Cost effectiveness of shortening screening interval or extending age range of NHS breast screening programme: computer simulation study

The Editor

Dear Sir

Boer and his colleagues (1) present the results of a simulation
exercise in which they investigate the cost effectiveness of changing the
parameters of the national breast screening programme. In particular, they
compare a two year screening interval in place of the current three year
interval. Their results should not pass without comment.

Firstly, their simulation model uses values for the natural history
of early breast cancer and the sensitivity of breast screening taken from
the Utrecht and Nijimegen programmes both of which started in the mid
1970's. The relevance for breast screening in the UK in the late 1990's is
not clear, bearing in mind that there is considerable between programme
variation in such basic parameters as the interval cancer rate and the
screening detection rate of small (<_10 mms="mms" invasive="invasive" cancers="cancers" _2.="_2." p="p"/> Secondly, their model fits the NW England results for the second
screening round badly, even though they have made some adjustment to the
assumed natural history values (doubling the sojourn time of T1a and T1b
cancers) to ensure that the NW England first round screening data are well
fitted. Their model predicts that at the second screening test after on
interval of three years, more than 60% of invasive cancers will be less
than or equal to 10 mms in diameter, and 9% greater than 20 mms. The
corresponding observed frequencies are 40% and 19%. Since the effect of
screening on breast cancer mortality is determined largely by the
reduction in T2+ cancers and the corresponding increase in T1a and T1b
cancers, the effect of gross inaccuracy in modelling these two quantities
is likely to be substantial. Reducing the screening interval will increase
the proportion of cancers that are screen detected, and among screen
detected should both increase the proportion of small cancers and decrease
the proportion of large cancers. The effect on predicted mortality of
poorly modelling the stage distribution of cancers detected at the second
or later screen, is thus likely to be even greater, with a two year
interval than a three year interval. The estimated marginal effect of
reducing the screening interval will then be unreliable to an unknown
extent.

Thirdly, no indication whatever is given of the uncertainty
surrounding the various estimates. The authors claim that the marginal
cost per life year gained of shortening the screening interval from three
to two years is £3545. This is a clear example of spurious precision, but
more seriously is likely to be highly misleading. Given the uncertainties
over the parameter values and the poor fit of the model itself, one would
have expected some sensitivity analyses at the very least. Quite plausible
alternative parameter values might well generate marginal cost values
several times greater than the quoted estimated of £3545. In most
situations the British Medical Journal quite rightly insists on
uncertainty estimates, usually in the form of confidence intervals. For an
article which the authors hope will make an impact on policy, omission of
uncertainty bounds renders it almost valueless.

One expects policy for the national breast screening programme to be
evidence based. The modelling presented by Boer and his colleagues uses
only a small portion of the available evidence on breast screening, and
none of the evidence from randomised trials or from relevant screening in
the UK. It fits the relevant parts of the NW region screening data badly,
and the resulting estimates are of unknown reliability. Future policy
decisions should be based on evidence directly related to the UK programme
itself. Such evidence will shortly be available from the results of the
multi centre randomised trial of different breast screening frequencies,
undertaken in Britain under the auspices of the UKCCCR.

N E Day PhD
Director of the Unit

1 Boer R, de Koning H, Threlfall A, Warmerdam P, Street A, Friedman E
and Woodman C. Cost effectiveness of shortening screening interval or
extending age range of NHS breast screening programme: computer
simulation study. British Medical Journal 1998; 317: 376-379.

2 Day N E, McCann J, Camilleri-Ferrante CC et al. Monitoring interval
cancers in breast screening programmes: the East Anglian experience. J
Med Screen 1995; 2: 180-5.