Histiocytic sarcoma is a malignant form of cancer that arises from histiocytes. Histiocytes are tissue-based white blood cells, or macrophages. This cancer can arise in many parts of the body because macrophages are found in all body tissues. This type of sarcoma is generally an aggressive cancer with a poor prognosis. It is rare in humans and occurs more frequently in certain dog species.
Primary histiocytic tumors have been found in joints of the arms and legs, the gastrointestinal tract, skin, bone marrow, the spleen, the central nervous system, the lungs and even the nasal cavity. Histiocytic cancer that is found in a single site is called histiocytic sarcoma. If the cancer spreads to other organs or distant lymph nodes, it is called disseminated histiocytic sarcoma.
"Histiocyte" is a general term relating to a group of cells that share the same cell lineage. All histiocytes begin in the bone marrow as stem cells. From the bone marrow, they become monocytes and migrate into the circulatory system. These cells then leave the blood and enter tissue, where they undergo differentiation into macrophages and become part of the immune system. Macrophages respond to and engulf foreign proteins in the body, such as viruses and bacteria.
Until the early 21st century, only small numbers of human cases had been mentioned in the literature. It is possible that previous cases were misdiagnosed as non-Hodgkins lymphoma. These tumors were first classified as histiocytic sarcomas in 1970 based on similarities of the cells to macrophages. After that time, research emphasis moved to a cytochemical and immunohistochemical categorization of the tumors.
It can be very difficult to diagnose this cancer, because it is similar to other histiocytic growths. Some of these growths are non-cancerous, such as hemophagocytic syndrome. Other similar growths are malignant, such as malignant histiocytosis or monocytic leukemia. One method used to diagnose this type of sarcoma is the hemoglobin scavenger receptor protein CD163. This protein identifies cells that have a histiocytic lineage with a greater degree of specificity.
Histiocytic sarcoma is recognized as occurring in several dog breeds as well as in humans. In Bernese Mountain Dogs, a genetic familial susceptibility to the cancer has been observed. Other dogs that seem predisposed to this disease include Flat-Coated Retrievers, Golden Retrievers and Rottweilers. Symptoms of the disorder in dogs include weight loss, low energy, limping and failure to eat. Chemotherapy has been mostly unsuccessful against this form of cancer, and most dogs succumb to the disease within months of diagnosis.
There is no test for early diagnosis of this condition, and the disease typically is advanced when diagnosis is made. After diagnosis, there is rapid progression of the cancer, especially if there is lymph node involvement. The best prognosis is for single small tumors found in limbs, where surgical removal can provide a good long-term outcome.
In humans, survival rates and the average length of survival are difficult to calculate because of the rarity of the condition. Some research has shown that patients who have an advanced stage of the disease have an average survival of about seven months after diagnosis, although it is possible to survive for many years with less-advanced stages of the disease. Tumor size has been found to be a predictor of survival, with tumors equal to or larger than about 1.4 inches (3.5 cm) having the worst outcome.