'Overdiagnosis' in About 20% of Common Cancers

About 20% of five common cancers are the result of "overdiagnosis," concludes a study from Australia that analyzed over 30 years of national healthcare data. The figures were 18% for women and 24% for men.

Overdiagnosis is defined as the diagnosis of cancer in people who would never have experienced symptoms or harm had the cancer remained undetected and untreated, the authors explain. It often results from population cancer screening programs conducted in healthy individuals. 

Although the concept of overdiagnosis, and estimates of it from population screening programs, have been reported before, this study offers a new perspective.

This is "the first time that the risk of overdiagnosis has been quantified across five cancers anywhere in the world," said first author Paul P. Glasziou, PhD, Bond University, Robina, Queensland, Australia, in a statement.

The authors looked at five of the seven cancers for which overdiagnosis has been documented: breast, thyroid, renal, and prostate cancers, and melanoma.

The results suggest that, overall, 18% of these five cancers in women were overdiagnosed in 2012, accounting for 22% of breast cancer cases and as many as 73% of thyroid cancers.

In men, 24% of these five cancers were overdiagnosed, including 42% of prostate cancers, 58% of melanomas, and 73% of thyroid cancers.

The study was published online December 19 in the Medical Journal of Australia.

The reasons for overdiagnosis differ by cancer type, the authors comment, with breast cancer overdiagnosis "largely attributable" to national screening, and that for prostate cancer and melanoma because of "opportunistic but extensive" screening.

Overdiagnosed renal cancers, they add, tend to be found during abdominal imaging, and these incidentalomas also account for a proportion of overdiagnosed thyroid cancers, alongside the "excessive investigation of thyroid function tests."

Glasziou commented in his statement that, when it comes to screening, "the problem is that some screening identifies abnormal cells that look like cancer but don't behave like cancer."

"However, reducing that problem is not easy, as some types of screening are important."

"Getting the balance right between too little and too much screening and testing will not be easy, but this is an important step," he said.

Elaborating to Medscape Medical News, he said that some screening, such as for neuroblastoma and chest X-rays for lung cancer, does not work, and "the screening that does often has modest benefits and must be weighed against the several harms."

He believes that, to reduce overdiagnosis, there will need to be changes in the naming and classification of cancers, as well as changes to clinical practice and, "in particular," changes to "recommendations about screening and how to screen."

These ideas have been aired over the last few years by other groups, including a proposal to take the word "cancer" out of some diagnoses, and proposals to limit screening to individuals who are at higher risk of cancer instead of screening whole populations.   

Study Details

For this study, researchers looked at data on cancer-specific diagnosis, cancer-specific and all-cause deaths, as well as population numbers, collated by the Australian Institute of Health and Welfare for men and women between 1982 and 2012.

The team then calculated and compared current and past lifetime risks for cancer, adjusting for changes in the prevalence of risk factors over the course of the study period, such as cumulative sun exposure or body mass index.

The results show that, in women, the lifetime risk of being diagnosed with breast cancer increased by 3.4% between 1982 and 2012, while the increase was 0.6% for renal cancer, 1.0% for thyroid cancer, and 5.1% for melanoma.

Researchers estimated that, in 2012, 22% of breast cancers (including 13% of invasive cases), 58% of renal cancers, 73% of thyroid cancers, and 54% of melanomas (including 15% of invasive cases) were overdiagnosed.

This equated to 18% of all diagnoses of these five cancers in women (8% of invasive cancers).

Invasive cancer risk increased by 8.6% in women during the study period, of which overdiagnosis of invasive breast, renal, and thyroid cancer, and melanoma explained 47% of the increase.

In men, the lifetime risk of being diagnosed with prostate cancer increased by 8.2% between 1982 and 2012, while the increase was 0.8% for renal cancer, 0.4% for thyroid cancer, and 8.0% for melanoma.

It was estimated that 42% of prostate cancers, 42% of renal cancers, 73% of thyroid cancers, and 58% of melanomas were overdiagnosed.

This equated to 24% of all diagnoses for these five cancers in men (16% of invasive cancers).

The risk of invasive cancer increased by 10.9% during the study period in men, with overdiagnosis of invasive prostate, renal, and thyroid cancers, and melanoma explaining 97% of the increase.

Public Education Campaigns

In their discussion of the results, the team points to recent cancer statistics from the UK showing 5-year survival rates for stage 1 breast cancer of 99%, 100% for stage 1 prostate cancer, 100% for stage 1 melanoma, 89% for stage 1 kidney cancer, and 88% for thyroid cancer of any stage.

These "very high survival rates," the team writes, provide "further evidence of probable overdiagnosis."

However, there has been a drive in recent years in the UK to diagnose patients with cancer at earlier stages to offer a better prognosis, which has been reflected by recent data suggesting few are now diagnosed at stage 4.

Glasziou agreed that this suggests a careful balance needs to be struck between public health campaigns urging early diagnosis and limiting overdiagnosis, but he emphasized that "earlier is not always better."

He highlighted the considerable harms that can be experienced by overdiagnosed patients undergoing cancer treatments.

Public education is needed, suggests coauthor Katy J. L. Bell, MD, Sydney School of Public Health, University of Sydney, New South Wales, Australia. "People still need to remain vigilant when it comes to early detection of cancers, however they need to be informed and engage in shared decision-making with their medical professionals about the harms of cancer screening and other associated procedures," she said in a statement.

The researchers received funding from the Australian National Health and Medical Research Council. The authors have reported no relevant financial relationships.

Med J Aust. Published online December 19, 2019. Full text

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Cite this: 'Overdiagnosis' in About 20% of Common Cancers - Medscape - Jan 30, 2020.