Why a growing number of cancers may best be left untreated

There are few things as upsetting as being told you have cancer. A provocative study published Sunday in the Medical Journal of Australia concludes some cancers may not be as harmful as once thought.

The study was led by Prof. Paul Glasziou, director of the Institute for Evidence-Based Healthcare at Bond University. The paper used data from the Australian Institute of Health and Welfare to assess lifetime risk of five cancers over a 30-year period.

Australians alive today are far more likely to be diagnosed with cancer than their parents were three decades ago.

The study concluded that Australians are increasingly being diagnosed with cancers that will do them little if any harm if left alone. In fact, the figures suggest that nearly one in four (24 per cent) so-called cancers in men were what doctors refer to as "overdiagnosed." In women, nearly one in five (18 per cent) cancers were overdiagnosed.  

Overdiagnosis means identifying problems that weren't causing symptoms and were never going to cause the patient harm.

The researchers found that, in men, 42 per cent of prostate cancers, 42 per cent of kidney cancers and 58 per cent of melanomas were overdiagnosed. In women, 22 per cent of breast cancers, 58 per cent of kidney cancers and 54 per cent of melanomas were overdiagnosed.

When it came to harmless cancers, thyroid cancer was in a class by itself. In men and in women, a staggering 73 per cent of all thyroid cancers were essentially harmless.

An overdiagnosis epidemic in Canada

The study is from Australia, but the results are highly relevant to Canada. A 2017 study published in the Canadian Medical Association Journal tracked a rapid increase in the incidence of thyroid cancer. In a 42-year period ending in 2012, the incidence jumped from four to 23.4 per 100,000 women, and from 1.5 to 7.2 per 100,000 men. No plausible cause for this was found, and researchers chalked that up to an overdiagnosis epidemic in Canada.

Overdiagnosis of prostate cancer in Canadian men has also been observed. The issue exists to some extent in kidney cancers, as well as melanoma.

Several factors may account for overdiagnosis of cancer. It might be due to increased rates of testing and from adopting high-sensitivity testing that detects very tiny cancers. It may also arise from efforts to broaden the definition of cancer so as to include more patients.

The overuse of tests can lead to detection of incidental abnormalities. For instance, the increased use of CT scans has led to the detection of small kidney cancers whose discovery is not related in any way to the reason why the CT scan was done in the first place.

Overdiagnosis of cancer carries some important implications. Harm may come from cancer treatment of patients who would never have had symptoms in their lifetime. There are patients who had a kidney removed because a CT scan detected a tiny cancer that would never have caused symptoms.

The key for patients and families is to appreciate that overdiagnosis is real. If you are newly diagnosed with cancer, ask your doctor if it's one of those cancers that get overdiagnosed. You may be glad you did.

Next month, White Coat, Black Art will tell you how the vast majority of patients with tiny papillary thyroid cancers get surgery even though they never needed it. Many will end up needing thyroid hormone replacement. They may even end up losing or damaging their parathyroid glands, which control the body's calcium levels.