In order to discuss the balance of benefit and radiation risk in a breast screening programme, it is necessary to have numerical values for the probability of breast cancer induction by X-rays, stratified by age. Various sets of such values have been used hitherto, mainly in relation to breast screening in the UK, both within the NHS Screening Programme and more generally for younger age groups. Further sets have recently been reported. These different sets of values are described and discussed, together with the effects of using additive or relative risk models, and the effect of using a dose and dose rate modifying factor (DDREF). Possible new radiation risk factors for breast cancer induction by X-rays, drawn from these sets, are identified. These are used to calculate fresh values of cancer detection/induction ratios, as an index of benefit/risk, for screening age women and for younger women with and without a family history of breast cancer.

Current cancer detection rates and dose levels in the UK Breast Screening Programme are used to compare numbers of cancers detected with numbers predicted to be induced by the screening process itself. Numbers of those detected are shown to exceed those induced by a large margin for women aged over 50 years. The associated benefit/risk ratio is also considered. For younger women this margin is progressively reduced but remains positive at least down to age 40 years, and possibly beyond. Women both with and without a family history of breast cancer are considered. Some implications for familial breast screening programmes are discussed. Some caution may be required before annual screening of women below the age of 35 years.

The numbers of cancers detected and induced in breast screening programmes are examined for two-view screening, and for a 2 year screening interval, in contrast to the single view screening at a 3 year interval of the UK Breast Screening Programme up until early 1995. Two-view screening is also considered for the 1 year interval and age range of the current UK age trial (40-47 years). The corresponding figures for screening of groups having a family history of breast cancer are calculated and discussed. Breast cancer induction data are taken from National Radiological Protection Board publications. Cancer detection rates are based on observed rates where available, and calculated rates otherwise. The results of calculations indicate cause for concern if screening is to be extended below the age of 30 years (or below 35 years in certain categories), or below 40 years of age if family history groups are shown in the future to have a generally increased susceptibility to ionizing radiation. The importance of restricting dose to 2 mGy per film (mean glandular dose for a standard breast thickness of 4.5 cm) is stressed, together with the need ot maintain maximum image quality. This is especially true for the family history groups, who should only be screened in centres within established screening programmes, or in centres with equally strict quality control procedures.

Measurements of compressed breast thickness during mammography is necessary for the calculation of breast dose. In theory, it should be possible to calculate breast thickness from the separation of images of radio-opaque markers attached to the compression paddle. Such a technique has been evaluated on two models of X-ray set using simulated and real breasts. The results show that breast thickness at a given position can be determined with an average error of about 2 mm by this method, which is likely to be adequate for dosimetry.