Histiocytic sarcoma in pets is largely a disease of middle-aged to older dogs, although the disease has been observed in dogs as young as 3 years of age.  Breed predispositions exist with Bernese Mountain Dogs, Rottweilers, Flat-coated Retrievers and Golden Retrievers being over-represented. The disease occurs sporadically in many other breeds, as well.

Histiocytic sacoma arises as a result of the excessive and abnormal proliferation of a population of histiocytes, a type of immune cell.  The underlying cause for this disease is unknown; however, hereditary factors are suspected based on the aforementioned breed predispositions.  Other factors, such as genetic and molecular mutations, as well as immune system dysregulation, are also postulated to be involved.

Because histiocytic sarcoma is a type of immune cancer, this type of cancer can be found in many locations throughout the body.  Therefore, the clinical signs of histiocytic sarcoma depend upon the tissue or organ affected, but are often non-specific such as depression or lethargy, inappetance and weight-loss.  Dogs with histiocytic sarcoma affecting the limbs or joints often present with an obvious mass and, many times, lameness. Dogs with lung involvement (either the original mass or spread of cancer) will often present with lethargy, exercise intolerance, dyspnea (difficulty breathing) and coughing.

Diagnosis:

A diagnosis of histiocytic sarcoma may be obtained with a fine needle aspirate of the mass and cytologic analysis.  In most cases, however, this cancer can be difficult to diagnose and the cytology will not provide enough detail of the cancer to provide a definitive diagnosis.  A tissue biopsy is often then required for a definitive diagnosis, and even then confirmation may require special stains.

Complete staging to know how far the cancer has spread is imperative for this cancer type.  Unfortunately, this cancer can spread many places and it is important to know the extent of disease prior to initiation of treatment for your oncologist to be able to determine the best treatments to offer and to know if those treatments are working against the cancer.

The most common sites for metastatic (disease spread) or disseminated disease are the lungs, lymph nodes, bone marrow, spleen, liver and brain.  Often the oncologists at WVRC will recommend chest x-rays, abdominal ultrasoundfine-needle aspirates, blood work, urinalysis, and, in certain cases, advanced imaging with MRI or CT-scan.

Treatment and Prognosis:

Histiocytic sarcoma may be treated according to two different therapeutic approaches, 1.) Definitive therapy, and 2.) Palliative therapy.

1. Definitive therapy

In most cases the definitive therapeutic approach provides the best clinical response and duration of remission (disappearance of observable cancer).  Having said this, however, despite even the very best of intentions and treatment, metastasis or dissemination frequently develop in dogs with histiocytic sarcoma, and the dogs ultimately succumb to their disease.

The curative intent approach involves surgical excision of the primary tumor with wide surgical margins.  Examples include, splenectomy in the case of splenic involvement, amputation in the case of an affected joint or limb, or lung lobectomy in the case of lung involvement.  Surgery is then followed by chemotherapy.  Radiation might also be considered post-operatively if clean margins were not achieved from surgery.

For dogs with histiocytic sarcoma affecting a joint or limb, surgical amputation of the affected limb is the most effective way of achieving local tumor control, as well as controlling the clinical signs (pain) associated with the tumor.  Histiocytic sarcoma of the joint or limb can be an extremely painful disease, and many dogs are toe-touching to three-legged lame at the time of initial presentation and diagnosis.

With the definitive treatment approach, chemotherapy is instituted post-operatively to delay the progression of cancer spread. The chemotherapeutic protocol currently recommended for histiocytic sarcoma incorporates a medication called CCNU (Lomustine) administered orally every 3 weeks for 5 treatments.

2. Palliative intent therapy

The palliative therapeutic approach may be considered in patients with disseminated or metastatic disease (cancer spread) at the time of initial diagnosis, or in patients where complete surgical removal of the primary tumor is not possible. The purpose of the palliative intent approach is to diminish the clinical signs associated with the tumor and to maintain a good quality of life.  Palliation may be achieved through a combination of surgery (amputation), chemotherapy, radiation therapy, and various oral analgesic agents and perhaps other pharmaceutical agents.

Prognosis

Without treatment, the clinical course for histiocytic sarcoma tends to be rapid.

The prognosis for localized articular (joint) or periarticular (near the joint) histiocytic sarcoma undergoing definitive therapy with amputation and/or radiation therapy followed with chemotherapy is a median of 12 months.  This means that in 50% of dogs they succumb to their disease before 12 months and the other 50% live longer than 12 months.

Dogs with splenic involvement treated with splenectomy and chemotherapy have a prognosis of 6-8 months.

It is difficult to provide an estimated median survival time for dogs treated with palliative therapy, because each individual patient’s situation, treatment and response to therapy can vary markedly.  In one study of dogs with histiocytic sarcoma treated with chemotherapy alone, the median survival time was 3-4 months, but this is extended with multi-modal therapy.

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