Cancer is a scary thing. It is a diagnosis that nobody wants to hear. The mere mention of the emotionally charged C-word provokes stress and anxiety in human patients and their loved ones.
Fortunately, dogs diagnosed with cancer do not understand their potentially dire circumstances, although sadly, their owners do. They often turn pale when I regretfully mention cancer as a possible cause of illness in their pets.
Cancer is not a single disease, but a group of syndromes with similar characteristics. Cancer is characterized by a condition called malignancy, in which cells begin to reproduce uncontrollably. Malignant cells often form discrete tumors that invade surrounding tissues, which may spread to distant organs such as the lungs or the brain. Some cancers, such as leukemia and lymphoma, do not always cause tumors to form. These disseminated cancers may spread throughout the blood and tissues of the body.
Cancer in its early stages often causes no symptoms. However, as tumors progress they may compromise affected structures and cause direct harm; bone cancers may cause bones to break, kidney cancers can cause kidney failure, intestinal cancers can block the passage of food through the intestines, and brain cancers can cause seizures and other neurological problems.
Cancers may be fragile, which can lead to rupture and life-threatening hemorrhage — this is especially common in cancers of the spleen in dogs.
Cancers may spread to nearby tissues, causing complications. For instance, some adrenal gland cancers have a proclivity to grow into a great vessel called the caudal vena cava, which can cause the vessel to burst, with fatal results.
Cancers can spread and cause problems in remote organs. Bone cancer may spread into the lungs, causing breathing difficulty. It may similarly spread to the brain, causing seizures or behavioral changes.
In many cases, cancers cause symptoms associated with the affected organs. In dogs, bone cancers often cause pain and subsequent limping on the affected limb. However, in many other cases cancers cause only vague symptoms such as lethargy, poor appetite, and weight loss.
All cancers are most treatable when they are caught early, but diagnosis is tricky. Currently, cancer is most frequently diagnosed through diagnostic imaging such as X-rays, CT scans, and ultrasound, which can identify discrete tumors. We confirm the diagnosis with microscopic analysis (histopathology) of cells from the suspect site. Unfortunately, the disease has to be significantly advanced for it to be identified in this fashion, and often, by the time a diagnosis is made, the cancer has spread to an extent that limits treatment.
One of the holy grails of medicine is a simple test that can identify cancer early, before symptoms have occurred and before a discrete tumor is identifiable. Cancer in such an early stage is called occult cancer. In theory, development of such a test should be possible.
The biochemistry of malignant cells differs from that of benign ones. They are likely to release unique chemicals (called biomarkers) into the blood stream. If we can identify and measure these biomarkers, simple blood tests should be able to identify occult cancer during routine checkups of seemingly healthy dogs.
Happily, a recent paper in Veterinary and Comparative Oncology shows that some progress is being made towards developing a simple blood test for some types of occult cancer in dogs. The study involved a large number of apparently healthy dogs. Two biomarkers (called TK1 and cCRP) were measured in the dogs. The dogs were followed over time and those that developed cancer were identified. It was determined that, by using a special algorithm, the previously measured levels of TK1 and cCRP could be predictive of cancer.
This research is still very much in its nascency. We are a ways off from commercial laboratory or point-of-care blood tests for occult cancer. However, the research is definitely potentially a step in the right direction.
Why did I say potentially? This research reminds me of a biomarker test for human cancer that has turned out to be one of the most controversial, and for many men, disastrous diagnostic tests ever developed.
I’m talking about the prostate-specific antigen, or PSA test. PSA is a biomarker released by the prostate; prostate tumors release PSA at higher levels than healthy prostate tissue. PSA levels above a certain threshold were deemed suspicious for prostate cancer.
PSA testing therefore became a routine recommendation in middle-aged and older men. But PSA testing had a flaw: too many false positives. (To understand a false positive, one need only think of car alarms; when one wakes you up at 3 a.m. with no burglar in sight, you are the victim of a false positive.)
High PSA levels led large numbers of men and their loved ones to suffer the stress of fearing they might have cancer. In addition to stress, for some men the PSA test led to extensive imaging and painful biopsies, or sometimes to removal of the prostate gland with complications such as incontinence and impotence.
In some instances the men involved did not have prostate cancer. Many other men had low grade tumors that were removed but would have been best left alone — such tumors were not likely to cause harm before the men succumbed to some other malady. PSA testing is now falling out of vogue; I know I won’t be getting the test any time soon.
Human experience with the PSA test indicates that caution should be used when biomarker assays for cancer are developed. However, it does not mean that we should abandon biomarker tests altogether. Well-developed, properly used biomarker tests for cancer have tremendous potential to advance both human and veterinary medicine.
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