Cancer screening for healthy people could be unnecessary
Updated: 2015-08-11 09:03
By Ronald Ng(HK Edition)
A cursory glance at any cancer treatment results would show that patients with early stage cancer have a much higher chance of living longer than patients with late stage cancer. This would lead to the conclusion that screening for cancers so they could be diagnosed earlier must be the right thing to do, right? Well, the answer is not so simple.
How do we measure cancer survival? The current theory argues that most cancer starts with a cell mutating to become cancerous. This cell then divides and its progeny then divide and divide and divide, creating more and more cancer cells, which ultimately kill a person. Since a cell is small, it cannot be detected. For this reason, we have no idea when that cancer cell first appears. The survival statistics of all cancers are therefore measured from the time of diagnosis to the time of death.
Let us say the body of one man, surnamed Wong, develops a cancer cell on Jan 1, 2000, and the body of another man, surnamed Chan, also develops a cancer cell on the same day. Both of them will not know that cancer has started. Let us say Wong develops symptoms of cancer on July 1, 2002, and is diagnosed to have cancer that day. He subsequently dies on July 1, 2003. So the statistics on his cancer will record him as having survived cancer for one year.
Now, say Chan goes for yearly screening, and because of that his cancer is discovered on Feb 1, 2002, earlier than Wong. However, Chan also dies on July 1, 2003. The statistical report on Wong will show he has survived cancer for one year and five months.
Although both start having cancer and die from it on the same day, one is reported to have survived longer than the other due to the way statistics are recorded and not due to any real improvements in survival due to cancer screening. Only if Chan had screening and lived significantly beyond July 1, 2003 could one say screening and early detection of cancer prolonged Chan's life.
Is there any way we could prove that early diagnosis by screening improves the likelihood of a person surviving? The only way to prove that is to have a large number of people having screening tests done and compare their survival to another large group of people who do not go for screening, and who only go to see a doctor when symptoms occur. Survival here is counted from the day they enter the study, and not from the day cancer is diagnosed, because if the survival is counted from the day cancer is diagnosed, the same bias (as illustrated by the case of Wong and Chan) will apply.
To date, screening for cancer has been found to be useful in prolonging lives only in breast cancer, cervical cancer and cancer of the large intestine.
Are there any negative pitfalls to doing unnecessary cancer screening? The answer is an emphatic yes. No matter how good the test is, there will be false positive results, meaning that a person might have tested positive without there being any disease. Once a person tests positive, more tests will have to be conducted. The tests might include expensive scans and even surgery.
Many years ago, a number of well-known cancer hospitals in the United States conducted a study on the use of chest X-ray to screen for lung cancer, and found that it was totally useless. Not only that, it led to many unnecessary chest operations because of the problem of false positives.
So what is the bottom line? The bottom line is to relax and enjoy life, and lead a healthy lifestyle. If you are a smoker, stop smoking immediately. Eat more vegetables and less fat. Do not waste your money on cancer screening except for those tests that have proven to be useful - breast cancer screening with mammography over the age of 50 or over the age of 40 if there is family history of breast cancer; regular Pap smears if the woman is sexually active; and regular check of stools for occult blood with follow up of colonoscopy if that is positive.
For those who advocate medical screening, please note that I mean seeing a doctor for tests when there are no symptoms. If there are any symptoms such as prolonged coughing, weight loss, feeling a lump somewhere, or seeing blood in the stool, going to see a doctor in those situations is not health screening, it is seeking a diagnosis to account for those symptoms.
So see a doctor if you feel something is not right, but if you feel yourself to be 100 percent healthy, do not opt for screening.
(HK Edition 08/11/2015 page9)