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Adrenal Medullary Hyperplasia: |
| Diffuse or nodular adrenal medullary hyperplasia appears to precede the development of pheochromocytoma in bulls with C-cell tumors of the thyroid gland. This diffuse proliferation of chromaffin cells is nonencapsulated but compresses the surrounding adrenal cortex. In bulls with prominent diffuse medullary hyperplasia, there are often a few small foci of intense nodular proliferation of medullary cells. |
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Pheochromocytomas: |
| These tumors of chromaffin cells are almost always located in the adrenal glands. They are the most common tumors in the adrenal medulla of animals; they develop most often in cattle, laboratory rats, and dogs, and are infrequent in other domestic animals. In bulls and rats, pheochromocytomas develop concurrently with calcitonin-secreting C-cell tumors of the thyroid gland, possibly as a neoplastic transformation of multiple types of endocrine cells of neuroectodermal origin in
the same individual. Malignant pheochromocytoma designates a medullary tumor that invades through the adrenal capsule into adjacent structures (eg, posterior vena cava) or metastasizes to distant sites (eg, liver, regional lymph nodes, or lungs), or both. Functional pheochromocytomas are reported infrequently in animals; however, several dogs and horses with pheochromocytomas have had tachycardia, edema, and cardiac hypertrophy attributed to excess catecholamine secretion. It
appears that horses may have a syndrome similar to the multiple endocrine neoplasia noted in humans with concurrent adrenal and thyroid disease. |
| Although size varies considerably, pheochromocytomas may be large (≥10 cm in diameter) and incorporate most of the affected adrenal. A small remnant of the adrenal gland often can be found at one pole. Smaller tumors are well encapsulated by a thin, compressed rim of adrenal cortex. Large pheochromocytomas are multilobular and variegated, and they may exert pressure on and invade adjacent tissues, particularly the vena cava and aorta. In dogs, ~50% of pheochromocytomas
metastasize to the liver, regional lymph nodes, spleen, and lungs. |
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