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| Journal of the New Zealand Medical Association,
08-June-2012, Vol 125 No 1356
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Colonoscopy requirements of population screening for
colorectal cancer in New Zealand
Terri Green, Ann Richardson, Susan Parry
Colorectal cancer is the second most common cause of cancer
registration (2,801 registrations in 2008, accounting for 14%
of all cancer registrations) and the second most common cause of cancer
death (1280 deaths in 2008, accounting for 15% of all deaths from
cancer) in New Zealand. Age-standardised colorectal cancer incidence rates are
lower for Māori than for non-Māori, and for females than for
males.1
The risk of colorectal cancer increases with age, and 90% of
all cases diagnosed are in people aged 50 years or
over.1 Although colorectal
cancer (CRC) incidence overall is forecast to decline in New Zealand, the
absolute number of people with CRC is expected to increase, because the effects
of growth and ageing of the population will more than offset the decline in
incidence.2
CRC mortality rates overall have also been declining, and
this decline is forecast to continue2 but
Māori CRC mortality rates have increased between 1980 and
19993 so that Māori and non-Māori
rates are comparable currently. If these trends continue, CRC mortality rates
among Māori will exceed non-Māori rates, with disparities increasing
over time. 4
CRC Mortality is higher in New Zealand than Australia and
most other countries. 5,6 It is suggested that
this is partly due to the higher incidence of CRC in New Zealand but that it may
also reflect poorer survival after diagnosis in NZ than
Australia. 5
Most colorectal cancers begin as adenomatous polyps, with
progression to cancer taking at least 5–10 years. This means that
detection at an early stage is possible. Treatment at an early stage is
associated with a better prognosis than treatment at a later stage, but this is
dependent on health services being able to offer timely and appropriate
treatment.7-9
Screening for CRC involves testing asymptomatic people to
identify those likely to have CRC. The most commonly used screening test is the
faecal occult blood test (FOBT), which requires people to put stool samples on a
card and send it to a laboratory to be tested for the presence of blood.
People with positive tests are offered colonoscopy to see if
they have CRC. Screening with a particular type of FOBT, guaiac FOBT, has been
shown in randomised controlled trials (RCTs) to reduce CRC mortality by about
15%.10,11
In 1997 the New Zealand National Health Committee convened a
working party to consider population screening for CRC in New Zealand. This
working party did not recommend population screening because of "the modest
potential benefit, the considerable commitment of health sector resources, and
the small but real potential for
harm".12,13
In 2005 the National Screening Unit of the Ministry of
Health convened an advisory group to revisit the issue of CRC screening, since
it had been several years since the previous report. There were also new results
from pilot programmes in the United Kingdom and Australia, and papers reporting
longer follow up from the randomised controlled trials of CRC screening.
The advisory group recommended that a feasibility study of
CRC screening using immunochemical faecal occult blood tests (FOBTi) be
considered and planning initiated. 14
The FOBTi test is not definitive and those with a positive
test result need to be referred for colonoscopy for a confirmatory diagnosis.
There is an ethical obligation to deliver this initial colonoscopy in a timely
manner.
The advisory group regarded a feasibility study as an
essential pre-requisite to any decision about screening in New Zealand in part
because existing colonoscopy capacity was insufficient to consistently deliver,
across the country, timely diagnostic colonoscopy for those with symptoms, or
timely surveillance procedures for those at increased risk of CRC. This was in
the absence of the additional demand that would be generated by a screening
programme. Concern about colonoscopy capacity has continued to be
raised.15,16
A pilot bowel screening programme was launched in the
Waitemata District Health Board region, in October 2011. The pilot programme
offers two-yearly FOBTi to eligible people aged 50–74 years, and will run
for 4 years. This paper focuses on the requirements for colonoscopy, should a
national screening programme be introduced, with FOBTi as the screening test. It
includes both the initial ‘referral’ colonoscopy following a
positive FOBTi test, and surveillance colonoscopy arising from adenomas found at
the initial colonoscopy.
MethodsStudy design—Estimates of the
New Zealand Population, base 2006, were obtained for the years 2011 to
2031. 17 Series 5 population projections, based
on medium fertility and life expectancy, was used in the modelling. The
estimated population aged 50–74 was 1.118 million in 2011 and 1.435
million in 2031.
FOBTi-based biennial screening of those aged
50–74 years, excluding those assumed to have already been diagnosed with
colorectal cancer, was modelled following a Markov process. This involves
patients moving from one ‘stage’ (e.g. being invited to screen) to
another ‘stage’ (e.g. participating in screening) according to
various probabilities. For example it was assumed that 60% of people would
‘move’ from being invited to being screened.
The stages included: the invitation to screen, the
initial screen, referral to colonoscopy, uptake of colonoscopy, outcome of
colonoscopy, adenoma surveillance and invitation to rescreen with FOBTi after 2
years, or after 5 years for those who had had a colonoscopy but no cancer or
adenoma had been found (see Figure 1). This process was started in 2011 and
stopped after 2031.
Figure 1. Faecal occult blood tests (FOBT)
screening diagram
 The model assumed that the initial screening would be
spread over the first 2 years of the programme. Thus half the population aged
50-74 were eligible for screening in year 1; the remainder became eligible in
year 2 except for those who had ‘aged out’ (became 75) or had died.
Those who had ‘aged into’ the eligible age range (turned 50) in year
2 also became eligible for screening. For subsequent years the model allowed for
‘aging in’ and ‘aging out’.
Surveillance colonoscopy of large adenomas (>10mm)
was at 3 and 6 years, and was at 5 years for small adenomas. Surveillance beyond
this was not modelled. The 2004 NZ Guidelines on which these surveillance
parameters were initially based, recommended the first surveillance colonoscopy
be performed at 3 years for those with adenomas size >10 mm and those with
greater than three adenomas.
The next surveillance procedure was recommended at
3–5 years if the colonoscopy was
negative.18 It was recognised that in practice
a proportion of patients with large adenomas would have the second surveillance
procedure at 5 years rather than 3 years, but on the other hand others following
removal of a large adenoma with advanced histology, would have surveillance
colonoscopy performed at one and 3 years, as had been recommended in the
recently released NZ Guidelines.
To model surveillance procedures at 3 and 6 years
following detection of a large adenoma, and to not model for surveillance beyond
6 years (which would certainly be required for a significant proportion) was
considered to best reflect the range of surveillance scenarios that could result
from the detection of large adenomas at the initial colonoscopy. Those
undergoing surveillance were returned to FOBTi screening 5 years after their
last normal colonoscopy.
The numbers of colonoscopies required each year, in
total and separately for the initial referral and for adenoma surveillance, were
calculated.
Base case scenario—For the base
case, FOBTi test positivity was assumed to be 6.4% for the initial screen based
on the Calvados, France FOBTi trial, 19 which
screened people aged 50–74. Positivity for re-screening was not available
and was estimated at 4.8% by assuming the same proportion of initial screen
positivity (75%), as occurred in the Italian (Florence) FOBTi trial.
20 [The positivities in that trial for first
and repeat screens were 4.4% and 3.3%.]
Uptake of FOBTi screening was assumed to be 60% based
on the Nottingham RCT for guiac based FOBT10
and uptake of referral colonoscopy was taken at 85%,19
and was assumed to be 100% for surveillance. Yield of large adenomas
(over 10mm) at colonoscopy was assumed to be 24%, and 20% for small
adenomas.19
Alternative scenarios—The model
was also run with 4% and 8% FOBTi positivity rates, and 70% FOBTi screening
participation rate. A further model run was undertaken for the base case
scenario, but with 90% participation in surveillance colonoscopy.
ResultsFor a FOBTi positivity rate of 6.4%, in the first year of a
programme (2011), a total of 18000 colonoscopies are required, building up to
27000 by year 7, and reaching 28000 after 20 years (year 2031) (see Figure 2).
As expected, there will be a high need for colonoscopy in
the first 2 years, for the prevalence round, following the first screen (18,000
in year 1 and over 19,000 in year 2).
Figure 2. Total colonoscopies for biennial
FOBTi screening 2011-2031
 Figure 3. FOBTi screening: referral and
surveillance colonoscopy 2011-2031
 Total colonoscopies are made up of ‘referral
colonoscopies’ (the first colonoscopy following a positive FOBTi) and
surveillance colonoscopies to follow up adenomas found (see Figure 3). Once the
prevalence round has passed, ‘referral’ colonoscopies, drop to 14000
and then show steady growth tracking the increase in the population, reaching
17000 after 20 years (see Figure 4).
Figure 4. FOBTi screening: Build up of referral
and surveillance colonoscopy 2011-2031
 There were four outcomes of the referral colonoscopy:
firstly those people found to have cancer, who were not modelled further;
secondly and thirdly those with large or small adenomas, who were followed up
with surveillance colonoscopy; fourthly those who had neither adenomas nor
cancer, who were returned to be re-screened after 5 years. Just over half of the
referral colonoscopies (i.e. 9000) would find neither adenomas nor cancer.
Adenomas were found in approximately 7000 people each year;
55% would have large adenomas and 45% small adenomas. Those with adenomas were
referred for surveillance colonoscopy. Surveillance starts at year 4 of the
programme requiring 4000 colonoscopies, and builds up to over 11,000
colonoscopies each year, by year 7 (Figure 4); 71% of these are for surveillance
of large adenomas, with the remainder for small adenomas (see Figure 5).
Figure 5. FOBTi screening: colonoscopy for
surveillance of large and small adenomas
 Sensitivity analysis—Table 1 shows
results for different values of the FOBTi positivity rate, and screening
participation. The number of colonoscopies is shown for year 1 of the programme
and for year 7 (corresponding to years 2011 to 2017). This spans the period
corresponding to the sharp rise in demand for colonoscopy services, which must
be planned for. After year 7, yearly demand increases, but at a much lower rate.
The most important parameter is the positivity of the FOBTi
test, since this determines the volume of referral colonoscopies. Reducing
positivity for the first screen to 4%, and 3% for subsequent screens, resulted
in 11,000 colonoscopies in year 1 increasing to 17,000 by year 7. Increasing the
positivity to 8% and 6% respectively for first and subsequent screens, increased
these values to 22,000 in year 1, and 33,000 in year 7.
The positivity rate determines both the number of cancers
and adenomas found. Higher positivity brings greater benefit, but increases the
number of colonoscopies required.
If participation in the FOBTi screening test increased from
60% to 70%, and assuming other parameters were as for the base case scenario
(including FOBTi test positivity of 6.4%) the number of colonoscopies required
in year 7 would be 31,000.
All values in Table 1 assume 85% compliance with the
referral colonoscopy, following a positive FOBTi, and 100% compliance with
surveillance colonoscopy. If participation in surveillance colonoscopy is
reduced to 90%, and assuming all other parameters are as for the base case
scenario, then total colonoscopies in year 7 reduce to 25,600. This includes
10,500 for surveillance.
Table 1. Sensitivity analysis - Colonoscopy
requirements (year 1 and year 7)
DiscussionThe benefit of a national screening programme for colorectal
cancer are achieved by detecting early stage CRC at colonoscopy performed as
follow-up to a positive FOBTi. However, at the initial referral colonoscopy over
40% of people will be found to have adenomas, which, according to current NZ
guidelines, require ongoing colonoscopic surveillance. There is an ethical
obligation for the initial confirmatory procedure and subsequent surveillance
procedures to be delivered in a timely manner.
The results show that the requirement for colonoscopy
following the introduction of a national screening programme is substantial. In
the first few years of a programme, most of the requirement for colonoscopy is
for the initial referral after a positive FOBTi, but by year 7, surveillance
colonoscopies will have built up and are estimated to account for 44% of the
total. Approximately 70% of this adenoma surveillance would be for large
adenomas, and 30% for small adenomas.
Colonoscopy capacity needs to expand to meet this demand. A
survey21 commissioned for the 2006 Advisory
group found that capacity had increased since the 1998 working group report, but
was still insufficient to consistently deliver, across the country, timely
diagnostic colonoscopy for those with symptoms or timely surveillance procedures
for those at increased risk of CRC. This was in the absence of the additional
demand that would be generated by a screening programme. The estimates in this
paper provide information on requirements under various scenarios, to support
capacity planning.
There are a number of limitations to our study. The rates of
adenoma yield were assumed constant over the screening age band (50–74
years). Yet adenoma prevalence increases with age (leading to a higher yield for
older people screened.22,23 On the other hand,
participation, which may decline with age, was also assumed constant. Thus there
may be some compensating effect of these two assumptions. Moreover the
parameters used in the modelling were themselves averages across age bands, and
therefore appropriate to generate total colonoscopies for the age band screened.
An important issue is the appropriateness of using
parameters based on overseas populations, when modelling the New Zealand
population. This applies to participation in screening, including for gender and
ethnicity subgroups. At present there is no information on the uptake of FOBTi
screening in New Zealand. It is anticipated that 60% of eligible people will
participate in the Waitemata pilot bowel screening programme. This pilot
programme started in October 2011.
Adenoma yield in New Zealand may also differ from that of
overseas populations. A study of 2,842 people undergoing colonoscopy in
Auckland, excluding those with indications associated with high or low adenoma
prevalence24 found that the prevalence of
histologically proven adenomas among 40–59 year olds was 8.7% for Maori
and 16.7% for non-Maori.
Surveillance of large adenomas after 6 years was not
included in the modelling. To model surveillance procedures at three and 6 years
following detection of a large adenoma, and to not model for surveillance beyond
6 years, was considered to best reflect the range of surveillance scenarios (as
described in the methods section) that could result from the detection of large
adenomas at the initial colonoscopy. However, discovery of further adenomas (at
3 or 6 years) would initiate a further sequence of surveillance for a proportion
of individuals and thus the results presented here could potentially be
conservative.
But this underestimation may compensate for the
overestimation due to the assumption of 100% compliance in surveillance assumed
for the base case scenario, when in fact compliance with surveillance
colonoscopy may decline with age as a consequence of comorbid health conditions.
Reducing participation in surveillance colonoscopy to 90% provides a further
estimate of the colonoscopy burden, with surveillance procedures now 41% of the
total.
This modelling has used parameter values from overseas
studies. The actual number of colonoscopies required for a national screening
programme in New Zealand, will depend on the participation for the initial
screen and then compliance with the first colonoscopy and subsequent
surveillance colonoscopy. The sensitivity analysis provides some estimates of
possible colonoscopy volumes with various parameter values.
The pilot bowel screening programme in Waitemata DHB
region should provide New Zealand specific information on many
of the parameters assumed for this modelling, and the model could be run again
to generate new estimates.
The number of colonoscopies also depends on adenoma
surveillance protocols and practice. The modelling was consistent with existent
NZ guidelines on adenoma surveillance but updated guidelines have recently been
released advocating an additional surveillance procedure at a year for
individuals with high risk adenomas.18 This
would further add to the surveillance burden. Current practice may also vary
around these guidelines with a consequent effect on total surveillance
colonoscopies.
Lack of adequate colonoscopy capacity to meet both the (new)
demand from a screening programme and the (existing) demand for people with
symptoms or at high risk runs the risk of compromising both demand streams.
Concern about meeting demand for colonoscopy has been expressed in other
countries, in Ireland which is planning the introduction of a screening
programme, 25 and in England, which established
a pilot study in 2000 and began national roll-out in 2006.
Research on the second round of screening in the English
pilot study reported, in relation to staff in endoscopy units, that
“managing screening-generated surveillance colonoscopies in a timely
manner while meeting diagnostic work (both Pilot and non-Pilot) was
challenging”. 26
Planning for a national screening programme in New Zealand
needs to take account of capacity requirements for surveillance colonoscopy, as
well as for the initial referral colonoscopy.
Surveillance colonoscopy need to be carefully managed and
guidelines for surveillance of low risk adenomas scrutinised to ensure that the
burden of colonoscopic surveillance following detection of adenomas does not
lead to unacceptable waiting times for the initial referral colonoscopy or for
procedures required for people with symptoms.
ConclusionRealising the benefits of a national screening programme for
colorectal cancer, using the immunochemical faecal occult blood based screening
test (FOBTi) requires provision of timely colonoscopy, for a confirmatory
diagnosis of CRC.
Total colonoscopy requirements of a screening programme,
including for adenoma surveillance are high and expansion of colonoscopy
services is required to meet this demand without compromising services for
people with symptoms. The demand depends on the positivity setting of the test.
Higher positivity will give a higher cancer yield but will require more referral
colonoscopies to detect CRC and for subsequent adenoma surveillance.
Surveillance following adenoma detection accounts for a
significant proportion of screening colonoscopies and needs to be carefully
managed so that it does not compromise the delivery of timely diagnostic
colonoscopy for people with symptoms or timely initial colonoscopy following a
positive FOBTi as part of a population CRC screening programme.
Colonoscopy volumes also depend on screening participation
rates and adenoma yield. When data becomes available from the pilot study, the
model can be rerun to give estimates more representative of the New Zealand
setting and population.
Competing interests: None
declared.
Author information: Terri Green, Senior
Lecturer, Department of Management, University of Canterbury, Christchurch; Ann
Richardson, Professor of Cancer Epidemiology, Health Sciences Centre, University
of Canterbury, Christchurch; Susan Parry, Gastroenterologist, Middlemore
Hospital—and Clinical Director of the NZ Familial Gastrointestinal Cancer
Registry, Auckland City Hospital, Auckland
Acknowledgements: The Ministry of Health
convened and supported the 2005 Advisory Group of which the three authors were
members. The Health Research Council provided project funding for the larger
Colorectal Cancer Control in New Zealand project. Some preliminary study
findings were presented at the 7th Health
Services Research Association of Australia and New Zealand conference, Adelaide,
December 2011, and Gastro 10, Annual Scientific Meeting, New Zealand Society of
Gastroenterology, Auckland, November 2010. Ann Richardson receives support from
the Wayne Francis Charitable Trust. We acknowledge Associate Professor Ian
Bissett for early discussions of ideas for this paper.
Correspondence: Dr Terri Green, Department
of Management, University of Canterbury, Private Bag 4800, Christchurch 8140,
New Zealand. Fax: +64 (0)3 3642020; email: terri.green@canterbury.ac.nz
References:
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