Understanding breast cancer screening

mammogram-breast-cancer-screeningIn order to understand breast cancer screening, it is important to explain some general principles of screening:

The disease screened for must be common. With 1 of 8 women suffering breast cancer during their life-time, this is clearly the case. While there are no accurate, recent statistics available for South Africa, our own cancer pick up rates indicate that contrary to official statements, breast cancer is as common in our population as in countries with established screening programs.

The disease must be sufficiently severe to warrant the screening effort. Nobody will dispute this for breast cancer.

A screening test must be available, that reliably picks up the signs of the disease. At the same time, a screening test must not generate too much “harms”. Mammography will pick up early signs of breast cancer in about 90 – 95% of cases, fulfilling the first condition. “Harms” mean that the test must not pick up signs of the disease in too many patients that with further work-up turn out to be false alarms. For mammography it is accepted, that less than 10% of women should be called back for further examinations. In our centre, about 4 – 5% of women are called back. It is also important, that while false alarms are inevitable, these can be sorted out with a minimum of cost and further harms done to the patient. Generally, less than 5% of women should have a biopsy to sort out findings. In our centre, it is about 2%. The vast majority of biopsies should be done as office procedures under mammographic or ultrasound guidance; only very rarely should a woman have to go to theatre to make a diagnosis. In our centre, only 2 in 100 biopsies are done as a theatre procedure. If a biopsy is performed, it must yield a diagnosis of cancer in 2 – 5 of 10 cases. In our centre, about 1 in 3 biopsies result in a diagnosis of cancer.

As new cases of the disease arise continuously, the screening test must be repeated at regular intervals. For mammographic screening, most cancer organizations recommend annual screening from age 40 onwards. In most governmental screening programs, the screening interval is 2 years for mainly financial reasons. Concerns about the radiation exposure are unfounded, as there is stringent supervision of mammography units by the Radiation Control Board in South Africa.

The screening test must pick up the disease so early, that it changes the course of the disease. In a well-run screening unit, about 1 in 5 cancers will be detected at a pre-invasive stage. This means, that the cancer is curable by surgery alone and does not require intensive treatment such as chemotherapy or radiotherapy. In our centre, one in 3 cancers is detected at the pre-invasive stage. More than half of the invasive cancers must be less than 2 cm in diameter. In our centre, about half of invasive cancers are less than 1 cm in size. The pre-invasive and these small invasive cancers can be cured in more than 90% of patients. It is well documented, that in regularly screened patients, the mortality of breast cancer is up to 50% lower than in those who do not undergo mammographic screening. Here, many misunderstandings arise. Breast cancer is a slowly growing cancer and effective therapy is available. This means, that the benefit of screening in the form of a reduced mortality becomes apparent only after about 10 years in those who underwent screening.

All the above shows that mammographic screening can change the course of breast cancer. It is unclear, however whether this is a given in South Africa. The effectiveness of screening is well documented in overseas screening programs, where stringent quality controls from regulatory authorities are applied. This is not the case in South Africa. Only a single unit in South Africa has ever published in peer-reviewed scientific journals its screening results and shown that it can provide screening at the quality of the best overseas units.

By Professor Justus Apffelstaedt, Associate Professor: University of Stellenbosch and Head of the Breast Clinic: Tygerberg Hospital

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