Did you know that the most common cancer among women in the Western world, cervical cancer, is caused by the Human papillomavirus (HPV)? And that the Hepatitis B virus is one major contributor to chronic liver cirrhosis, that can ultimately lead to hepatocellular cancer, the most common form of liver cancer in the world? Both cancers could potentially be substantially controlled — or even eradicated — by vaccination. This is not too different from vaccinating against chickenpox, polio, or the flu. But, who would have thought that one can get vaccinated against cancer?
Prevention
As with infectious diseases, vaccination against these cancers should be done before exposure to the pathogenic virus. This means early childhood and adolescence for HPV. Both boys and girls need to be vaccinated since the same HPV virus is known to cause cancers other than cervical cancer, and either gender may transmit the virus. As for Hepatitis B virus, healthcare workers and those at risk of exposure to blood, blood products, or human body secretions need to be preferentially vaccinated.
Despite the advances in controlling cervical and liver cancer, the uptake of such vaccines worldwide remains low, partly due to cost (mainly in the developing world) and due to misguided social stigma (in the developed world).
Most cancers, however, are not caused by viruses. In time, as scientists discover unique genetic properties of other cancers, it should be possible to similarly develop strategies that use unique signature DNA or RNA or tumor-specific peptides as cancer vaccines.
Early detection
The concept of early detection of cancer heavily depends upon reliable tests that can positively predict the presence of the cancer (high positive predictive value) and have a low likelihood of being non-specifically positive (low false positive rate).
A good cancer screening test needs to identify those patients who actually have a malignancy in early stages, and not have a high false positive rate (be sensitive and specific). The PSA blood test as a routine screening test for prostate cancer for example, remains controversial. Men need to carefully consider their individual risk-benefit ratio before electing to undergo PSA testing.
Today, the chest X-ray is giving way to the CT scan, which can identify masses and tumors as small as 5mm. The low-dose helical CT scan has thus been shown to be a useful screening test for lung cancer for a specific subset of smokers.
The CT scan, too, is giving way to the CT/PET, which can identify masses that are growing rapidly and thus potentially malignant. However, despite popular belief, the CT/PET is not a good tool for early detection of cancers, even though it is very useful to differentiate malignant tumors from benign ones.
The plain mammogram, which was the most sensitive early screening test for breast cancer, is giving way to digital 3-D tomosynthesis, which is better able to pick up lesions in women with dense breast or fibrocystic changes. And in some cases, one may even have to revert to breast MRI to detect small tumors. Yet, the relevance of mammography continues to be challenged and the age of onset and frequency of screening for breast cancer is often different in different countries. Those with a family history of breast cancer should abide by more stringent screening guidelines in consultation with doctors.
A colonoscopy accompanied by biopsy of a visualized abnormality has a very high sensitivity and specificity, yet is often avoided by many because it requires a procedure that some find uncomfortable or embarrassing. A new DNA test for colon cancer is gaining popularity since it only requires a stool specimen. However, if found positive, those patients still need to undergo a colonoscopy to confirm the diagnosis.
The future
So, the question remains, is there any test that can detect all cancers at once, such as a whole-body digital CT/PET? The answer is no, because different tissues have different properties and are surrounded by different normal organs. Thus, a CT scan can view abdominal organs very well, but is not a good test for colon cancer since colon tumors are hidden in the folds of colonic tissue. It cannot pick up pancreatic cancer early either, because small pancreatic lesions may be inconspicuously hidden between abdominal organs in the vicinity.
The advent of genomic medicine may provide new hope for the early detection of most cancers. Medicine is evolving and the era of molecular diagnosis is ushering in the ability to identify cancer-causing genes even in the smallest droplet of a person’s blood. All tumors in the body need access to blood vessels in order to grow. They also shed cells as they grow, and these cells shed their DNA into the bloodstream. If we can identify genetic markers of cancer, we could search for and identify these tiny tumor DNA fragments in a drop of a patient’s blood.
This capability is around the corner. However, not every person with a tumor will develop cancer, since the human immune system may destroy such small tumor deposits before they become detectable tumors. Such patients will not die of cancer even though they may have a positive circulating tumor DNA test. And becoming aware of one’s tumor DNA fingerprint can be a source of great anxiety — akin to a reading of your palm by a fortune teller. What information should one act upon, and what should one ignore?
For now, the accompanying guidelines of the American Cancer Society provide useful information about the currently-accepted cancer screening tests. It is important to consult a doctor to gauge one’s personal circumstances and risk in order to best apply these guidelines. Thus, family history and personal risk factors such as smoking and obesity, need to be taken into consideration in selecting the ideal cancer screening test for any given individual.
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Dr. Mansoor Saleh is Professor of Medicine at the O’Neal Comprehensive Cancer Center, University of Alabama, and Founding Director of the new Cancer Centre at the Aga Khan University Hospital, Nairobi.
