Everything You Need to Know About Histiocytic Tumors in Pets

Histiocytic diseases are a complicated group of disorders we face in veterinary medicine.  The terminology can be overwhelming, and owners seeking information can easily become frustrated when trying to understand their pets’ diagnosis.

Many different diseases include the word “histiocytic” or some variant of the term, lending to the intricacy surrounding the diagnosis. Though difficult, I thought it important to attempt to break this complicated topic into simplistic terms.

Histiocytic diseases arise from histiocytes, which are immune cells produced in the bone marrow. These cells travel in the bloodstream as monocytes and then enter into different tissues, where they will mature into histiocytes. The three major categories of histiocytes in tissues are dendritic cells, macrophages, and Langerhan’s cells. Identification of the different subtypes of cells can provide a great deal of information as to the precise etiology of a particular histiocytic disorder.

When I am presented with a case of a pet diagnosed with a “histiocytic disorder,” I first try to understand if the disease fits into one of two broad categories, either representing a reactive or a neoplastic histiocytic condition. This often requires a biopsy of affected tissue, so I will urge owners to consider this, especially in cases where the exact nature of the disease is uncertain.

Reactive histiocytic diseases are non-malignant conditions, meaning they are not considered cancers per se. However, they still represent excessive proliferation of mixed reactive immune cells. In this example, malignant means something that spreads throughout the body in an uncontrolled fashion.

The two main subtypes of reactive histiocytic diseases are cutaneous histiocytosis (CH) and systemic histiocytosis (SH).  These are typically considered diseases of a dysregulated immune system and are often treated by veterinary dermatologists with immunosuppressive medications and supplements. Though not true cancers, these conditions can severely affect a pet’s quality of life, and in advanced cases, even cause significant morbidity or even be fatal.

Neoplastic histiocytic diseases are also disorders of unregulated growth of immune cells. Though not intuitive, some neoplastic diseases are considered benign while others are malignant. The distinguishing feature between the two would be determined upon features seen on biopsy or fine needle aspirate cytology. Whether the tumor remains localized within one anatomical area (benign) or can spread to distant sites in the body (malignant) will determine the diagnosis.

The quintessential example of a benign neoplastic histiocytic tumor would be a histiocytoma. These are tumors that are typically located in the superficial layers of the skin on the head, neck, ears, or limbs of young dogs. Histiocytomas are considered benign because they very rarely spread from their site of origin to other sites in the body.

Histiocytomas are readily diagnosed via needle aspiration cytology. Spontaneous regression of these tumors is common; therefore immediate surgical removal is not always indicated. Surgery may be recommended in cases where tumors do not resolve, or when they are irritating to the pet (or in some cases, to the owner).

Malignant histiocytic tumors are neoplastic masses falling under the “truly cancerous” category. Neoplastic histiocytic tumors originating at a single site in the body are referred to as localized histiocytic sarcomas (LHS).  They can arise within many different organs of the body but are more commonly found in the skin, spleen, lymph nodes, lung, bone marrow, brain, and the tissue surrounding the joints of the limbs. 

Localized histiocytic sarcoma has the best prognosis if treated early by wide surgical excision. As a tumor can arise in many different tissues, surgical removal could entail amputation of an affected limb, removal of an entire effected lung lobe, or excision of a skin mass, depending on where the growth originated.

When a localized histiocytic sarcoma tumor spreads to distant sites in the body, beyond the lymph node located closest to its tissue of origin, the disease is termed disseminated histiocytic sarcoma (DHS).

In some animals, multiple histiocytic tumors are diagnosed simultaneously in several areas of the body (e.g., in the skin and in internal organs and lungs at the same time). Some will refer to this condition as malignant histiocytosis (MH). However, I personally feel this terminology is rather outdated, and I still prefer using disseminated histiocytic sarcoma in such cases.

Where it becomes incredibly confusing is when we consider how both localized histiocytic sarcoma and disseminated histiocytic sarcoma tumors are capable of widespread metastasis (spread), hence with time, the two syndromes virtually merge. This makes it nearly impossible to differentiate true cases of disseminated histiocytic sarcoma versus cases of massive spread of a localized histiocytic sarcoma.

The way I see it, it’s often the proverbial “chicken or egg” question when deciding if a pet has a localized histiocytic sarcoma that can spread throughout the body versus disseminated histiocytic sarcoma where multiple tumors arose and were detected at the same time. As we will see next week, we would usually approach treating either condition the same way, so it may not matter in the end.

Histiocytic sarcoma occurs more commonly in Bernese mountain dogs, Rottweilers, Golden retrievers, and Flat-coated retrievers. As is typical for most cancers, little information is known in cats, but both localized and disseminated forms of the disease are known to occur in our feline patients.

A diagnosis of histiocytic sarcoma can be devastating for owners. The first and most important steps are to take a deep breath, pause, and consider the information you are given. Seeking referral to a veterinary oncologist may be the best plan of action for many owners in order to feel equipped to make the best decision for their pets and to better understand the disease and all of the available options.

In next week’s article I will discuss staging, treatment options, and prognosis for histiocytic sarcoma in veterinary patients.

Dr. Joanne Intile

Image: XiXinXing / Shutterstock