Malignant Histiocytosis and Histiocytic Sarcoma

Rhea V. Morgan, DVM, DACVIM (Small Animal), DACVO

BASIC INFORMATION Biopsy of solitary tumors may be needed to reach a defini- Description tive diagnosis. Sophisticated immunohistochemical stains may be Histiocytes are tissue cells that interact with white blood cells when done on biopsy samples to confirm that the cells are actually his- tissues are inflamed. Malignant histiocytosis (MH) is the wide- tiocytes and not white blood cells. spread proliferation of malignant histiocytes in various organs of the body. When the cancerous histiocytes form a solid tumor, it is TREATMENT AND FOLLOW-UP called histiocytic sarcoma (HS). HS can metastasize (spread else- where in the body) and become disseminated. When this happens, Treatment Options HS is hard to differentiate from MH. Chemotherapy is the main treatment option for MH and dis- Causes seminated HS, but it is not often successful. Drugs such as The cause of MH and HS in most animals is unknown; however, lomustine, doxorubicin, cyclophosphamide, vincristine, pred- it is a genetic trait in the Bernese Mountain dog. The dog breeds nisone, mitoxantrone, dacarbazine, and etoposide may be tried. most often affected include the flat-coated retriever, Bernese Some drugs are used alone; others are used together in a spe- Mountain dog, Labrador retriever, and Rottweiler. Both conditions cific protocol. occur predominantly in male dogs 6-11 years of age, but any age Solitary HS are treated by aggressive surgical removal, with and gender may be affected. These conditions are rare in the cat. incorporation of wide margins of apparently normal tissues that surround the tumor. Surgery may be followed by radiation therapy Clinical Signs in some cases. Surgery for tumors on the legs may involve ampu- Signs of MH include loss of appetite, lethargy, fever, and dramatic tation of the entire limb. The benefit of follow-up chemotherapy weight loss. Other signs may be present, depending on the organs has not been well defined in these cases. involved. On physical examination, the liver, spleen, and some lymph nodes (glands) may be enlarged. Follow-up Care Solitary HS are rapidly growing tumors that develop most Follow-up visits and repeated laboratory tests are used to monitor often under the skin and within the soft tissues of the legs. HS may response to treatment and side effects of the medications. X-rays, also arise in the spleen, lungs, brain, or nasal cavity and around abdominal ultrasounds, and bone marrow aspirations may also be joints. Unlike dogs with MH, those with HS do not often act ill, repeated. Periodic monitoring is usually required for the rest of the especially early in the clinical course. Surface tumors or growths life of the dog. under the skin may simply be discovered by the owner. Eventually, Prognosis tumors in muscles and around joints may cause lameness, pain, Dogs with solitary HS of the skin or underlying tissues that are and swelling. Internal tumors of the liver and spleen may cause treated with aggressive surgery have the best prognosis. Some of signs similar to those of MH. these tumors do not recur or metastasize. Metastasis can occur in as many as 60% of these cases, however. Prognosis for inter- Diagnostic Tests nal HS is poor. Dogs with tumors of the spleen, liver, or brain Routine laboratory tests and x-rays of the chest and abdomen are may succumb to their disease within a few weeks, and fewer than often performed to investigate the clinical signs. Common laboratory 20% survive for 1 year. Dogs with HS around the joint also have a abnormalities include anemia, low platelet counts, low blood protein poor prognosis, with average survival times of 6 months following levels, abnormal liver function tests, and evidence of mild jaundice. amputation, and metastatic rates of about 90%. A fine-needle aspiration may be done on visible solitary Prognosis for MH is guarded (uncertain) to grave. Although tumors and may reveal abnormal histiocytes. Other tests that may remissions can be achieved with chemotherapy, they are often be recommended include an abdominal ultrasound, aspiration of short (less than 100 days). Typical survival times are 3-5 months abdominal masses under ultrasound guidance, bone marrow aspi- (range, 8-884 days). Only a few dogs with MH survive for 1 year ration or biopsy, and measurement of blood ferritin, which is often or longer. elevated in MH. It is important to evaluate the dog thoroughly to Because malignant histiocytic diseases are genetic and inher- determine whether an HS has metastasized and what organs are ited in the Bernese Mountain dog, parents and offspring of affected involved with MH. dogs should not be bred.