Interval cancers are those that occur following a negative screening episode, in the interval before the
next invitation to screening is due. For faecal occult blood testing interval cancers may occur following
a negative FOBT, or following a positive test result with negative further assessment (colonoscopy).
Rates of interval cancers reflect both the sensitivity of the screening test (false negatives), and the
incidence of newly-arising cases not present at the time of screening. With increasing time since negative
test, the rate and proportion of the latter will increase. In the absence of repeat screening, incidence
rates would eventually reach the background level again. Rates of interval cancers should
therefore be presented by time period (years) since previous screen.
Rates of interval cancers will depend on the underlying incidence in the population. They will also depend
on the extent of selection bias, whereby rates in those not participating in screening differ from
the general population rates.
For this reason it is important that (age- and gender-specific) incidence
rates in non-responders are also monitored, to allow for the underlying incidence in responders to be
estimated.
Background incidence rates can be estimated from rates prior to the introduction of screening (although
time trends need to be considered) or from areas not covered by the screening programme
(when geographic differences need to be considered).
The interval cancer rate can therefore be expressed as a proportion of the background incidence rate,
standardised for age and gender, by dividing the number of interval cancers in the specific age/gender
group (I) by the ones expected based on the background incidence for that age/gender group (C), or
as a proportion of the background incidence rate adjusted for non-participants (C*). The adjusted rate
can be calculated as:
C* = (C – (1 – P) N) / P
P: participation rate
N: rate in non-responders
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