Report in English with a Dutch summary (KCE reports 45A)

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144 Screening for Colorectal Cancer: Appendices KCE reports vol.45 All parameters and variables included in the model are presented in table 30. Uncertainty is accounted for in the model by including all estimated variables with their respective distributions. Estimates and distributions are derived from literature or from expert opinion if no data were directly available from literature. This approach allows probabilistic sensitivity analyses and the construction of confidence intervals around the point estimates for total costs and costs per CRC detected resulting from the model. Parameters for which precise data are available, such as costs of specific procedures, are included in the model without a distribution. The basis for the assumed value and distribution of each variable in the model is briefly explained in the following paragraphs. 8.1.1.1 Exclusion of individuals at high risk Mass screening is targeted at individuals at average risk. Therefore individuals at increased or high risk should not be included in mass screening but offered regular health care. In both the GP and mailing system, individuals at high risk are excluded from mass screening. In the GP system the GP performs a prescreening risk-stratification and informs the patients about the eligibility for mass screening. In the mailing system, the accompanying letter will clearly state that individuals who are already being followed-up for their increased CRC risk should not participate. In case of doubt they would be adviced to contact their GP. We do not have precise data on the actual number of individuals that will be excluded for this reason. A few studies estimated the prevalence of having a family history of CRC in at least one first degree relative at 5 to 10% (see chapter 3.5). In the literature it is assumed that about 25 to 30% of cancers occur in individuals at increased risk. Assuming on average a doubling of the risk in this group, this would correspond to a proportion of 14 to 18% of the population that should be excluded from mass screening. We therefore used a point estimate of 16% with an uncertainty ranging from 14% to 18% in a Betadistribution. 8.1.1.2 Participation 8.1.1.3 Compliance Participation is defined as going to the GP in the GP system or returning the test kit to the laboratory in the mailing system. In RCTs participation to FOBT in the first round of screening ranges from 60% to 69,5% (see table 23). The Cochrane meta-analysis 24 reported 67%. Real world experiences in different countries (see table 28), however, indicate more variation in participation, ranging in Europe from 20% in the Czech Republic to 75% in Finland. In chapter 5 we showed that the median participation rate to programmatic offers of FOBT is between 40 and 50%, and that approximately 50% can be obtained with minimal prompting. For the budget impact model we used a point estimate of 45% with an uncertainty ranging from 15% to 75% in a Beta-distribution. Compliance is defined as accepting colonoscopy after a positive FOBT result. In our model, compliance with colonoscopy is assumed to be 87,5%, with uncertainty limits ranging from 80% to 95% in a Beta-distribution. This assumption is based on experience in other countries (see chapter 7), on expert opinion and supported by evidence from the Burgundy and the Funen trial that report a compliance rate with colonoscopy after a positive FOBT of 85% and 82% respectively (see table 29).
KCE reports vol.45 Screening for Colorectal Cancer: Appendices 145 8.1.1.4 Sensitivity Whereas we judged that participation rates from RCTs could not be directly extrapolated to real life screening conditions, this is probably less so for other parameters of screening performance. Sensitivity estimates from the RCTs that used unrehydrated gFOBT 362, 358, 351, 363, 207, 208 range from 41% in France to 81% in Göteborg and Minnesota. In our model, we used a point estimate of 50% for sensitivity of FOBT and a range from 40 tot 80% in a Beta-distribution. The proportion of CRC missed (false negatives) is one minus sensitivity. 8.1.1.5 Positivity rate The positivity rate, or the percentage of positive FOBT results, was taken from the same RCTs using unrehydrated gFOBT 362, 358, 351, 363, 207, 208. Specifically reported positivity rates for the first round of screening ranged from 1,9% in Göteborg to 2,4% in Minnesota, with two trials reporting 2,1% positivity rate in the first round (Nottingham and Burgundy). Therefore we chose 2,1% as a point estimate for the first round of screening with uncertainty ranging from 1,9 to 2,4% in a skewed Beta-distribution. For subsequent rounds of screening a lower point estimate of 1,5% was assumed with uncertainty ranging from 1,4 to 1,6 in a Beta-distribution based on information from RCTs (see tables 29 and 35). 8.1.1.6 Colonoscopy detection rates (Cancer, Adenoma, negative) After a positive FOBT, colonoscopy is used as the golden standard to evaluate the colon. During this evaluation not only CRC is found but also adenomata or other disorders. From the RCTs, CRC and adenomata 10mm are reported. Table 29 gives an overview of the (colonoscopic) findings in 3 RCTs over all screening rounds combined 348, 207, 208 and from a study of Allison 593 that compared the performance of several FOBT tests and of a combination of tests by identifying screened patients who had colorectal neoplasms diagnosed (carcinoma or a polyp 10mm in diameter) in the two years after screening. It should be appreciated that combined with the reported positivity rates of FOBT, those numbers lead to an apparent incidence that is two to three times higher than the incidence reported in cancer registries. This is due to the inherent property of screening that it increases apparent incidence by detecting cancer earlier. Table 29: Observed performance characteristics of FOBT and colonoscopy Burgundy Nottingham Funen Allison Colonoscopy after pos FOBT 85% 82% Reported Sensitivity 41% 64% 46% 37% Positivity rate (average) 1,5% 1,5% 1,5% 2,46% CRC after positive FOBT 9,9% 11,5% 10,4% 6,6% Adenoma (>10mm) after positive FOBT 14,3% 34,6% 22,2% 16,67% CRC incidence in those screened 0,15% 0,18% 0,15% 0,16% CRC incidence in those screened corrected for sensitivity of FOBT 0,36% 0,28% 0,33% 0,43% Approximations of detection rates after positive FOBT made for the Dutch consensus development meeting used similar estimates 2: 10% for CRC and 30% for adenoma. In a French survey of colonoscopies (unpublished but presentation available at the SFED website 315) the incidence was 4% for CRC and 35% for all polyps, but these colonoscopies were carried out for various reasons and not
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