Analysis of breast cancer mortality and stage distribution by age for the Health Insurance Plan clinical trial.

The Health Insurance Plan (HIP) of Greater New York conducted a clinical trial to determine if screening for breast cancer with mammography and clinical examination would decrease breast cancer mortality. The extent of disease at diagnosis among breast cancers detected by screening and the effect of screening on breast cancer mortality have been evaluated in the cohort of all HIP women diagnosed with breast cancer within 6 years of entry into the trial and followed at least 18 years after trial entry. Six years was the earliest time at which the number of cases diagnosed in the control group was equal to the number of cases diagnosed in the study group. In the cohorts of women 40-49 and 50-64 years of age at entry, shifts were significant to lower stages for screen-detected cases. As a result, the study group women in each age cohort had significantly lower breast cancer mortality than control group women when statistical analyses were restricted to data from cases only. In the 40-49 age-at-entry cohort, the reduced breast cancer mortality in the study group appears to result from lower mortality in stage I cases as well as from earlier case detection, and this may explain differences between the two age-at-entry cohorts in the length of follow-up time required to demonstrate a mortality reduction due to screening.