These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer". Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding about tumors and their treatment in animals is improving all the time.
We understand that this can be a very worrying time. If you have any questions please do not hesitate to ask us.
What are neuroendocrine cells and chemoreceptors?
Neuroendocrine cells produce specialized chemical substances called "neuroendocrine hormones". These hormones affect the rates of specific chemical reactions in nearby cells or in other tissues throughout the body. Neuroendocrine hormones interact with the nervous system and other types of hormones to integrate and co-ordinate a wide variety of activities that maintain internal stability of the body.
"Neuroendocrine hormones... integrate and co-ordinate a wide variety of activities that maintain internal stability of the body."
Chemoreceptors are neuroendocrine cells that are receptive or sensitive to changes in blood carbon dioxide, oxygen and acidity. They are widely distributed in the body but tumors develop principally in the aortic and carotid bodies in domestic animals. The carotid body is in the neck. The aortic body is at the base of the heart. They formed from multiple small collections of cells embedded with nervous tissue in the outer walls of the carotid artery and aorta.
What are chemodectomas?
Chemodectomas are tumors of the chemoreceptors. They include both benign (non-spreading) adenomas and malignant carcinomas. Malignant tumors are locally invasive and approximately 30% metastasize to other sites in the body. Both benign and malignant tumors are difficult to remove surgically as they wrap around major blood vessels. Tumors may be multiple.
What do we know about the cause?
The reason why a particular pet may develop this, or any cancer, is not straightforward. Cancer is often the culmination of a series of circumstances that come together for the unfortunate individual.
Cancer is essentially the result of non-lethal genetic damage (mutation) to cells with "external" contributory factors that may include radiation, chemicals, hormones and infections. The mutated cells upset the normal regulation of cell death and replacement. They do this by activating growth-promoting oncogenes (cancer genes), inactivating suppressor genes and altering the genes that regulate normal, programmed cell death (apoptosis).
Why has my animal developed this cancer?
Some animals have a greater tendency (genetic susceptibility) to cancer. Some breeds have far more cancers than others, often of specific types. The more divisions a cell undergoes, the more probable is a mutation so cancer is more common in older animals.
Chemodectomas are most common in short-nosed dogs so it is suggested that low oxygen concentrations combined with a genetic predisposition may cause these cancers. Further evidence for this theory is that people who live at high altitudes have ten times as many of these tumors as people who live at sea level. This implies that prolonged stimulation of the cells of the gland is the major cause of development of this type of cancer. Cancer induction is a multi-step process called tumor progression. Some cancers never progress past the first stages so remain benign. Others progress rapidly. There is a continuous spectrum from benign to malignant cancer.
Are these common tumors?
These are all rare tumors, primarily recognized in dogs and very infrequently in cats. Chemodectomas have a breed predilection for short-nosed dogs, particularly the Boxer and Boston terrier. Most are eight years of age or older and males are more often affected than bitches. Tumors are most common in the aortic body but in 65% of cases, there are tumors in both carotid and aortic bodies.
How will these cancers affect my pet?
The neuroendocrine cells of chemodectomas are non-functional, so clinical problems are primarily associated with their enlargement and consequent compression of the adjacent organs. Carotid body tumors interfere with swallowing and compress the large veins and arteries of the neck. Aortic body tumors compress the major blood vessels around the heart so produce signs otherwise associated with cardiac disease (shortness of breath, coughing, and edema or soft swelling of the tissues under the skin of the neck).
How are these cancers diagnosed?
With carotid body tumors, the swelling in the neck indicates a problem. Echocardiography (cardiac ultrasound) or specialized aortography techniques may demonstrate the presence of aortic body tumors.
Accurate diagnosis and therefore prediction of behavior (prognosis) rely upon microscopic examination of tumor tissue (histopathology). For this procedure, your veterinarian will submit either a small part of the mass (biopsy) or the whole tumor to a veterinary pathologist at a specialized diagnostic laboratory. These tumors break apart easily because they are highly vascular, and they also wrap around large blood vessels so they are extremely difficult to remove completely. It is rarely possible to determine if the cancer has been fully removed.
What types of treatment are available?
Treatment is surgical removal of the lump(s).
Can these cancers disappear without treatment?
It is not common, but the loss of blood supply to a cancer can make the cells die. Unfortunately, the disappearance of the cancer is rarely complete.
How can I nurse my pet?
After surgery, you will need to prevent your pet from interfering with the incision site, which needs to be kept clean and dry. Report any loss of stitches or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask.
How will I know how the cancer will behave?
The histopathology report will give your veterinarian the diagnosis that helps to indicate how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence or metastasis (distant spread).
When will I know if the cancer is permanently cured?
'Cured' has to be a guarded term in dealing with any cancer. In 65% of chemodectomas cases, there are concurrent carotid and aortic body tumors. Both benign and malignant tumors are surgically challenging to remove because they envelop or wrap around major vessels. Total cure is therefore uncertain even when tumors are benign.
"Total cure is therefore uncertain even when tumors are benign."
Aortic body tumors grow slowly and exert pressure on the large vessels. Malignant tumors often invade the heart and adjacent vessels. When they metastasize, secondary tumors are most frequent in the lung and liver. The histopathology report should indicate whether this is probable in your pet.
Carotid body tumors are more malignant than aortic body tumors and metastasize (spread elsewhere in the body) in approximately 30% of cases. Metastases may be in the lung, lymph nodes and other organs.
Are there any risks to my family or other pets?
No, these are not infectious tumors and are not transmitted from pet to pet or from pets to people.