Screening for breast cancer

There are few areas in imaging specifically and in medicine in general, fraught with more controversy than screening for breast cancer. Due to the emotive issues surrounding the diagnosis, the scientific literature on breast screening and its issues reaches the lay press quickly and is sometimes reproduced in non scientific, often potentially inaccurate terms. 

This debate is potentially interminable (and as such out of the scope of this site and its aims) so this is the radiological perspective from a source that has been doing senology in real life for almost 25 years.

Mammography is not a perfect study and it does not find all cancers in all women. But until a cure is found, this is the best universally available tool we have to find breast cancers reliably and reproducibly early in the disease process.

The rationale behind screening

The rationale behind screening for breast cancer is the universally accepted dictum that (all other factors being equal)  early diagnosis results in increased survival. The smaller the cancer at the time of diagnosis, the better the 5 year survival of breast cancer.

In general, if  a breast cancer is found at less than a 1cm diameter, the 5 year survival is the same as the general population at large. Screening for breast cancer is cost and risk effective, feasible and evidence based.

Historical background 

Many involved in the current discussion on "over-diagnosis" have not had the advantage of seeing the development and evolution of therapy in breast cancer. The increased survival in breast cancer is due in part to earlier diagnosis but also improved treatment. For those who want to read and ponder an insightful review of how far we have come since the 1960's, reference 12 is a very insightful succinct review of the state of mammography from one of the pioneers in the field, Dr Ed Sickles.

The perviously used radical surgical therapies for breast cancer are now a thing of the past. Lymphoedema used to be a very significant, dreaded complication of mastectomy and is now a rare occurrence in clinical practice.

Not only are more women surviving this disease, they are doing it with cosmetically acceptable results that do not degrade and mutilate them in an effort to ensure survival. This is a direct and undeniable result of the use of screening mammography. This fact does not enter into the ding dong  Battle of Articles which dominates the literature. Women now return to the world to resume their lives fully and live out productive lives without the radical surgical interventions previously used to try and stem the disease and its results.

The initial literature support came form the Two Counties Trial in Sweden. Follow up for the trial participants continues and the data support the continued use of screening to detect breast cancer at an early stage when the therapy is potentially curative and cosmetically acceptable. The Two Counties trail and the subsequent Health Insurance Plan (HIP) study in New York showed a decrease in deaths from breast cancer in those aged 40-74. A direct result of the HIP study was that screening was introduced in the USA.

Since screening was introduced in the US, the mortality rate from breast cancer has declined by 30%; it had stayed constant from the Second World War.

The concept of risk

As it pertains to screening and breast cancer, there is significant confusion among the general population and many professionals about the concept of risk.

Depending on which source you use, about 10% of the breast cancers seen have a positive first line family history of breast cancer. This means that 90% or more of the cases will be spontaneous mutations with no family history.

A common occurrence seen regularly is the reaction when someone is told they have a breast malignancy and invariably the reaction is "but I don't have a  family history" or "no one in my family has breast cancer". This unfortunate state of affairs is further compounded by the potentially term "average risk" when used in the context of breast cancer. This is a misnomer. Women with a positive family history are at exceptional risk; women with no family history are at risk because of their gender. The unthinking use of "average risk" inadvertently implies that we should concentrate our efforts at the 10% or that the 90% with no history can somehow relax and not get breast imaging. We cannot ignore the 90% of the population who would otherwise have a false sense of security under the umbrella of "average" risk. There is no such thing.

What about those under 50?

As a general rule, biologically, cancers in younger women tend to grow faster and hence metastasize  earlier. The lag period in the time before a lesion is picked up is therefore potentially shorter in women under 50 than in post menopausal women. Some of the cancers seen in screening in those over 50 were actually potentially diagnosable when the patient was under 50¹⁰. 

The downside of the interminable debate is that it confuses patients and their physicians with contradictory news reports and literature bombarding them from all sides. It detracts energy and resources away from the issue at hand which is to find a cure for this disease.

Criteria for a screening test

There are three steps necessary to prove that screening mammography  is effective in breast cancer⁷:

1. The test can find cancers when they are smaller than without the test: mammography has succeeded here without doubt. To such an extent that the new buzzword of "overdiagnosis" has now arisen.

2. randomized, controlled trials must show a reduction in mortality:  there is no argument here in the accepted medical literature. Seven trials have demonstrated this^2,9.

3. when the test is introduced into the general population, the death rate declines: the number of deaths per thousand of the population. Screening mammography has reduced the death rate of women^8,9.

Overdiagnosis

This is the newest challenge to the use of screening mammography to save lives and give potentially curative,  cosmetically acceptable therapy to the second biggest killer of women worldwide. In many respects "overdiagnosis" is likely also applicable with other malignancies like prostate. Almost predictably, breast screening is highlighted.

The definition of overdiagnosis is epidemiologic not pathologic i.e. a breast cancer (or prostate)  would not have been diagnosed during a patient's lifetime if screening had not taken place. Unfortunately, there are no pathological or imaging features to distinguish the progressive cancers we see, manage every day and which we know will likely kill eventually from those that are non-progressive and do not eventually kill the patient. In practical terms, overdiagnosis will likely represent cases of low grade DCIS and those cases of indolent IDC that on review were actually present on mammograms of years ago but only now diagnoses for whatever reason (as a general rule, the rate of growth of an IDC can be an indication of how aggressive the tumour is ). All radiologists have seen cancers like this: they have been there for years in an invariably elderly lady and they don't seem to change much with time. Maybe we need to re evaluate how we manage confirmed low grade DCIS or smal slow growing indolent cancers is elderly patients with co-morbid conditions.

Some breast cancers will not kill. In autopsy series, up to 2% of female autopsies have breast cancer. But the cancers that will not kill are not only those we find on screening; they also include large palpable cancers¹¹.

For a comprehensive overview of the current thinking on breast screening, there is a wonderful, comprehensive review supplement in the Journal of Medical Screening Sept 212. Its well worth owning and reading if this is your field of work. Highly recommended¹⁴