Interval Cancers in FOBT-based CRC Screening Program
Interval Cancers in FOBT-based CRC Screening Program
The demonstration pilot in Scotland was based on the Nottingham randomised trial, as this was considered by the UK National Screening Committee to be the most appropriate for the UK population. The methodology used for the pilot has been described in detail in earlier publications. Briefly, screening in Scotland was carried out in Tayside, Grampian and Fife (with a total population of about 1.3 million) and was based on biennial GFOBT using a test kit (Hema-screen; Immunostics Inc, Ocean, New Jersey, USA) biochemically identical with that used in the randomised trials carried out in Nottingham, England, Funen, Denmark and Göteborg, Sweden. The test was designed to collect two samples from each of three separate bowel motions on to guaiac gum-impregnated filter paper through windows in the test kit card. The card was developed and read within 14 days in a laboratory specifically set up for the pilot and accredited to ISO 15189 standards. If five to six windows were positive on the initial GFOBT (strong positive), colonoscopy was offered. If one to four windows were positive (weak positive), a further GFOBT was requested, and, if any of the subsequent windows were positive, a colonoscopy was offered. Minor changes to this algorithm took place during the pilot and these have been detailed elsewhere.
Male and female residents in the three pilot NHS Boards aged between 50 and 69 years were identified by the Community Health Index and invited to participate by having a test kit and instructions sent in the mail. Community Health Index acts as a unique identification number for every person registered with a general practitioner or having some contact with the NHS in Scotland. It is made up of date of birth followed by a four-digit number as a unique identifier.
For the purposes of this study, the following definitions were used: a screen-detected cancer was defined as a cancer diagnosed at colonoscopy triggered by a final positive screening test result; an interval cancer was defined as a cancer diagnosed within 2 years of a negative test result; a missed cancer was defined as a cancer diagnosed within 2 years of a negative colonoscopy triggered by a positive test result; a non-screened cancer was defined as a cancer arising in the Scottish population not offered screening (ie, in a resident outside the pilot NHS Boards) during the same time period and in the same age range (50–69 years).
In order to categorise the cancers as above, all screened records from the demonstration pilot were linked with confirmed colorectal cancer records in the Scottish Cancer Registry (SCR) for the time periods relating to each round. This allowed the calculation of the time between the final screening test result and the date that the colorectal cancer was diagnosed. Cases where the individual defaulted before a complete GFOBT, refused investigation treatment, was declared unfit for colonoscopy, was undergoing symptomatic investigations, was admitted as an emergency, or died between screening investigations were categorised as 'miscellaneous', and were excluded from analysis.
Cancers arising in individuals who chose not to participate in the screening programme were not considered in this study. The prognosis associated with such cancers is already known to be particularly poor, presumably as they arise in people who are likely to have poor general health by virtue of their non-attendance.
The Dukes' staging system was used to describe the stage of the cancers studied as recorded by the SCR. Although polyp cancers were identifiable separately in the screen-detected group, it was not possible to do this reliably from the registry data. All polyp cancers were therefore included in the stage A category for the purpose of this analysis. Survival times were calculated for all individuals with colorectal cancers diagnosed within the time periods of the three rounds of the demonstration pilot (with life status as at 31 December 2009). For these groups, survival curves were estimated for all causes of death and for colorectal cancer deaths.
The SCR uses multiple sources, and routine indicators of data quality suggest that case ascertainment is very nearly complete. One study carried out in 1992 yielded an estimate of completeness of ascertainment of colorectal cancers of 98.5%, and a more recent study of breast cancer spanning the period 1978–2000 obtained similar results.
Methods
The demonstration pilot in Scotland was based on the Nottingham randomised trial, as this was considered by the UK National Screening Committee to be the most appropriate for the UK population. The methodology used for the pilot has been described in detail in earlier publications. Briefly, screening in Scotland was carried out in Tayside, Grampian and Fife (with a total population of about 1.3 million) and was based on biennial GFOBT using a test kit (Hema-screen; Immunostics Inc, Ocean, New Jersey, USA) biochemically identical with that used in the randomised trials carried out in Nottingham, England, Funen, Denmark and Göteborg, Sweden. The test was designed to collect two samples from each of three separate bowel motions on to guaiac gum-impregnated filter paper through windows in the test kit card. The card was developed and read within 14 days in a laboratory specifically set up for the pilot and accredited to ISO 15189 standards. If five to six windows were positive on the initial GFOBT (strong positive), colonoscopy was offered. If one to four windows were positive (weak positive), a further GFOBT was requested, and, if any of the subsequent windows were positive, a colonoscopy was offered. Minor changes to this algorithm took place during the pilot and these have been detailed elsewhere.
Male and female residents in the three pilot NHS Boards aged between 50 and 69 years were identified by the Community Health Index and invited to participate by having a test kit and instructions sent in the mail. Community Health Index acts as a unique identification number for every person registered with a general practitioner or having some contact with the NHS in Scotland. It is made up of date of birth followed by a four-digit number as a unique identifier.
For the purposes of this study, the following definitions were used: a screen-detected cancer was defined as a cancer diagnosed at colonoscopy triggered by a final positive screening test result; an interval cancer was defined as a cancer diagnosed within 2 years of a negative test result; a missed cancer was defined as a cancer diagnosed within 2 years of a negative colonoscopy triggered by a positive test result; a non-screened cancer was defined as a cancer arising in the Scottish population not offered screening (ie, in a resident outside the pilot NHS Boards) during the same time period and in the same age range (50–69 years).
In order to categorise the cancers as above, all screened records from the demonstration pilot were linked with confirmed colorectal cancer records in the Scottish Cancer Registry (SCR) for the time periods relating to each round. This allowed the calculation of the time between the final screening test result and the date that the colorectal cancer was diagnosed. Cases where the individual defaulted before a complete GFOBT, refused investigation treatment, was declared unfit for colonoscopy, was undergoing symptomatic investigations, was admitted as an emergency, or died between screening investigations were categorised as 'miscellaneous', and were excluded from analysis.
Cancers arising in individuals who chose not to participate in the screening programme were not considered in this study. The prognosis associated with such cancers is already known to be particularly poor, presumably as they arise in people who are likely to have poor general health by virtue of their non-attendance.
The Dukes' staging system was used to describe the stage of the cancers studied as recorded by the SCR. Although polyp cancers were identifiable separately in the screen-detected group, it was not possible to do this reliably from the registry data. All polyp cancers were therefore included in the stage A category for the purpose of this analysis. Survival times were calculated for all individuals with colorectal cancers diagnosed within the time periods of the three rounds of the demonstration pilot (with life status as at 31 December 2009). For these groups, survival curves were estimated for all causes of death and for colorectal cancer deaths.
The SCR uses multiple sources, and routine indicators of data quality suggest that case ascertainment is very nearly complete. One study carried out in 1992 yielded an estimate of completeness of ascertainment of colorectal cancers of 98.5%, and a more recent study of breast cancer spanning the period 1978–2000 obtained similar results.
