Specificity of screening in United Kingdom trial of early detection of breast cancer

Abstract

Objectives: To study the specificity of screening for breast cancer by clinical examination with or without mammography and to estimate the extra breast biopsies resulting from a population screening programme.

Design: Non-randomised, population based study.

Setting: Two screening districts (Edinburgh and Guildford) and four comparison districts (Dundee, Oxford, Southmead, and Stoke).

Subjects: 49,956 women aged 45-64 in the screening districts and 127,109 women in the comparison districts.

Interventions: The screening districts offered women annual screening by clinical examination, with mammography in alternate years for seven years.

Main outcome measures: Numbers of true positive, false positive, true negative, and false negative results; specificity and predictive value of screening; numbers of benign and malignant biopsy specimens.

Results: At their first mammographic and clinical screen 94% (30,035/31,997) of women without breast cancer were correctly classified as negative; 6% (1962) were referred for further investigation, but only 321 (1%) required a biopsy to establish that the suspicious lesion was not malignant. At subsequent screens specificity improved to 96%, and only 0.4% of women without cancer received biopsy. After the first screen the ratio of benign to malignant biopsy specimens was the same as that among women in the comparison centres, but because mammographic screening increased the number of women with both malignant and benign disease referred the number of biopsies was increased up to twofold in the years women were offered screening by mammography.

Conclusion: Our provision of a prompt, highly specialised assessment of women with suspicious lesions at screening may have contributed to the relatively low specificities, while at the same time probably mitigating the adverse effects of low specificity.