Overdiagnosis of breast cancer not uncommon report new studies
Breast cancer is the leading cause of cancer deaths and the second leading cause of death (behind coronary heart disease) in in the United States. Many women regularly undergo a screening mammogram for the early detection of the disease. According to two new studies, a significant number of these women are diagnosed with breast cancer, which may not require treatment at present; this situation is known as overdiagnosis.
The findings from the Norwegian Screening Program were reported in the April 3 issue of the Annals of Internal Medicine. Researchers from Harvard University (Boston, Massachusetts, the Karolinska Institutet (Stockholm, Sweden), and Oslo University Hospital (Rikshospitalet, Oslo, Norway) noted that overdiagnosis not only poses a major ethical problem but also encumbers the patient and the healthcare system.
The authors defined overdiagnosis as the percentage of cases of cancer that would not have become clinically apparent in a woman's lifetime without screening, which are detected on a screening mammogram. They note that defining the actual percentage of overdiagnosis is hampered by a lack of studies that compare the incidence of breast cancer detected with and without mammogram screening. Thus, the objective of their study was to estimate the percentage of overdiagnosis of breast cancer attributable to mammography screening. They wrote: "Mammography screening increases breast cancer incidence owing to earlier detection of cancer that would otherwise have been diagnosed later in life and to overdiagnosis of cancer that would not have been identified clinically in a lifetime."
The researchers reviewed data from a nationwide mammography screening program in Norway for women between the ages of 50 to 69 years of age. The program was gradually implemented from 1996 to 2005. They reviewed the incidence of invasive breast cancer from 1996 to 2005 in counties where the screening program was implemented and compared that percentage to that of counties where the program was not yet implemented. They also reviewed the incidence during the previous decade to check for a change in breast cancer incidence over time. The percentage of overdiagnosis was calculated by accounting for the expected decrease in incidence following discontinuation of screening after age 69 years (approach 1) and by comparing incidence in the current screening group with incidence among women two and five years older in the historical screening groups, accounting for average lead time (approach 2).
The study group comprised 39,888 patients with invasive breast cancer; 7,793 were diagnosed after the screening program began. The researchers estimated the rate of overdiagnosis attributable to the program to be 18-25% for approach 1 and 15%-20% for approach 2. Therefore, they found the percentage of overdiagnosis to be 15%-25% of cancer cases; this percentage meant that from six to 10 women were overdiagnosed for every 2,500 women in the study group.
Another study, which was published online last January in the Medical Journal of Australia by researchers at Monash University (Melbourne, Australia), had similar findings. Breast cancer researchers Robin Bell, PhD and Robert Burton, MD requested that women who participated in the publicly-funded BreastScreen program should be presented with a more balanced view regarding the benefits and risks of breast screening. They noted that their analysis found that improvements in cancer treatments, rather than early detection through screening, were likely to have caused the 21-28% reduction in breast cancer deaths since the program began in 1991. A 2010 study reported that for every 2,000 women who underwent screening over a 10-year period, one would have her life prolonged; however, 10 healthy women would be diagnosed as breast cancer patients and treated unnecessarily.
The Cancer Council has promoted the need for women to be informed about the risks and benefits of screening, including the uncertainty of overdiagnosis. However, despite that viewpoint, the council notes that breast screening has contributed substantially to an overall drop in breast cancer deaths. It reported that three evaluations of mammography screening for women aged 50-69 years had had reduced breast cancer mortality by 30-47%.
Dr. Bell noted that, in her opinion, the benefits of the BreastScreen program were overstated. “This comes down to the balance of harm versus benefits… My view is that women need to be given more balanced information about the BreastScreen program when invited to be screened. Overdiagnosis amounts to women having a small, slow-growing cancer being diagnosed and treated, where in her lifetime that cancer may not have required treatment."
Dr. Bell noted that the impact of breast screening was diminishing as the outcome of treatment for breast cancer improved and the balance of benefit to harm of breast screening was becoming less favorable. "This has serious implications for health policymakers," she said.
Take home message:
Overdiagnosis is probably even more common in the United States than in Norway, because US radiologists are more likely than European radiologists to report mammographic abnormalities, and because US women often start annual mammography screening at 40 years of age, whereas Norwegian women start biennial screening at 50 years of age.