also different from sarcoma.

Does a C result on the HS risk test mean my dog has cancer?

No: the HS risk-variant test is a statistical breeding tool, rather than an individual cancer diagnosis. The result is considered alongside other breeding traits. A tumour in a symptomatic dog is investigated separately with fine-needle sampling or biopsy even if the risk result is known. The risk-test result is presented as a genetic index.

Should a localised sarcoma be removed?

Wide surgical removal is often recommended for a localised skin, joint or bone sarcoma. In some cases, this requires amputation. Before deciding, disease elsewhere in the body is investigated because spread changes treatment options and the outlook.

How long can a dog live with histiocytic sarcoma?

Survival depends on disease form and treatment possibilities. With disseminated disease, the timescale is often 4–6 months. Some effectively treated localised cases may live an average of 1.5–3 years. Survival with the haemophagocytic form is often under 2 months.

Can taking a biopsy spread or worsen the tumour?

A tissue sample does not accelerate histiocytic sarcoma growth. It helps confirm tumour type before treatment. Histiocytic sarcoma cells can resemble other cancers, so additional stains or other detailed tests may be needed to choose the right treatment.