INFLUENCE OF NUMBER OF VIEWS AND MAMMOGRAPHIC FILM DENSITY ON THE DETECTION OF INVASIVE CANCERS - RESULTS FROM THE NHS BREAST SCREENING-PROGRAM

The National Health Service Breast Screening Programme (NHSBSP) has re commended the adoption of two view mammography at the prevalent screen , and the use of a target film density in the range 1.4-1.8. The aim o f this study was to review the impact of number of views and optical d ensity on the detection of invasive cancers. The last four annual retu rns for screening centres in the NHSBSP have been analysed retrospecti vely for 2 827 342 women aged 50-64 years attending their first (preva lent) screening examination. The detection of invasive cancers was ass essed in relation to the number of views and film density using the ag e adjusted, Standardized Detection Ratio measure of screening performa nce. Typical film densities were reported for each screening year by l ocal physicists, and the average value for all mammography sets at eac h programme calculated, and found to vary from 0.85 to 1.85. The mean him density across the NHSBSP rose progressively from 1.30 (SD=0.21) i n 1991/2 to 1.57 (SD=0.12) in 1994/5. Programmes using single view mam mography (MLO) and an optical density less than 1.4 detected 76% (95% CI 74-79%) of the expected invasive cancers. Programmes using two view mammography (MLO and CC) and an optical density equal to or greater t han 1.4 detected 95% (95% CI 92-98%) of the expected invasive cancers. In 1994/95 when more programmes used the recommended screening modes, the NHSBSP detected 96% (95% CI 92-101%) of the expected invasive can cers at prevalent screening. The detection of invasive cancers was hig hest where programmes used two views with a film density in the range 1.4-1.8. The results provide evidence of the benefit of the recommende d protocol for prevalent screening and indicate that from 1995/96 when all programmes will be using the recommended protocol, it is likely t hat the detection rates and interval cancer rates from prevalent scree ns in the NHSBSP will be close to the figures in the Swedish-Two Count y Trial.