Secondary prevention: screening for breast cancer.

Screening healthy women in order to detect the earliest signs of breast cancer may offer a possibility of curing many breast cancer patients who would be incurable if left until they developed symptoms. However, the natural history of breast cancer is very variable, probably indicating a wide spectrum of different growth rates of the tumour. Therefore it cannot necessarily be assumed that cancers detected by screening at an apparently early stage will behave in the same way as symptomatic cancers at that stage. To prove that screening enables cancer to be cured one needs to compare the number of deaths in a group of women who have been offered screening with those in a comparable group who have not. Unlike the situation in clinical trials of different treatments, comparison of the survival of screen-detected cancers with symptom-detected cancers is inadequate proof, because of selection bias, lead-time bias and length-bias. One randomized controlled trial of screening for breast cancer has so far been published and this shows that women in the group who were offered screening suffered one third fewer deaths from breast cancer than women in the control group, the difference persisting for up to 14 years from the first screening invitation. Further trials are now under way in Canada and various European countries, hoping to confirm this finding and to explore various other issues. Of the screening test methods currently available mammography seems the most sensitive and specific and its radiation hazard is now of almost negligible proportions provided that regular careful monitoring of the equipment is carried out. However it may have the disadvantage of overdiagnosing cases of borderline non-invasive neoplasia which might not progress to invasive cancer within the woman's lifetime. Clinical examination of the breasts is a less satisfactory test in older women but it may be useful in premenopausal women in whom mammography is less sensitive. The validity of self-examination of the breasts by women themselves is still largely unknown, but it is unlikely that compliance with regular breast self-examination will be as high as women's acceptance of screening. Further research is required into the optimal frequency of screening and into its cost-effectiveness.