Issues of Screening
What is a baseline mammogram?
A woman's first screening mammogram is often termed a "baseline" mammogram. The films from this mammogram are kept in a woman's medical file. All images from future mammography tests are compared to the baseline mammogram. These first images give the interpreting physician a standard of the nature and composition of the individual's healthy breast (assuming the baseline results are normal). If, for example, an unusual structure is detected during a screening mammogram and the same structure appears on the woman's baseline mammogram from 15 years earlier -- the same shape, size, location -- the interpreting physician can deduce that this structure is a normal component of that woman's breast. If you have had a mammogram, or ever get one, make sure that all future physicians have access to the films. Women who do not have a baseline mammogram, or whose previous films are not available, are more likely to require additional follow-up procedures to examine, analyze, and identify what may appear to be an abnormality.
Does early detection through mammography improve survival rates?
Yes. Earlier cancers respond better to treatment. The smaller cancers diagnosed by mammography alone are more likely to be early-stage cancers. Mammography screening trials show that detection of early cancers by mammography can result in fewer deaths from breast cancer among postmenopausal women. The amount of reduction has varied from 24% (Swedish trials) to 13–21% (Edinburgh trial) to no difference (Canadian trial).
Routine mammography is only one factor that affects survival rates. The most important factor is the quality of available medical care. Even before the introduction of mass screening, regions and countries differed greatly in their breast cancer awareness, in the frequency with which women sought and received breast examinations, and in the average size of breast lumps detected by physical exam. The size of the lump is important, because long-term survival is extremely high if cancers are detected when they are small (less than 20 mm, or approximately 0.75 inches, in diameter).
Two studies show significant differences in the size at which breast cancers are usually detected. In Swedish screening trials, women assigned to the mammography group had breast cancers detected at an average size of 22 mm. Women in the control group received their usual medical care, and cancers were found at an average size of 28 mm (larger than 1inch). The women in the mammography group had considerably smaller cancers, and their death rate from breast cancer was 24% lower.
However, it is important to note that when quality breast exams are given, there is little difference in detection rates. In Canadian screening trials, women assigned to the mammography group had breast cancers found at an average size of 16 mm. Control group women aged 50-59 received annual breast exams from experienced physicians; their cancers were found at an average size of 19 mm. There was little difference in the average tumor size between the groups, and no difference in the death rates from breast cancer.
Does routine mammography in addition to usual medical care improve cancer survival?
Cancers found by mammography only (those too small to be felt even by an expert physician) are more likely to be early-stage cancers. However, size alone does not guarantee that the cancer is in an early stage, because the behavior of the cancer cells is more important than the size of the tumor.
If a woman gets routine and thorough breast exams, she is less likely to benefit from mammograms than if she does not get such exams. An experienced physician is able to detect even small cancers (smaller than 10 mm) during a comprehensive breast exam. The addition of mammography to an annual exam by a breast specialist may not increase the survival rate for breast cancer by a large amount.
