Croakey is old enough to remember the days when anyone who raised questions about the potential for mammographic screening for breast cancer to have a downside was treated with all the derision and scorn usually reserved for dangerous heretics.
Thankfully, the debate has matured quite a bit since those days. We are now hearing a somewhat more balanced discussion which pays greater attention to some of the complexities involved. One such complexity, as illustrated by a new study, is the potential for screening to lead to over-diagnosis and unnecessary treatment.
Alex Barratt, Associate Professor in the School of Public Health at the University of Sydney, and a co-author of the new study, reports on its findings:
“The idea of over-diagnosis of invasive breast cancer in a mammography screening programme is disturbing, counter-intuitive and confusing. But there is accumulating evidence that over-diagnosis of breast cancer is substantial and is the biggest downside of mammography screening.
We have just published a study which estimates over-diagnosis of breast cancer in NSW since the introduction of BreastScreenNSW. We estimated over-diagnosis at 30-42%.
This means that through earlier detection of breast cancer, about 25% (23-29%) of all NSW women diagnosed with breast cancer are undergoing cancer treatments (such as lumpectomy, mastectomy, radiation therapy and endocrine therapy) for cancers that would not have manifested in their lives.
The benefit of mammography screening is a reduction in the risk of dying from breast cancer. The price though is an increased risk of having breast cancer detected, because screening finds more cancers, including cancers which are so slow growing they would never be found without screening.
We did this study because we noticed that the incidence of invasive breast cancer among women 50-69 years (the target age group) almost doubled with the introduction of screening (from 150/100,000 per year to just under 300/100,000). This steep increase in incidence was not observed in women outside the screening range (ie less than 40 years and over 80 years). It was apparent, but less steep in the partially screened age groups (40-49 and 70-79).
The increase in incidence in women 50-69 years could be caused by changes in risk factors such as Hormone Replacement Therapy, obesity, and nulliparity. All of these increased over the same years as BreastScreen was rolled out, in women of the same age. However, we adjusted our results for these increases; if anything we over adjusted for them. Our study is the first to formally adjust for these risk factors.
Incidence also goes up when screening is introduced because of lead time – the earlier detection achieved by screening. We also adjusted for lead time; if anything we over-adjusted for lead time.
Even after these adjustments, we found 30-42% excess incidence over that expected with screening. This finding is very compelling evidence that over-diagnosis due to screening is behind the increasing breast cancer incidence in women 50-69 years of age.
This is in addition to the detection of DCIS (Ductal Carcinoma In Situ) by screening mammography. DCIS comprises about 18% of all “cancers” found by BreastScreen NSW and a very substantial proportion of that is likely to be over-diagnosis too.
DCIS very rarely presents clinically, and detection and treatment of screen detected DCIS has not caused a decline in breast cancer incidence (as occurs with cervical cancer screening and the detection of precancerous cervical lesions).
Over-diagnosis is a very significant downside of mammography screening. In addition to the treatment risks and side effects, there is the psychological trauma of a cancer diagnosis, and its implications for insurance and cancer risk assessment in other family members.
What does it all mean? Firstly policy makers need to be aware that over-diagnosis in breast cancer screening is substantial.
It needs to be considered when assessing any expansion of the screening program, or the introduction of another cancer screening program (which may also be affected by over-diagnosis), such as a breast cancer MRI screening program or a lung cancer screening program.
Secondly, breast screening programs need to review whether their informed c onsent information is adequate, or needs to be upgraded to provide information on overdiagnosis as well as on mortality reduction. (see here for estimates of outcomes of screening mammography for Australian women).
Thirdly, we need better tests – either a better screening test that doesn’t cause over-diagnosis or a better triage test to sort the aggressive from the indolent screen detected breast cancers.”