Review and Diagnosis of Aortic Body Tumors in Dogs and Cats

Shannon M Bement, DVM; Kenneth S. Latimer, DVM, PhD; Heather L. Tarpley, DVM; Holly A. Moore, DVM

Class of 2005 (Bement), Department of Pathology (Latimer, Tarpley, Moore), College of Veterinary Medicine, The University of Georgia, Athens, GA 30602-7388

Introduction

Neoplasia of the heart is rare. The most common neoplasms affecting the heart include hemangiosarcoma and heart base tumors, specifically aortic body tumors.³ In the current literature, the term "heart base tumor" can refer to any neoplasm in the region of the heart base, including chemodectoma (aortic body tumor), ectopic thyroid or parathyroid carcinomas, hemangiosarcoma, lymphoma, or metastatic tumors (Fig. 1).^1,2,3 "Heart base tumor" is also often used synonymously with chemodectoma, aortic body tumor, or cardiac paraganglioma (Fig. 1).

Figure 1. Classification of heart-associated tumors

Chemodectomas are neoplasms of the chemoreceptor organs. These organs are of neuroendocrine origin and arise embryologically from the neural crest.² These chemoreceptor organs are responsible for detecting changes in blood oxygen levels, carbon dioxide levels, pH, and blood temperature.^1,2 They act through the autonomic nervous system to cause appropriate changes in respiratory and cardiac function via parasympathetic nerves. There are several chemoreceptor organs throughout the body, but the aortic body and the carotid body are the most common sites for development of neoplasia in dogs and cats. The aortic body is located within the pericardium at the base of the heart, typically between the aortic arch and the pulmonary artery. The carotid body is located at the bifurcation of the carotid arteries and typical presentation is a neck mass.^1,2,5 Aortic body tumors are reported to occur 4 to 5 times more frequently than carotid body tumors in dogs.^2,8 There have only been 6 cases of feline aortic body tumors reported in the literature, and even fewer cases of carotid body tumors.^8,9 The main focus of this article will be aortic body tumors.

Incidence

Aortic body tumors can be benign or malignant. Malignant tumors are less common, but they may invade the heart and vessel walls.⁸ Metastatic disease has been reported to affect the lungs, lymph nodes, liver, bone, and other organs.^1,6 In one study of 357 cases of chemoreceptor tumors in dogs, only 12% of aortic body tumors had distant metastases, most frequently to the liver and lungs.² In cats, metastatic disease seems to be more common, with 3 of 6 cases exhibiting metastasis.⁹

Signalment

The average age of dogs and cats at the time of diagnosis of aortic body tumors is 10 years. Reports indicate that brachycephalic breeds of dogs (Bulldogs, Boxers, Boston Terriers) are more commonly affected with aortic body tumors.^1,2,8 Theories suggest that the chronic hypoxia associated with these breeds over-stimulates the chemoreceptor organs causing hyperplasia and an increased incidence of neoplasia.² Some reports indicate that male dogs have a higher incidence of disease than females, though this has not been demonstrated in all studies reported for aortic body tumors.^1,8 There is no indication of breed or sex predisposition in cats.⁹ The incidence of concurrent neoplasia associated with aortic body tumors is common. Approximately 52% of animals with an aortic body tumor also have another type of neoplasia. The most commonly associated forms of neoplasia are interstitial cell tumors and thyroid carcinomas.^1,2,8

Clinical Presentation

Disease caused by an aortic body tumor is most commonly associated with local compression of the right side of the heart leading to right- sided congestive heart failure.^3,9 Pathology is usually due to tumor enlargement that restricts ventricular filling, leadingto poor cardiac output. The tumor also results in pericardial effusion and cardiac tamponade, which further exacerbate the compression. Clinical signs associated with aortic body tumors may involve any signs consistent with right-sided heart failure. History of the affected patient may include cough, dyspnea, anorexia, weight loss, vomiting, diarrhea, weakness, or abdominal distention.^1,8 On physical examination, the patient may appear lethargic, dyspneic, or cyanotic. A holosystolic murmur, arrhythmia (gallop rhythm, tachycardia), or altered lung sounds may be heard on thoracic ausculation.^3,8 Abdominal distention or subcutaneous edema around the head, neck, and forelimbs may be apparent.⁸ These clinical signs may range from subtle to severe. The tumor also may be found incidentally when evaluating other medical conditions.

Diagnosis

Diagnosis of an aortic body tumor is based on signalment, history, clinical signs, physical examination, and several ancillary tests. A minimum data base includes a complete blood cell count, biochemical profile, urinalysis, electrocardiogram, thoracic and abdominal radiographs, and thoracic and abdominal ultrasound examination. Although blood or urine tests are very important in evaluating concurrent disease processes, there are no specific abnormalities that indicate the presence of an aortic body tumor.

Electrocardiogram (ECG)

An ECG can be helpful in identifying the presence of pericardial effusion and/or myocardial disease.⁸ Arrhythmias including tachycardia and ventricular premature contractions may be detected, suggesting myocardial damage. Electrical alternans, if seen, is an indication of pericardial effusion.⁸

Radiographs

Thoracic and abdominal radiographs will help to identify the heart based mass, metastatic disease, and secondary disease associated with the primary tumor. Thoracic radiographs may indicate a variably sized mass at the base of the heart associated with the aorta and pulmonary artery (Fig. 2). Right-sided cardiac enlargement and pericardial effusion may be present. Depending on the size of the tumor, the trachea may be deviated dorsally. Metastases to the lungs also can be evaluated when left and right lateral and ventral-dorsal thoracic views are obtained. Abdominal radiographs will evaluate all of the abdominal organs and specifically the liver and spleen, which may appear congested or contain metastases.

Figure 2. Lateral thoracic radiographic projection of heart-base tumor with marked dorsal tracheal elevation.

Thoracic and Abdominal Ultrasound

Thoracic ultrasound examination or echocardiography can be used to obtain precise tumor measurements and determine the degree of involvement with the heart and associated vessels (Fig. 3). This will help to determine if surgical excision is possible. The echocardiogram can detect pericardial effusion and the extent of heart failure or compromise. This technique also provides the opportunity to obtain an ultrasound-guided aspirate of the mass or of pericardial fluid. These samples can be analyzed for cells indicative of an aortic body tumor. Abdominal ultrasound examination is useful in evaluating the abdominal cavity and internal organs for disease and staging.¹²

Figure 3. Ultrasound imaging of a heart-base tumor in a dog. The neoplasm is adjacent to the aorta (black circular area) at the base of the heart (top) (image courtesy of the Department of Anatomy and Radiology, College of Veterinary Medicine, University of Georgia).

Computed Tomography (CT)

More advanced imaging techniques, when available, are helpful in evaluating the extent of disease. This is very helpful when determining if a tumor is surgically resectable. With CT, the vessels and heart wall are imaged in slice form, allowing better anatomical evaluation of the tumor.

Thoracic Exploratory and Biopsy

A biopsy specimen is necessary for the definitive diagnosis of an aortic body tumor. Complete tumor resection is ideal but rarely possible due to significant envelopment of the vasculature by the neoplasm. Grossly, a typical aortic body tumor measures approximately a 4 to6 cm in diameter and appears as a single or nodular, smooth, firm, encapsulated mass that surrounds or invades the aorta and/or pulmonary artery (Fig. 4).^1,8 The tumor has a mottled gray appearance on cut surface.^1,8 Published reports indicate that tumors can range in size from 1-13cm in length.⁸

Figure 4. A heart-base tumor (top) distorts the appearance of the heart and envelops the great vessels (image courtesy of Noah’s Arkive, University of Georgia).

The microscopic appearance of an aortic body tumor is very similar to normal chemoreceptor organs. Clusters of neoplastic, endocrine-like cells are divided by well vacularized connective tissue trabeculae. Neoplastic cells have a uniform appearance. Individual cells have a round to oval nucleus and eosinophilic cytoplasm that sometimes has vacuoles.^1,4,7,8,9 Scattered, large giant cells can also be seen. Hemorrhage and necrosis are often evident.^1,4 Malignancy of an aortic body tumor is difficult to evaluate microscopically because malignant tumors may have rare mitotic figures and benign tumors may have many mitotic figures.^1,4,8

Cytologic Features

Aortic body tumors are exfoliative and can yield cells that are highly suggestive of aortic body tumors.⁹ Cytology can be obtained by ultrasound guided biopsy of the heart based mass. Clotting profiles should be evaluated prior to aspiration. Several aspirates should be obtained if possible, because cytologic slides are commonly nondiagnostic because the neoplastic cells are easily lysed. ⁹ Fine-needle aspirates may consist of blood, lysed cells, bare nuclei, and intact epithelial cells. In cellular samples, aggregates of endocrine-like cells will predominate. Anisocytosis and anisokaryosis are often present. Individual cells have a dark, round, centrally placed nucleus with a fine to moderate granular chromatin pattern. Nucleoli may be distinct, faintly visible, or absent. There may be a moderate amount cytoplasm that is lightly basophilic and may contain vacuoles. A differential diagnosis for this cytologic appearance is an ectopic thyroid carcinoma.⁹

Figure 5. Cytology of an heart-base tumor with neuroendocrine-like cells, free nuclei, and erythrocytes.

Treatment

Surgical excision is the treatment of choice for aortic body tumors, but the location and usual vasculature invasion make surgical removal difficult.^8,11 Radiation treatment has been used in a limited number of cases; however, the success of this treatment regimen requires further research.⁸ Research has been done which indicates that palliative pericardectomy alone done at the time of biopsy can greatly improve the quality of life and life expectancy. This procedure has been performed in dogs with and without pericardial effusion at the time of surgery. In one study of 24 dogs, the median survival time of following pericardectomy was 730 days, whereas the median survival time of dogs not receiving a pericardectomy was 42 days.¹¹ In another study of 25 dogs, the results were essentially the same in that dogs treated via pericardectomy survived significantly longer than those that did not have this elective procedure.¹⁰

Prognosis

The prognosis for animals with aortic body tumors is guarded to fair.^8,9 Average survival time in dogs treated medically is 4 months. The average survival time for dogs with a palliative pericardectomy is 22 months. ^10,11 In cats, the length of survival ranges from 0 to 19 months, depending on type of treatment employed.⁹

References

1. Johnson KH: Aortic body tumors in the dog. J Am Vet Med Assoc 152:154-160, 1968.

2. Hayes HM, Sass B: Chemoreceptor neoplasia: A study of the epidemiological features of 357 canine cases. Zentralbl Veterinarmed [A] 35:401-408, 1988.

3. Ware WA, Hopper DL: Cardiac tumors in dogs: 1982-1995. J Vet Intern Med 13:95-103, 1999.

4. Evans MG, Lana DP, McMichael TL: Aortic body tumor with adjacent ectopic thyroid tissue in a dog. J Comp Pathol 96:237-240, 1986.

5. Dean MJ, Strafuss AC: Carotid body tumor in the dog: A review and report of 4 cases. J Am Vet Med Assoc 166:1003-1006, 1975.

6. Szczech GM, Blevins WE, Carlton WW, et al: Chemodectoma with metastasis to bone in a dog. J Am Vet Med Assoc 162:376-378, 1973.

7. Tillson DM, Fingland RB, Andrews GA: Chemodectoma in a cat. J Am Anim Hosp Assoc 30:586-590, 1994.

8. Owen TJ, Bruyette DS, Layton CE: Chemodectoma in Dogs. Compend Contin Educ Pract Vet 18:253-264, 1996.

9. Caruso KJ, Cowell RL, Upton ML, et al: Intrathoracic Mass in a Cat. Vet Clin Path 31:193-195, 2002

10. Vicari ED, Brown DC, Holt DE, Brockman DJ: Survival times of and prognostic indicators for dogs with heart base masses: 25 cases (1986-1999). J Am Vet Med Assoc 219:485-487, 2001.

11. Ehrhart N, Ehrhart EJ, Willis J, et al: Analysis of Factors Affecting Survival in Dogs with Aortic Body Tumors. Vet Surg 31:44-48, 2002.

12. McEntree MC: Contemporary Tumor Imaging: Optimizing treatment decisions. Proceedings, ACVC 2001. www.vin.com.

Acknowledgement

The image "Unser Freund - Der Hund of Boxer" is from the Encore Editions website at the Boxer page.

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