Histiocytic Sarcoma (HS) in Dogs
Histiocytic Sarcoma (HS) is a highly aggressive malignant cancer that originates from dendritic cells or macrophages, both types of white blood cells. It can present either as a localized tumor, affecting a single organ, or as a disseminated form that spreads throughout multiple organs and tissues. A rare subtype, Hemophagocytic HS (HHS), originates in the spleen or bone marrow and is particularly aggressive. Though HS is rare in cats, it is more commonly found in dogs, particularly in certain breeds.
Incidence and Breed Prevalence
HS is a rare cancer, making up less than 1% of all canine cancers. However, some breeds are genetically predisposed, including:
– Bernese Mountain Dogs (BMD)
– Flat-Coated Retrievers (FCR), particularly susceptible to periarticular HS (PAHS)
– Other breeds at risk include Rottweilers, Miniature Schnauzers, and Pembroke Welsh Corgis.
Clinical Signs of HS
Symptoms vary depending on whether the cancer is localized or has spread.
– Localized HS: Commonly involves the skin, spleen, or lymph nodes, often presenting as a visible mass. When the tumor occurs in the joints, it can cause lameness.
– Disseminated HS: This form involves multiple organs and causes a wide array of systemic symptoms:
– Lungs: If the tumor spreads to the lungs, it can cause dyspnea (difficulty breathing) and coughing. Pulmonary involvement is common in advanced cases.
– Liver and Spleen: Tumors in the liver and spleen can lead to hepatosplenomegaly (enlargement of these organs), causing abdominal distension, lethargy, and weight loss. These organs are commonly affected in the disseminated form of HS.
– Bone Marrow: Involvement of the bone marrow can result in anemia (leading to pale gums and weakness), thrombocytopenia (causing bleeding tendencies), and pancytopenia (a reduction in all blood cell types).
– Lymph Nodes: Enlarged lymph nodes may be palpable, especially in cases where HS has spread to the lymphatic system.
– Brain: When HS affects the brain, it can cause neurological symptoms such as seizures or ataxia (loss of coordination).
Other notable symptoms across both forms of HS include weakness, fever, anorexia, and in severe cases, collapse due to organ failure.
Diagnosis of Histiocytic Sarcoma
1. Cytology and Histopathology:
Diagnosis typically requires a tissue biopsy. HS cells are usually large and pleomorphic, with notable irregularities. Cytology often reveals high mitotic activity, and biopsy is required to confirm the presence of cancerous cells.
2. Imaging:
Diagnostic imaging such as X-rays, ultrasounds, CT scans, or MRIs can help determine tumor size, spread, and metastasis, particularly in the lungs and organs like the liver and spleen.
3. Laboratory Tests:
Blood tests often reveal anemia, thrombocytopenia, and elevated liver enzymes. Bone marrow infiltration can lead to pancytopenia, and hyperferritinemia may indicate HS.
Treatment Options for Histiocytic Sarcoma
1. Surgery:
For localized tumors, surgery is often the first treatment option. This may include amputation for periarticular HS, splenectomy, or lung lobectomy for affected organs. However, the metastatic rate remains high (e.g., 91% in periarticular cases), so surgery alone is not always curative.
2. Radiation Therapy:
Palliative radiation can be used to manage non-resectable tumors or those in sensitive areas like the joints. Radiation can extend survival and improve quality of life, with a combination of radiation and chemotherapy providing a median survival time of 7 months.
3. Chemotherapy:
– Lomustine (CCNU): The most effective chemotherapy agent for HS, with response rates between 29% and 46%. In dogs with localized HS, surgery followed by lomustine extended median survival to 19 months.
– Doxorubicin: When combined with lomustine, it increases the response rate to 58% and delays progression by approximately 6 months.
– Other drugs, such as dacarbazine, epirubicin, and vinorelbine, have been used as second-line or rescue treatments.
4. Palliative Care:
In cases where aggressive treatment is not feasible, corticosteroids may be used for symptom relief, although they are associated with shorter survival times.
Prognosis for Dogs with Histiocytic Sarcoma
The prognosis for HS varies greatly depending on whether the disease is localized or disseminated.
– Localized HS: Dogs undergoing surgery combined with chemotherapy have a median survival time of approximately 1 year. For periarticular HS (PAHS), the median survival is 13 months.
– Disseminated HS: Prognosis is much poorer, with a median survival time of 4.2 months. Hemophagocytic HS (HHS), the most aggressive form, leads to survival times ranging from days to 1–2 months, even with treatment.
Negative prognostic factors include the use of corticosteroids, anemia, thrombocytopenia, and hypoalbuminemia. These factors, especially in cases involving HHS, tend to shorten survival times.
Conclusion
Histiocytic Sarcoma is an aggressive cancer that primarily affects dogs, with certain breeds like Bernese Mountain Dogs and Flat-Coated Retrievers at higher risk. Early detection and a combination of surgical, radiation, and chemotherapy treatments can extend survival times, especially for localized cases. However, disseminated forms of HS carry a significantly worse prognosis. Regular monitoring and comprehensive treatment strategies are crucial for managing this challenging disease.
