Histiocytic Sarcoma in Cats and Dogs
Histiocytoma (benign tumor)
Histiocytomas are common benign (noncancerous) tumors that occur in dogs younger than 3 years old. They are usually
solitary (single lesions) but multiple histiocytomas can occur in some pets, especially shar-pei dogs. Under the
microscope, the histiocytoma cells can be difficult to distinguish from other types of cells such as
lymphoma and a
technique called
immunohistochemistry (a process of localizing proteins in cells and tissues using antibodies)  is typically
applied to obtain a definitive diagnosis. Solitary histiocytomas can spontaneously  disappear within 6 weeks and multiple
histiocytomas within 10 months.

Histiocytic sarcoma (malignant tumor)
Histiocytic sarcomas are malignant (cancerous) soft tissue sarcoma tumors that arise from histiocytic cells (e.g.
macrophages and dendritic cells) which are a part of the immune system. The diagnosis of histiocytic sarcoma is
confusing and can be controversial. The immunohistochemical analysis of these tumors suggests that the tumor cells
originate from the dendritic cells. There are two types of distinct types of histiocytic sarcomas based on clinical findings
and biological behavior of tumor cells: localized and disseminated. The symptoms of histiocytic sarcoma will vary
depending on the tumor's location, but nonspecific symptoms such as lethargy, weakness, lack of appetite, and weight
loss are common.

Localized histiocytic sarcoma
Localized histiocytic sarcomas are locally aggressive and have a moderate-to-high risk to metastasize (spread) to distant
organs, namely the regional lymph nodes, lungs, spleen, liver, and kidneys. This tumor subtype requires
immunohistochemistry for definitive diagnosis. Malignant histiocytic sarcomas tend to occur in flat-coated retriever,
rotttweiler, golden retriever, and Labrador retriever, and usually affect middle-aged to older dogs. This type of tumor is
usually present as a single mass on the skin or under the skin of the pet's limbs but other locations have been reported
such as the spleen, lung and periarticular tissue.

To differentiate this tumor from the disseminated histiocytic sarcoma, thorough tests are usually done such as X-rays,
abdominal ultrasound, regional lymph node biopsy/aspiration and bone marrow cytology.

Aggressive surgery is the recommended course of treatment for local tumors. Radiation therapy can be useful in the
cases where the tumor could not be completely surgically removed in order to kill any remaining cancer cells at the site of
surgery. It is not currently known whether
chemotherapy following surgery has any benefit on overall survival.

The prognosis for pets diagnosed with localized histiocytic sarcomas of the skin or subcutis (layer under the skin)  is not
known. In a very small study of 5 dogs, no local recurrence (coming back) and no metastasis were observed in the dogs
treated with surgery alone. The prognosis for dogs with localized histiocytic sarcoma on internal organs such as the
spleen is poor with a median survival time of 1 month and 0%-20% of diagnosed dogs surviving 1 year.

Disseminated histiocytic sarcoma
Disseminated histiocytic sarcomas are malignant tumors that typically involve the lymph nodes, bone marrow, lungs,
spleen, and liver. They are also known as malignant histiocytosis and tend to occur more frequently in Bernese mountain
dogs, rottweilers, golden retrievers, Labrador retrievers, and flat-coated retrievers. Unlike dogs with localized histiocytic
sarcomas, disseminated sarcoma can cause symptoms which will depend on the affected organ systems. Anemia
(deficiency of red blood cells), thrombocytopenia (low platelet count) and leukocytosis (high white blood cell count) are
relatively common. Disseminated histiocytic sarcoma are differentiated from localized sarcomas by the presence of
symptoms rather than histology, immunohistochemistry or ultrasound examination. Treatment is usually unrewarding but
some partial response has been observed with lomustine-based
chemotherapy. The prognosis is generally poor with a
median overall survival time of 128 days and 0% of pets are alive 1 year after diagnosis.

Does cancer cause pain in cats and dogs?
Pain is common in pets with cancer, with some tumors causing more pain than others. In addition to pain caused by the
actual tumors, pets will also experience pain associated with cancer treatments such as surgery, radiation therapy or
chemotherapy. Untreated pain decreases the pet's quality of life, and prolongs recovery from the illness, treatment or
injury. It is, therefore, essential that veterinary teams that are taking care of pets with cancer should also play a vital role
in educating pet owners about recognizing and managing pain in their pets. The best way to manage cancer pain in pets
is to prevent it, a term referred to as preemptive pain management. This strategy anticipates pain ahead of time and
administers pain medication before the pet actually experiences pain, thus ensuring the pet's maximum comfort.

To learn more about how to recognize pain in pets with cancer and what cancer pain management options are available
for your pet, please visit the
Cancer Pain Management section.

Is nutritional support important for pets with cancer?
Cancer cachexia (a term referring to progressive severe weight loss) is frequently observed in pets with cancer. Pets with
cancer lose weight partly because of lack of appetite and partly because of cancer-induced altered metabolism. Some of
the causes for decreased appetite are related to the cancer itself (for example, tumors may physically interfere with food
chewing, swallowing, and digestion process) and some may be related to the side effects of cancer treatment (for
example, some chemotherapy drugs cause nausea and vomiting, and radiation therapy can cause mouth inflammation).

Proper nutrition while undergoing cancer treatment is essential to maintain your pet's strength, improve survival times,
quality of life and maximize response to therapy. Adequate nutritional support was shown to decrease the duration of
hospitalization, reduce post-surgery complications and enhance the healing process. Additionally, pets with cancer need
to be fed diets specifically designed to provide maximum benefit and nutritional support for the patient. To learn more,
please visit the
Cancer Nutrition section.

How do I find a qualified veterinary oncologist?
To locate a qualified veterinary oncologist in your area who can discuss with you appropriate cancer treatment plan for
your pet's cancer condition, please visit the "
Locate a veterinary oncologist" section.  

Are there any clinical trials investigating new treatments?
There are several clinical trials ongoing for dogs with soft tissue sarcomas, which are partially funded by the institutions.
To learn more about these trials, please visit the
Clinical Trials for Soft Tissue Sarcoma in Dogs section.

Additionally, there are several clinical trials available for cats and dogs with any tumor type for which your pet may qualify.
To learn more these trials (which are partially or fully funded by the institutions), please visit the
Dog Clinical Trials (any
tumor type) or Cat Clinical Trials (any tumor type) section.  

To learn more about veterinary clinical trials in general, please visit the
Pet Clinical Trials section.

Additional online resources

  • Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
  • Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins,
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 2/21/2017
Histiocytic sarcoma
Comprehensive guide to cancer diagnosis and treatment in cats and dogs