Objective: To evaluate the effectiveness of a mass screening programme for breast cancer in a French population where hormone replacement therapy (HRT) is common and where mammography is prescribed outside the programme for asymptomatic women.
Methods: From 1993 to 1996 inclusive, 41,062 women underwent a first test and 48,275 a second or third test in the Bouches du Rhône programme. Their HRT status was ascertained at the time of the test. False positive and false negative tests were identified at one year follow up. The incidence of interval cancers was estimated up to three years after the screening test.
Results: The odds of being detected at screening rather than as an interval cancer within one year of the test was five times greater among non-users of HRT than among users (odds ratio (OR) 5.14 (confidence interval 2.5 to 11.8)). This high reduction in sensitivity among users (71% v 92%) was associated with a very small reduction in specificity at the incident screen only. The incidence of interval cancers among HRT users was 3.5 times that of non-users within the first year after the test and 1.7 times during the following two years.
Conclusions: When the early results of a programme have been used to measure its effectiveness they should be reassessed in populations where HRT is in widespread use. As interval cancers had better prognostic factors in HRT users than in non-users, the efficacy of a screening programme in such populations should be the subject of further studies.