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The Lancet Oncology, Volume 12, Issue 12, Pages 1118 - 1124, November 2011

Natural History of breast cancers detected in the Swedish mammography screening
programme: a cohort study


Dr Per-Henrik Zahl DrMedSci a , Prof Peter C Gøtzsche DrMedSci b, Prof Jan Mæhlen DrMedSci c

Background

The natural history of screen-detected breast cancers is not well understood. A previous analysis of the incidence
change during the introduction of the Norwegian screening programme in the late 1990s suggested that the natural
history of many screen-detected invasive breast cancers is to regress spontaneously but the study was possibly
confounded by use of hormone replacement therapy in the population. We did a similar analysis of data collected
during an earlier period when few women were exposed to hormone replacement therapy.

Methods

We compared cumulative breast cancer incidence in age-matched cohorts of women living in seven Swedish
counties before and after the initiation of public mammography screening between 1986 and 1990. Women aged
40—49 years were invited to screening every year and women aged 50—74 years were invited every 2 years. A
screened group including all women aged 40—69 years (n=328 927) was followed-up for 6 years after the first
invitation to the programme. A control group including all women in the same age range (n=317 404) was also
followed-up for 6 years—4 years without screening and 2 years when they entered the screening programme.
Screening attendance was much the same in both groups (close to 80%). Counts of incident invasive breast
cancers were obtained from the Swedish Cancer Registry (in-situ cancers were excluded).

Findings

Before the age-matched controls were invited to be screened at the end of their follow-up period,
the 4-year
cumulative incidence of invasive breast cancer was significantly higher in the screened group (982 per 100
000) than it was in the control group (658 per 100 000)
(relative risk [RR] 1·49, 95% CI 1·41—1·58). Even after
prevalence screening in the control group, the screened group had higher 6-year cumulative incidence of invasive
breast cancer (1443 per 100 000 vs 1269 per 100 000; RR 1·14, 1·10—1·18).

Interpretation
Because the cumulative incidence among controls did not reach that of the screened group, we believe that many
invasive breast cancers detected by repeated mammography screening do not persist to be detected by screening
at the end of 6 years, suggesting that the natural course of many of the screen-detected invasive breast cancers is
to spontaneously regress.

Funding: None.

The mammography-screened
group was diagnosed with 35%
more invasive breast cancers
than the unsceened group.
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