Variations in individual radiation dose in a breast screening programme and consequences for the balance between associated risk and benefit.

In a breast screening programme there is a small but inevitable risk of inducing some breast cancers that may appear many years later. This risk has to be compared with the benefits of detecting existing cancers at an early stage. In the current UK screening programme, for women aged 50-64 years and with a 3-year screening interval, the numbers detected greatly exceed the numbers predicted to be induced. For the individual, only those with very large and thick breasts who also have many views taken during the screening process appear to be at any appreciable risk, and the number for whom the risk of induction exceeds the probability of cancer detection is of the order of less than 1 per million, a risk level normally considered negligible. Results are also presented for younger age groups, and for a range of dose levels. Down to age 35 years, induction exceeds detection in less than about 1% of those screened, if a 3-yearly interval is maintained. The effect of shorter screening intervals is briefly considered. All results are based on repeat screening rounds (incidence rounds) using a single view and not on the initial (prevalence) round, and use UK data for breast cancer incidence. The calculation requires a number of assumptions and is intended as a preliminary one, pending more data on doses and numbers of women requiring specific numbers of films in screening programmes, but sufficient to promote discussion of the appropriate age range for breast screening.