- 1 Bowel cancer screening
- 2 Bowel cancer screening tools
- 3 Role of GPs in screening
- 4 Benefits and risks of bowel cancer screening
- 5 Screening people at increased risk
- 6 Notes
- 7 References
Bowel cancer screening
Finding bowel cancer or precancerous conditions early is a key to reducing bowel cancer mortality and morbidity. Bowel cancer is one of only three cancers for which population-based screening is recommended; the others are breast cancer and cervical cancer. (For more information, see the Principles of screening chapter of the National Cancer Prevention Policy.)
Population-based bowel cancer screening with the immunochemical faecal occult blood test (iFOBT) in Australia is managed through the National Bowel Cancer Screening Program (NBCSP), see the Policy context section. FOBT is the recommended screening tool due to its high sensitivity and specificity, acceptability among the public and cost-effectiveness. For information on bowel cancer screening in Australia through NBCSP, see the Policy context section.
The two objectives of bowel cancer screening are:
- to prevent cancer by identifying and removing precancerous, advanced adenomas; and
- to diagnose and treat early-stage, curable cancers.
Research shows that most bowel cancers develop from adenomatous polyps (adenomas); a small adenoma may grow into advanced cancer over an estimated 10 years. While common in middle aged and older people, most adenomas (around 19 out of 20) never progress to cancer.
Adenomas are described as 'advanced' if they have certain features such as larger size (10 mm or more in diameter). These features indicate a greater likelihood of progression to localised cancer and the risk of developing cancer elsewhere in the large bowel. Once cancer has developed, growth may occur gradually over 12 to 24 months or even longer. Most people with advanced adenomas and many with early-stage bowel cancer experience no obvious symptoms.
Bowel cancer survival is highly dependent on the pathological stage of tumour development at the time of diagnosis, as well as the treatments that follow. Earlier diagnosis is strongly associated with better prognosis. Based on international data, five-year case survival rates for bowel cancer range from 84–94% when diagnosed at stage A/stage I, when the cancer is contained within the bowel wall, to 6–19% when diagnosed at stage D/stage IV, when the cancer cannot be removed by surgery or has spread to other areas of the body.
- 88–92% for stage A/I, when the cancer is contained within the bowel wall;
- 70–83% for stage B/II, when the cancer has extended through the bowel wall, but with no lymph nodes affected;
- 43–53% for stage C/III, when the cancer is present in the lymph nodes;
- 7–19% for stage D/IV, when the cancer cannot be removed by surgery or has spread to other areas of the body.
There is evidence of an upward trend in survival figures over time in Australia, due to a number of factors including improvements in surgical technique and more widespread use of adjuvant chemotherapy and radiotherapy.
Bowel cancer screening tools
Faecal occult blood test
Cancers in the large bowel tend to produce low-grade bleeding, so detecting blood in small (often invisible) concentrations in faeces is the basis for using FOBT to screen for bowel cancer. FOBT can detect blood in faeces by identifying the chemical or immunochemical properties of haem and haemoglobin (substances found in blood).
There are two types of FOBT: the guaiac test (gFOBT) detects chemical activity in haem; the immunochemical test (iFOBT) uses globin antibodies to detect blood. The guaiac test relies on the user avoiding red meat and vitamin C supplements for several days before use; the immunochemical test requires no change in diet or medication from the user.
Of those who have a follow-up colonoscopy around 4.3% have bowel cancer, 23% have an advanced adenoma; and 25% will have a precancerous growth called a non-advanced adenoma. This means around 52% of people with a positive iFOBT will have some type of neoplastic lesion (adenoma or cancer). The remainder usually have a noncancerous condition such as haemorrhoids. The likelihood of finding bowel cancer is 12 to 40 times greater in someone who tests positive for FOBT.
The sensitivity of iFOBT for advanced adenoma ranges from 24-50%. Based on a collection of a single sample, iFOBT has a sensitivity of 50% for stage I bowel cancer and 70% for stage II. It is noted that collection of two samples (as used in the NBCSP) would be expected to increase the sensitivity of the test. The sensitivity for all bowel cancers ranges from 60-85%. The specificity of iFOBT for bowel cancer is around 95%.
Population screening with FOBT is recommended in the clinical practice Guidelines for the Prevention, Early Detection and Management of Colorectal Cancer developed by Cancer Council Australia’s Australian Cancer Network and endorsed by the National Health and Medical Research Council (NHMRC). The guidelines recommend organised screening with FOBT, performed at least once every two years for the Australian population over 50 years of age. A large Australian population-based prospective study found that people who reported ever being screened with FOBT had a 40% reduced risk of developing bowel cancer for at least four years after screening. The authors note that their data cannot be used to directly evaluate bowel cancer screening effectiveness due to a number of limitations, but their findings are broadly consistent with the results of randomised controlled trials.
Acceptability of FOBT as a screening tool is high. A survey of WA residents in the target age group for the NBCSP found that, among those not yet invited to participate in screening, 83% stated they were ‘very/quite’ likely to participate if invited to do so and 94% were ‘very/quite’ likely to undergo the follow-up colonoscopy if their FOBT result was positive.
Sigmoidoscopy involves tube examination of the rectum and the lower part of the colon (i.e. the section of large bowel closest to the anus). The sigmoidoscope may be rigid (best suited for examining the rectum) or flexible (reaching into the lower part, but unable to examine the upper part of the colon).
Flexible sigmoidoscopy allows examination of the area where 55% to 60% of bowel cancers and advanced adenomas occur. When abnormalities are detected, a tissue sample (biopsy) can be collected for pathological examination.
Evidence from several case-control studies indicated that screening by flexible sigmoidoscopy should lead to a substantial reduction in bowel cancer mortality. Major randomised controlled trials using flexible sigmoidoscopy for screening were then conducted in the US, UK, Italy and Norway. Progress results showed sigmoidoscopy to be feasible, safe and well accepted within the setting of three trials.
The final results of the UK trial were reported in April 2010, with bowel cancer incidence being reduced by 23% and mortality by 31% in the group offered sigmoidoscopy. In 2011, the Italian trial reported incidence was reduced by 18% and mortality by a statistically non-significant 22%. In 2012, the US trial reported incidence was reduced by 21% and mortality by 26% after a median follow-up of 11.9 years.
The randomised controlled trial in Norway, the only trial that was population-based, showed that screening by flexible sigmoidoscopy reduced bowel cancer incidence by 20% and mortality by 27% on an intention to screen basis.
While a meta-analysis of randomised controlled trials published in 2012 provided Level I evidence for efficacy of flexible sigmoidoscopy as a screening tool for bowel cancer, evidence based on population screening is limited. High cost, low acceptability and concerns about impact on colonoscopy services are significant issues for its use on a population level in Australia.
A colonoscope is similar to a flexible sigmoidoscope, but is much longer and able to examine the entire length of the large bowel; studies have shown that colonoscopy has around 95% sensitivity for detection of cancer. Colonoscopy also allows biopsies to be taken from suspected abnormalities as well as enabling most adenomas and some polypoid cancers to be removed during examination.
Colonoscopy sensitivity for detection of adenomas varies according to their size and site within the large bowel. As with sigmoidoscopy, sensitivity for detection of lesions depends on the proficiency of the examiner, again highlighting the need for comprehensive training and the auditing of outcomes. In 2009, the Australian Department of Health and Ageing released detailed recommendations on key performance indicators for colonoscopy. The National Bowel Cancer Screening Program Quality Working Group recommends standards, objectives and performance indicators for colonoscopy use in Australia.
Colonoscopy is the recommended follow-up test for those with positive findings at FOBT or screening sigmoidoscopy. It is also recommended as the primary tool for cancer surveillance in people with an increased risk of bowel cancer . When it is not possible to examine the total length of the bowel by colonoscopy, CT colonography should be performed.
There is no high-level evidence to support use of colonoscopy in population screening currently available. However, two randomised controlled trials are underway. Initial findings from the Spanish study reported people were more likely to participate in screening with FOBT than with colonoscopy. The number of bowel cancers detected were similar in the two groups, but more adenomas were identified in the colonoscopy group.
The acceptability of colonoscopy as a screening tool is limited by the procedure’s invasive nature and the need for vigorous bowel preparation and sedation.
Plasma and faecal DNA testing
DNA biomarker testing, using plasma (blood) or faecal DNA samples, is an emerging technology for the detection of bowel cancer. A number of DNA modifications associated with bowel cancer and its precursors have been identified. These are both genetic (usually mutational changes to the genes) and epigenetic (changes to gene expression, usually by promoter methylation).
DNA biomarker tests for bowel cancer using plasma and faecal stool samples are available, however to date, the performance and cost-effectiveness of such tests have not been evaluated for population screening.
A recently published assessment of faecal DNA testing on a sample of 9989 people tested before screening colonoscopy reported a sensitivity of 42% for advanced adenoma and 93% for bowel cancer (stage I-III), with no difference between stages. However, specificity for this test is only 86.6%. Another study on the automation of DNA testing on faecal samples reported a sensitivity of 57% for advanced adenoma, 95% for stage I cancer and 100% for stage II cancer, and specificity of 90%.
In a preliminary evaluation of plasma DNA testing, sensitivity was 7% for advanced adenoma, 29% for stage I cancer and 68% for stage II two cancer; specificity was 94%.
In 2008, the U.S. Preventive Services Task Force concluded that current evidence was insufficient to assess the benefits and harms of faecal DNA testing for screening and that a high monetary cost per test was likely.
The poor sensitivity of plasma DNA testing for advanced adenoma and for stage I cancer makes the test inappropriate for population screening. Population screening using faecal DNA testing would lead to a high number of unnecessary colonoscopies, due to the test’s low specificity. In comparison, the good sensitivity and specificity, cost-effectiveness and availability of iFOBT make it the most effective screening tool currently available. See below for more.
Conclusion: why FOBT is the recommended screening tool
As early as 1997, population-based screening with FOBT was demonstrated to be clinically effective in reducing bowel cancer mortality. The Australian Health Technology Advisory Committee concluded that pilot programs were required to determine the most effective approach in terms of feasibility, acceptability among the target population and cost-effectiveness. Studies indicated that more Australians would participate in FOBT using the immunochemical test than the guaiac test.
Subsequent evidence, including from the pilot studies for the Australian screening program and clinical practice guidelines, support the use of iFOBT as the recommended population screening tool for bowel cancer in Australia. The most recent clinical practice guidelines for the prevention, early detection and management of colorectal cancer strongly recommend population screening using FOBT for Australians aged 50 and over, at least every two years.
As early detection is the primary objective of screening, GPs are urged to recommend iFOBT as a screening tool for asymptomatic patients with no family history of bowel cancer. GPs are also urged to encourage Australians turning 50, 55, 60, 65, 70, 72 and 74 to participate in the NBCSP, as they are eligible for free screening with an iFOBT kit mailed to their homes. All Australians aged 50 to 74 will be eligible for screening every two years by 2020. See the Policy context section of this chapter for more information on the NBCSP.
Colonoscopy has been considered as a potential screening, rather than diagnostic tool, due to its high sensitivity for detecting bowel cancers and advanced adenomas. However, the overall appropriateness of colonoscopy as the primary tool in population screening remains unclear. And the feasibility of providing colonoscopy for the five million asymptomatic Australians at average risk of bowel cancer is also doubtful, given the high cost of the procedure, workforce limitations and other logistical issues.
Role of GPs in screening
GPs have an important role at critical points in the screening process. These include:
- determining the appropriateness of screening for individual patients (e.g. those with significant co-morbidities, or those who have recently undergone screening outside the national program);
- assessing high-risk individuals and managing them according to NHMRC guidelines;
- receiving FOBT results where the participant has nominated a GP;
- managing participants with a positive FOBT, such as organising colonoscopy through the usual care pathway; and
- notifying the central registry of outcomes.
- bleeding from the back passage or any sign of blood in a bowel motion;
- an unexplained and persistent change in bowel actions;
- unexplained tiredness;
- iron deficiency anaemia (identified through a blood test) – which could be linked to unexplained tiredness (as above) and can be symptomatic of bowel cancer;
- lower abdominal pain; and
- a persistent feeling of fullness.
Recommendation from a GP has been shown to be the most significant factor in encouraging people to screen for bowel cancer with FOBT. One survey showed more than 90% of respondents would be “likely” or “very likely” to have an FOBT every two years if advised by a doctor. Pilot program invitees who did not participate reported a greater likelihood of doing so if it was recommended by a GP.
GP involvement is therefore critical to optimal participation in bowel cancer screening programs. While there is evidence that GPs support bowel cancer screening, they have articulated a need for further education on the issue.
Benefits and risks of bowel cancer screening
Screening is effective at detecting bowel cancers early, when they are easier to treat.A study of SA residents invited to participate in the NBCSP found that invitees were more likely to have stage A lesions compared with all other patients (35% compared with 19%), and half as likely to have stage D cancer (5% compared with 12%). Among those who participated in screening this difference was more pronounced. Before population screening was introduced, monitoring systems in South Australia found that only 15% of bowel cancers were detected at their earliest point, stage A. A Biogrid Australia analysis of data on 1628 cases of bowel cancer diagnosed between 2006 and 2008, almost triple the number of stage A cancers were detected through the NBCSP, compared with those diagnosed through symptoms (40% compared with 14%).
These findings are consistent with highest-level international evidence showing that bowel cancer screening on a population basis significantly reduces mortality and morbidity. The most obvious benefit of screening for bowel cancer is the potential for reducing mortality in the recommended age group – people age 50 and over – by between 30% and 40%.*
An independent analysis published in 2011 estimated that a complete NBCSP would save up to 500 lives each year. This estimate of mortality benefit is conservative, as it is based on low participation rates.
A number of studies have shown that bowel cancer screening using FOBT is cost-effective. See the Policy context section of this chapter for more information.
Potential adverse effects
The most significant potential adverse effect associated with screening asymptomatic people for bowel cancer is the risk of physical harm incurred through colonoscopy. Colonoscopy is performed as a day case procedure and usually requires sedation; it can produce severe complications such as perforation, haemorrhage or death and carries a remote risk of transmitting infections. In a review of six prospective studies of colonoscopy, about one in 1000 patients suffered perforation, three in 1000 suffered major haemorrhage, and between one and three in 10,000 died as a result of the procedure. In two more recent studies, the findings were similar, overall morbidity being 0.4%. A review of a large Australian hospital experience supported the conclusions of these other studies and reported a mortality rate of 0.004% in outpatients having the procedure.
Bowel cancer screening may also result in adverse psychological and physical effects due to false positive tests (i.e. a preliminary indication that cancer may be present, which is not confirmed through investigation). People who present with bowel lesions that are ultimately found to be benign may experience significant initial distress and anxiety. Studies have shown, however, that anxiety usually abates when individuals learn they do not have bowel cancer, with no evidence of long-term harm after screening.
Thus, timely follow-up is critical for people who test positive to FOBT. This is particularly important as a high number of people receiving positive FOBT results are not diagnosed with cancer. A recent NBCSP monitoring report has shown that only one in every 32 people who underwent a colonoscopy to follow up a positive screening result are diagnosed with a confirmed or suspected cancer, and one in 17 found to have advanced adenomas. For more information, see Program data.
The potential for these harms must be weighed against the significant benefits of population-based screening. On balance, this is the case for screening with FOBT.
Screening people at increased risk
The Australian Health Technology Advisory Committee’s report on bowel cancer screening recommended that a national approach to screening be complemented by policy for groups at increased risk of bowel cancer: e.g. individuals with a family history of bowel cancer, or a personal history of bowel adenoma, bowel cancer or inflammatory bowel disease.
GPs are well-placed to determine bowel cancer risk on the basis of family history and to instigate appropriate management according to the guidelines (see Role of GPs in screening for more information).
The Clinical practice guidelines for the prevention, early detection and management of colorectal cancer defines three categories of people in relation to risk for bowel cancer based on their family history of the disease:
- in the first category, people who have just one first-degree relative with bowel cancer are advised to have the same screening as those at average risk, provided their relative was diagnosed with cancer at or over the age of 55;
- the second category includes people with two or more first-degree relatives with bowel cancer (on the same side of the family) or one first-degree relative diagnosed under the age of 55 years. Screening is generally based on periodic (usually every five years) colonoscopy because of their elevated cancer risk; and
- the third category covers members of families with definite or suspected familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC). People with FAP usually have hundreds of small polyps throughout their bowel, some of which become malignant if not removed. HNPCC is not associated with polyposis; however, like FAP, it is caused by an inherited gene mutation. The place of genetic testing and recommendations about surveillance and prophylactic surgery are described in chapter 7 of the guidelines.
Detailed recommendations for surveillance by colonoscopy in those with past bowel cancer or adenoma are also set out in the guidelines.
* ↑ It is difficult to accurately predict the mortality benefit of screening in Australia, due to a number of variables and assumptions. Using available data, we estimate that a fully implemented NBCSP would reduce bowel cancer mortality in the screening target group by between 30% and 40%, based on a 60% participation rate if the program were fully implemented and supported by communications strategies, FOBT sensitivity for 85% of cancers and 60% for advanced adenomas and efficiencies in the program ensuring appropriate investigation of participants testing positive.
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