An adrenal tumor is any benign or malignant mass on one or both of the adrenal glands. The tumor can be functional, which means that it stimulates unregulated secretion of one or more adrenal hormones, or nonfunctional, which means that it does not cause abnormally high levels of adrenal hormones to enter circulation.
To understand the potential ramifications of adrenal tumors, it is important to have an understanding of what the adrenal glands are, and what they do when working normally. The adrenal glands are tiny, paired structures located in the abdomen, just above each kidney. They are located near the part of the kidneys that is closest to the dog's head. The adrenal glands make, release and regulate a number of important hormones. The outer part of the adrenals, called the adrenal cortex, produces corticosteroid hormones, including mineralocorticoids, glucocorticoids and the adrenal sex hormones progesterone, estrogen and androgens.
Mineralocorticoids, the most potent of which is aldosterone, play a key role in regulating the levels of sodium, potassium and other essential electrolytes in the blood. They are essential to maintenance of adequate internal fluid volume, normal cardiac output and proper blood pressure.
Glucocorticoids, the most important of which is cortisol, are critical to regulating metabolism of carbohydrates, proteins and fats, which makes them important for maintenance of proper blood sugar levels. They also are intimately involved with managing inflammatory reactions and suppressing the immune system. Glucocorticoids' immunosuppressive traits help heal wounds and fight infection.
The innermost part of the adrenal glands, called the adrenal medulla, secretes epinephrine (adrenaline) and norepinephrine (noradrenaline). Both of these hormones increase heart rate and blood pressure, in addition to performing other functions.
Production and release of adrenal hormones are regulated by the pituitary gland, which is located deep inside of the brain. The pituitary gland produces adrenocorticotropic hormone (ACTH). In healthy dogs, ACTH is released from the pituitary gland when circulating levels of corticosteroids are low. ACTH stimulates the adrenal glands to produce and release more adrenal hormones. When blood levels of corticosteroids are normal, the pituitary gland stops secreting ACTH, which "turns off" the adrenal glands' secretion of steroids.
Causes of Adrenal Cancer
As with other types of cancer, the causes of adrenal gland tumors are not well understood. In companion dogs, adrenal tumors are split fairly equally between adenomas, which are benign or non-malignant masses, and adenocarcinomas, which are malignant. A malignant tumor is one that tends to progressively worsen with time and, if not treated or removed, usually will result in death of the affected animal. Malignant masses typically are highly invasive, meaning nearby tissue is affected, and metastatic, which means that the cancerous cells spread from one part of the body to others.
Even though they are not malignant, adrenal gland adenomas are either functional or non-functional. Functional adenomas and adenocarcinomas most commonly cause the adrenal cortex to secrete an over-abundance of cortisol, a glucocorticoid hormone. Normally, when there is an abnormally large amount of cortisol in the blood stream, the pituitary gland reduces its production and secretion of adrenocorticotropic hormone (ACTH). This, in turn, typically causes the adrenal glands to reduce their production and secretion of cortisol and other corticosteroid hormones.
However, when there is a functional tumor on one or both of the adrenal glands, they will continue to produce and release hormones regardless of the presence or absence of the usual ACTH stimulus. Functional adrenal tumors usually are found only on one of the two adrenal glands. As a functional tumor grows, it "ramps up" the affected adrenal gland's unregulated release of steroids into the blood stream. As this happens, the other adrenal gland (called the "contralateral" adrenal gland) starts to shrivel up and decreases dramatically in size. This happens because the overload of corticosteroids in the dog's blood stream tells the pituitary gland to stop releasing ACTH, which leaves the contralateral adrenal gland with nothing to do, and so it withers away.
Non-functional adenomas are benign growths on one or both of the adrenal glands that do not affect the glands' production and release of steroid hormones. Functional adenomas, as well as adenocarcinomas, almost always cause uncontrolled over-production and release of adrenal corticosteroids into circulation.
There is no easy way to distinguish functional adrenal adenomas from adenocarcinomas. However, adenocarcinomas tend to be a bit larger than adenomas, when looked at using X-ray or abdominal ultrasound techniques. In dogs, the right and left adrenal glands seem to be affected by adrenal tumors of both types with equal frequency.
Prevention of Adrenal Tumors
Because veterinary science does not yet fully understand the causes of adrenal gland adenomas and adenocarcinomas, there is no realistic way to describe a sound preventative protocol.
When an adrenal tumor is suspected, many attending veterinarians will refer the owner and her dog to a veterinary teaching hospital or other highly-specialized referral center, where more advanced diagnostic and treatment options are available.
How Adrenal Tumors Affect Dogs
If an adrenal gland tumor is not functional, that is, it is not causing the adrenal gland to secrete abnormally large amounts of corticosteroid hormones, the affected dog usually will have no symptoms of discomfort or disease. The mass will simply be lingering on or inside of the adrenal gland, without causing any perceptible consequences. However, when the tumor is functional, meaning it does cause the affected adrenal gland to secrete abnormally large amounts of adrenal hormones into the blood stream, the dog will feel quite ill. Most dogs with functional adenomas or adenocarcinomas become weak and lethargic, lose their appetite and feel excessively thirsty. These are fairly non-specific signs which often develop with many other diseases and disorders. This makes adrenal tumors especially difficult to diagnose.
Symptoms of Adrenal Tumors
Because tumors of the adrenal gland most commonly afflict middle-aged and older dogs, many owners who see symptoms are fooled into thinking that their beloved pets are simply experiencing the natural effects of aging. The signs that owners and their veterinarians may notice when a dog has a functional adrenal gland tumor include one or more of the following, depending on which part of the adrenal gland is affected:
Increased volume of urine output (polyuria)
Increased thirst and excessive water intake (polydipsia)
Increased appetite and food intake (polyphagia; affected dogs are often ravenous)
Weight gain, frequently to the point of obesity
Abdominal enlargement (pendulous, distended abdomen; pot-bellied appearance)
Hair loss (alopecia; usually patchy, symmetrical and bilateral)
Excessive Panting; often when lying down and resting
Clitoral enlargement in females (clitoral hypertrophy)
Testicular enlargement in males (testicular hypertrophy)
Loss of normal reproductive cycling in females (anestrus)
Thin, fragile skin that tears easily
Poor coat condition
Lack of coordination (ataxia)
Neurological signs (circling, aimless wandering, pacing, bumping into walls or furniture, falling down for no apparent reason)
Poor wound healing
Dogs at Increased Risk
Middle-aged and older dogs are much more likely to develop symptoms from adrenal gland tumors than are younger dogs, regardless of whether the masses are benign adenomas or malignant adenocarcinomas. Females and large-to-giant breed dogs have an elevated risk of developing adrenal tumors. German Shepherds, Labrador Retrievers, Dachshunds, Poodles and some breeds from the Terrier Group seem to be predisposed to adrenal gland tumors, although the reasons for these associations are not well-understood, even by experts on the subject.
Adrenal cancer is a mystery to most people. However, with the tools of modern veterinary medicine, it can be diagnosed and treated, or at least managed.
How Adrenal Cancer is Diagnosed
Most veterinarians presented with a dog showing signs consistent with excess secretion of one or more of the adrenal steroid hormones will initially conduct routine blood work (a complete blood count and serum biochemistry panel) and a urinalysis. The results of these tests may suggest an adrenal gland disorder, if there is a functional tumor on one or both adrenal glands. Thoracic radiographs (chest X-rays) can be taken to look for any spread of cancer (metastasis), enlarged abdominal organs and/or mineralized adrenal glands. Abdominal ultrasound may be recommended to assess the size of each adrenal gland. The veterinarian will be looking for enlargement or irregularity of one of the glands and a shrunken or smaller contralateral adrenal gland, which would be suggestive of cancer.
A number of diagnostic tests based on blood samples are available to determine whether a functional adrenal tumor, once diagnosed, is secreting cortisol, aldosterone or adrenal sex steroid hormones. Abdominal computed tomography (CT scan) or magnetic resonance imaging (MRI) can be quite useful to detect any local tissue invasion from a metastatic adrenal tumor. If surgical removal of the mass is accomplished, submitting it to a diagnostic laboratory for microscopic analysis (histopathology) is the most definitive way to diagnose the precise type of adrenal cancer that is present.
While arriving at a definitive diagnosis of adrenal gland cancer can be somewhat time-consuming, almost all of these tumors can ultimately be diagnosed and, hopefully, treated or at least managed.
As with most cancers, the goals of treating adrenal gland cancer are to remove the primary neoplastic tissue, remove or destroy any sites of metastasis, relieve any discomfort suffered by the patient and restore or prolong a good quality of life.
Dogs with adrenal cancer may need to be stabilized medically before they can safely be taken to surgery for attempted removal of the affected gland. Pre-surgical medical management may include administration of certain drugs to inhibit adrenal production of cortisol and/or other medical therapy to reduce hypertension (high blood pressure) if it is present.
The preferred treatment for dogs with adrenal tumors is to remove them surgically. There is no reliable way to determine unequivocally whether an adrenal mass has become malignant and metastasized (spread) to other body tissues. However, if an adrenal tumor is found and the dog has symptoms of uncontrolled release of adrenal steroids, surgery probably will be recommended. Medical management and supportive care are also important to address steroid-secreting tumors, with or without attempted or effective surgical removal. Medical management long-term can be as costly as, or even exceed the expense of, the initial surgical treatment.
Dogs with adrenal cancer that has not spread, and that recover well following surgical excision of the tumor, have a fair prognosis, possibly of months to years post-surgery. Unfortunately, if the cancer has metastasized, the prognosis is guarded to poor.