[Benefits and limitations of mass screening. The natural history of breast cancer].

Most mortalities caused by breast cancers are due to metastases. A breast cancer gives rise to lymphatic spread, which is at the origin of nodal involvement, and to distant dissemination, usually through the bloodstream. Nodal involvement, which generally occurs before metastatic dissemination, appears not to be the cause of the dissemination but rather an index of the likelihood of a tumor to metastasize. This likelihood is influenced by the histological grade and the growth rate of the tumor. Despite the variations in biological characteristics of breast tumors and the likelihood of metastatic dissemination, the existence of a relationship between tumor size and the probability of metastatic dissemination enables the calculation of how many disseminations could be avoided through early diagnosis and thus the gain in human life that screening could hope to achieve. However, in reality, the effective gain is lesser because the rate of participation is lower than 100% and the existence of false negatives (undetected cancers) reduces the number of detected cancers. Furthermore, a high rate of false positives increases the anxiety of women because they provoke unnecessary examinations. Screening is worthwhile only if the increase in human life outweighs the economic and social costs (anxiety, going to appointments) that it may produce. It is therefore necessary to improve the benefits (fewer false negatives) and to decrease the social and psychological costs (fewer false positives). This can be done by the implementation of rigorous quality assurance, systematic training of health care personnel, a follow-up of women who have been screened, and an annual assessment of screening results. Screening in France since 1994 has shown a marked improvement since the implementation of guidelines prepared by the national screening committee which emphasize the need for quality control and ongoing training of staff. However, the results vary greatly from one county to another and are often below European standards, although some regions in France are close to achieving these. This shows that the system of screening in France, despite its organization involving many private and public radiology departments, can give results equal to those in countries in which screening is performed by a small number of specialized units. The French system requires regional structures of orientation and evaluation. To be effective, these structures must be guided by written legislation and regulations, otherwise they will be unable to overcome the difficulties currently faced by the implementation of mass screening.