The hallmark of this disease is systemic proliferation and organ infiltration of large atypical cells of histiocytic origin.
These cells can be multinucleated and display phagocytosis of erythrocytes and leukocytes. The distribution of these cells
includes the lung, bone marrow (Photo 4), lymph nodes, liver, spleen and central nervous system (Photos 5 and 6). Clinical
signs include lethargy, anorexia, fever, weight loss as well as respiratory and neurologic signs. Anemia, thrombocytopenia,
pancytopenia and hypercalcemia also can be seen.
Photo 5: Diffuse histiocytic sarcoma with pulmonary mass.
DHS is similar to MH but is used to designate a localized histiocytic sarcoma that spread to distant sites. As with CH and
SH, DHS and MH can appear clinically similar and can be difficult to distinguish as separate entities. The distinction may
not be clinically significant as the staging, treatment and prognosis for both diseases is similar.
Photo 6: Aspirate of pulmonary mass.
Staging of a dog suspected of having MH or DHS includes a CBC, chemistry profile, thoracic radiographs (three views), abdominal
ultrasound, aspirate of any affected organ and a bone marrow aspirate. The prognosis for dogs with MH or DHS is considered
guarded. In the past, treatment has been unrewarding; however, many of these dogs were treated with lymphoma-type protocols.
Most recently, oncologists have reported responses with lomustine (CCNU), doxorubicin, cyclophosphamide, Doxil and prednisone.
Photo 7: Localized histiocytic sarcoma.
This disease is also called localized histiocytic sarcoma (LHS). The breed disposition is similar to that seen with DHS. The
most common location for these tumors is the subcutis, but they have been seen in other locations, such as the spleen. Many
of these tumors occur on the limbs and are often periarticular (Photos 7 and 8). Periarticular tumors have been misdiagnosed
as synovial cell sarcoma. They are locally invasive and can metastasize. The metastatic rate has been reported as high as
50 percent (Moore 2002, Fidel 2006). When LHS spreads beyond the local lymph nodes, it is considered DHS.
Staging for this disease would be similar to DHS. If these tumors are localized, aggressive treatment should be considered.
For local control, surgery and/or radiation therapy would be recommended. In one study, dogs that had complete local control
with surgery had no evidence of metastasis or local recurrence for an unspecified period (Moore 2002). Amputation is recommended
for periarticular tumors. There are few published reports regarding the efficacy of radiation therapy for these tumors, but
one study demonstrated that these tumors are responsive to radiation therapy even when treating gross disease (Fidel, 2006).
The type of radiation protocol would vary depending on the clinical picture as well as the goal of treatment. Chemotherapy
also plays a role in the treatment of these tumors and is recommended for non-resectable or metastatic tumors as well as an
adjuvant therapy to surgery and/or radiation therapy. As with MH and DHS, there are no large published reports regarding the
efficacy of chemotherapy, but responses have been documented using lomustine (CCNU), doxorubicin, cyclophosphamide, Doxil
Photo 8: Aspirate of tarsal lesion.
Malignant fibrous histiocytoma
This tumor is thought to be of mesenchymal, rather than histiocytic, origin; but this has not been proven with immunohistochemical
stains or molecular methods. It has been suggested that malignant fibrous histiocytoma may arise from a more primitive precursor
that has the ability to differentiate either into mesenchymal cells or histiocytes (Kerlin 1996). Most pathologists classify
this tumor as a soft tissue sarcoma. There is also a giant cell variant of this tumor that is called giant cell tumor of the
Dr. Cronin earned her DVM degree from Cornell University in 1990. She completed an internship at the Animal Medical Center
in New York and a medical oncology residency at North Carolina State University. She is a diplomate of the American College
of Veterinary Internal Medicine in oncology. After completing her residency, she was lecturer at the University of Pennsylvania
Veterinary Teaching Hospital and a medical oncologist at Angell Memorial Animal Hospital in Boston. In 2001, she co-founded
the New England Veterinary Oncology Group in Waltham, Mass.