Hemangiopericytoma in Dogs

Hemangiopericytoma is a common type of mesenchymal tumor that affect dogs and is part of a larger family of tumors called soft tissue sarcomas. Tumors originate in pericyte cells which are fibroblastic connective tissue cells. Pericytes are a common cell that is part of the blood-brain barrier as well as numerous other vessels. They arrange along blood vessels such as capillaries and venules; and are a particularly interesting type of cell because they remain in an embryonic state until the body needs them to form other more specific types of cells that support the muscles and the vascular system. When the body needs one of these specific types of cells, the pericyte jumps into action and grows to meet the body’s needs.

Sometimes a pericyte cell develops a chromosomal abnormality and divides improperly, which results in a tumor rather than a healthy cell that the body can use. While any dog can develop a hemangiopericytoma, it is more commonly found in middle-aged to older dogs and is also more common in larger breed dogs. There is no sex predilection. It’s estimated that about twenty percent of these tumors metastasize (spread).

Signs And Symptoms Of Hemangiopericytoma

In dogs, hemangiopericytomas can occur anywhere in the body but tumors primarily arise in subcutaneous tissue (just beneath the skin) in the limbs (dog’s legs) or anywhere on the outside body of the dog. However, they can also appear in other body parts such as the spinal cord or the brain. If you notice any of these signs or symptoms in your dog, it’s essential to take them to your primary care veterinarian for an examination as soon as possible.

• A mass or nodule that grows gradually or quickly over weeks, months or even years. High-grade tumors usually grow very rapidly.
• Growths may vary in size and the way they present – from soft to firm – and sometimes they fluctuate.
• The site where tumors are present may appear as ulcerated (bleeding and sore), bald spots, or pigmented differently from the rest of the affected area.

Diagnosis and Staging

A work-up for canine hemangiopericytoma requires a thorough physical examination by a veterinarian, diagnostic tests such as blood work, and imaging like X-rays and possibly an ultrasound. A fine needle aspirate can be employed which would tell your vet if the mass is a soft tissue sarcoma. A biopsy is necessary to confirm the exact diagnosis of a hemangiopericytoma and will determine the aggressiveness of the tumor. Other diagnostic tests will validate the overall health of your dog including any other concurrent conditions as well as checking for metastatic (spread of) disease. Diagnostics include:

• Blood work such as a complete blood count (CBC) and a chemistry profile
• Urine sample
• Biopsy or fine needle aspirate (FNA) to confirm the diagnosis and tumor grade
• Three-view chest X-rays
• An abdominal ultrasound, CT scan, and/or MRI may also be recommended

The results of the imaging can tell your veterinarian if the cancer has spread, where it has spread to, and how deeply rooted the tumor is. Once your veterinarian has a clear diagnosis, treatment options can be considered.

Other masses that can present the same way as a hemangiopericytoma include other soft tissue sarcomas (fibrosarcoma), hemangiosarcoma, and mast cell tumors, to name a few.

Treating Hemangiopericytoma

When caught early, especially for low-grade tumors, hemangiopericytoma is highly treatable. These tumors tend to stay in one area of the body, rooting themselves, and growing. They are most problematic when they are located near important structures and grow to the point where they affect these areas like the spinal cord or in the axillary (“under arm”) region. The larger the tumor grows, the harder it is to remove it, so early detection is very important. Surgery is usually the first option to remove as much of the tumor as possible. Once removed, the goal is to slow the regrowth of the tumor as much as possible.

Treatment options for hemangiopericytoma depend on the size, location, and grade of the tumor. Your veterinarian may also discuss the age of your dog with you as it relates to the speed at which the tumor is growing. Hemangiopericytoma tumors typically grow very slowly so older dogs may pass away from other causes unrelated to the tumor. As always, discuss the pros and cons of each treatment option with your vet. The following are the most common treatment options.

Surgery

Surgery is often the mainstay of treatment. The goal is to obtain wide, clean margins all around the mass as well as in the deep plane of the surgical site. In cases with wide, clean margins, surgery is often curative. However, surgery may not be able to get all the cancer cells therefore the tumor can grow back. In this case, following surgery with radiation therapy (see below) can be quite successful. Occasionally, surgery is not possible due to the location or the size of the growth. For these cases, radiation therapy or chemotherapy may be recommended in an attempt to shrink the tumor as much as possible.

Radiation Therapy

Radiation therapy uses high-energy particles to target cancer cells, killing those cells or putting the cells in a quiescent (stable, not growing) phase. There are 2 forms of radiation:

Stereotactic Radiation Therapy

Hemangiopericytoma is highly responsive to stereotactic radiation therapy (SRS/SRT) and has a high success rate estimated at 80-90%¹. SRS/SRT can be used for cases in which microscopic cell are present after surgery or on the actual mass itself. A patient typically needs 1-3 treatments. Most dogs have minimal to no side effects with this type of radiation therapy.

Conventionally Fractionated Radiation Therapy (CFRT)

As with other soft tissue sarcomas with residual disease post-operatively getting RT, there is a high cure rate. With CFRT, a patient may need 16-21 fractions (treatments). There is a 50% chance of local side effects with this form of radiation therapy.

Chemotherapy

Chemotherapy can be used for more aggressive tumors, for those that have metastasized, and for those tumors in which the family elects some form of treatment but declines surgery and/or RT. Chemotherapy has a 50% chance of slowing down the growth of a soft tissue sarcoma such as hemangiopericytoma. Chemo pills are typically given, often on an every-other-day basis. Periodic rechecks including a physical exam and blood work are necessary with your veterinary oncologist.

Amputation

If the tumor is located on a limb or on the distal part of the tail, your vet may discuss amputation. Since surgery for tumors in these areas can be difficult to obtain clean margins, amputation may offer a cure. With amputation, wide margins are often feasible. With each recurrence, hemangiopericytomas tend to be more deeply rooted and harder to remove.

Metastasis

Metastasis (spread) can happen with a hemangiopericytoma, but it is rare. It’s estimated that these tumors metastasize in one out of every five cases. The lungs and the regional lymph nodes are the most common areas for metastasis. Ultimately, the goal of treatment is to remove or shrink the main tumor to ensure that it doesn’t spread throughout the body. If any part of the cancer is left behind, it can continue to grow and cause issues for your dog.

Whatever the treatment path chosen, regular follow-up visits with your veterinarian including imaging will help monitor your dog’s progress and how well the treatments are working.

The Prognosis For Dogs With Hemangiopericytoma

The prognosis for dogs with hemangiopericytoma depends on several factors, including the tumor’s size, location, and aggressiveness (grade). Aggressiveness will be determined histologically, by how the cancer cells look under a microscope, by a veterinary pathologist. Dogs with small tumors, tumors that are completely removed, or those with a low to intermediate grade receiving radiation therapy (with or without surgery) often have a good prognosis. Dogs with larger, more aggressive tumors or tumors that have spread may have a much more guarded prognosis.

Regular check-ups and monitoring are essential to observe the area where the tumor was removed and to ensure it does not recur or spread. Pet parents should also keep an eye out for any changes in their dog’s health, such as new lumps, bumps, or signs of pain, and report them to their veterinarian promptly.

Sources:
¹ VIN