Hemangiopericytomas of Dogs

Colin Hurd, DVM; Perry J. Bain, DVM, PhD; Heather L. Tarpley, DVM; Melanie E. Johnson, DVM; Pauline M. Rakich, DVM, PhD; Kenneth S. Latimer, DVM, PhD

Class of 2004 (Hurd), Department of Pathology (Bain, Tarpley, Johnson, Latimer), and Athens Veterinary Diagnostic Laboratory, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

Hemangiopericytomas are common neoplasms that are found predominantly in the skin and subcutis of the dog. These neoplasms are presumed to arise from pericytes that surround small blood vessels; however, the actual cell lineage is unknown. Although the tumor’s name suggests a benign growth, these neoplasms have malignant nature, featuring aggressive growth, extensive local invasion, and infrequent metastasis.² Hemangiopericytomas often are classified with schwannomas and neurofibromas as spindle cell tumors of canine soft tissue.³

Clinical Features

Hemangiopericytomas are seen most commonly in older (average age of 10 years), medium to large breed dogs. This neoplasm is rarely seen in cats. Grossly, hemangiopericytomas can be soft, firm, or rubbery and are frequently mistaken for lipomas. They also may be nodular and locally invasive. Hemangiopericytomas occur most frequently on the extremities, but may arise on the trunk (Figs. 1 and 2).¹

Figure 1. Irish setter with two masses on the left lateral thigh. Both masses were diagnosed as hemangiopericytomas (image courtesy of Noah’s Arkive, University of Georgia).	Figure 2. Hemangiopericytoma on the hindlimb of an English Setter. The skin over the surface of the neoplasm is ulcerated (image courtesy of Noah’s Arkive, University of Georgia).

Diagnosis

A presumptive diagnosis of hemangiopericytoma can sometimes be made cytologically. Fine-needle aspirates of these neoplasms may contain spindloid to polyhedral cells with light gray, wispy cytoplasm and a round to oval nucleus that may contain one or two nucleoli (Fig. 3). Some neoplastic cells will have cytoplasm with a veil-like appearance (Fig. 4). A few binucleated and multinucleated cells and scattered small lymphocytes also may be observed.

Figure 3. Neoplastic cells from a fine-needle aspirate of an hemangiopericytoma have a spindleoid appearance. Nucleoli are present but difficult to visualize (Wright stain).	Figure 4. Fine-needle aspirate of an hemangiopericytoma in a dog. Some of these cells have light gray cytoplasm that resembles a veil (Wright stain).

Definitive diagnosis of hemangiopericytoma usually requires surgical biopsy and histopathology. Microscopically, hemangiopericytomas have a characteristic "fingerprint pattern" appearance (Fig 5 and 6).¹ This "fingerprint" is composed of multiple layers of spindle cells arranged in a concentric whorls that frequently (but not always) surrounds a central blood vessel.¹ However, fibrosarcomas, cutaneous fibrous histiocytomas, and schwannomas also may exhibit whorling patterns. Scattered aggregates of small lymphocytes also may be observed within these neoplasms. Hemangiopericytomas with two or more mitoses per ten 45x fields of view usually are locally invasive and difficult to surgically resect.

Figure 5. Low magnification of an hemangiopericytoma showing the classical onion skin or whorling appearance (hematoxylin and eosin stain).	Figure 6. Higher magnification of the same neoplasm. Individual neoplastic cells are spindleoid with a round to oval nucleus, occasional nucleolus, and attenuated eosinophilic cytoplasm. A few small lymphocytes (dark round cells) also are present (hematoxylin and eosin stain).

Currently, there are no immunohistochemical (IHC) markers for the specific identification of hemangiopericytomas or that would differentiated them from other sarcomas. Hemangiopericytomas do not stain for vascular markers. However, IHC may be useful to exclude other relatively undifferentiated sarcomas (such as hemangiosarcoma) from the differential diagnosis.¹

Treatment

The preferred initial treatment of hemangiopericytoma is surgical excision with wide margins. Hemangiopericytomas have characteristics typical of soft tissue sarcomas. They often have a pseudocapsule, out of which extend multiple microscopic tendrils of neoplastic cells. Incomplete resection of the entire neoplasm will result in recurrence; therefore, wide surgical margins are essential. However, surgery intervention can only control approximately 70% of these neoplasms.¹

If the hemangiopericytoma recurs after attempted surgical excision, other treatment options include further resection the neoplasm, limb amputation, surgical debulking of the neoplasm followed by radiation therapy, or no further attempt at treatment. Because microscopic tendrils of the neoplasm may be inadvertently left following primary resection, the recurrent tumor usually is more invasive than the original neoplasm.

Radiation therapy can be used after attempted surgical excision if clean margins are not obtained. The neoplasm’s remission time may be somewhat prolonged after radiation.

Prognosis

Although most hemangiopericytomas do not metastasize, they do have a high incidence of recurrence. Furthermore, tumor recurrence usually is associated with a greater degree of infiltration into the surrounding tissues. Thus, the clinical prognosis is better in those instances where the neoplasm can be excised cleanly. If the entire neoplasm is removed, the patient may have an excellent prognosis. However, if microscopic tendrils of neoplastic cells are left behind, the clinical prognosis is poorer depending upon how rapidly the tumor returns. Neoplasms that grow slowly may not cause the patient significant problems. Conversely, neoplasms that grow rapidly may require further treatment.² Because chemotherapy is not considered useful, there are no other treatment options besides wide surgical excision and radiation therapy.

References

1. Mazzei M, Millanta F, et. al.: Haemangiopericytoma: Histological spectrum, immunohistochemical characterization and prognosis. Vet Dermatol 13:15-21, 2002.

2. Morrison WB: Cancer in Dogs and Cats: Medical and Surgical Management, 2nd ed. Jackson Hole, Teton NewMedia, 2002, pp. 83, 251,483-484.

3. Yager JA, Wilcock BP: Color Atlas and Text of Surgical Pathology of the Dog and Cat, vol. 1, 1994, pp. 287,289-290,686-691.

Acknowledgement

"Up Here Boss", watercolor of a field English Setter by Ron Dotson, is from his web site Ron Dotson Art and is used with permission.

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