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 generic information about cancer screening, see the Principles of screening chapter.)
Approximately 15-20% of bowel cancers arise from the serrated pathway, however, most bowel cancers develop from adenomatous polyps (adenomas); a small adenoma may grow into advanced cancer over an estimated 20-25 years on average. While common in middle aged and older people, 40% of advanced adenomas progress to cancer.
Adenomas are described as 'advanced' if they have certain features such as larger size (10 mm or more in diameter), contains villous growth pattern, or high-grade dysplasia. 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. Patients with bowel cancer detected via participation in NBCSP were diagnosed at an earlier stage than patients with symptomatic cancers. In a global report, the 5-year survival data for Australian adults diagnosed with colon or rectum was 70.7% and 71%, respectively.
Australian five-year survival rates for bowel cancer, based on South Australian and Western Australian data, are:
- 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
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.
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). iFOBT is the recommended screening tool due to its high sensitivity and specificity, acceptability among the public and cost-effectiveness. For information on NBCSP, see the Policy context section.
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 detects 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 gFOBT relies on the user avoiding red meat and vitamin C supplements for several days before use; the iFOBT requires no change in diet or medication from the user.
In 2016, of those who had a follow-up colonoscopy after a positive screening result, 16% had some type of neoplastic lesion (3.8% had bowel cancer, 6% have an advanced adenoma, 6% have a precancerous growth called a non-advanced adenoma).
The sensitivity of iFOBT for advanced adenoma ranges from 16-64%. Furthermore, the sensitivity of iFOBT ranges from 53-100% and specificity is around 93% for detecting colorectal cancer.
Acceptability of iFOBT 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 iFOBT result was positive.
The Clinical Practice Guidelines for the Prevention, Early Detection and Management of Colorectal Cancer, endorsed by the National Health and Medical Research Council (NHMRC), recommends organised population screening where iFOBT is performed every two years for the Australian population aged between 50 and 74 years. A cost-effectiveness analysis further supports the use of iFOBT in a population screening program as biennial iFOBT screening was found to be highly cost-effective in Australia. It was also the most cost-effective approach when compared with other screening technologies (including colonoscopy, computed tomographic colonography, DNA tests and flexible sigmoidoscopy) under all screening scenarios considered.
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. See role of GPs in screening for more information.
Screening age range and intervals
The clinical recommendations assessed different age range scenarios and found that the 50-74 age range was most cost-effective and had a balanced ratio of benefits to harms. Reducing the starting age to 45 years would present with a less favourable benefits-to-harms balance and require more colonoscopies for each extra cancer death prevented. Screening after 74 years was not cost-effective and not recommended. Therefore, bowel cancer screening is recommended for people aged 50-74 years.
Recommendations were made to keep screening frequency unchanged. Testing with iFOBT and other screening tools (including colonoscopy, computed tomographic colonography, DNA tests and flexible sigmoidoscopy) at various alternative intervals (including annual) were compared to biennial testing with iFOBT. The latter was found to be the most cost-effective approach for bowel cancer screening in Australia.
Benefits and risks of bowel cancer screening
Screening is effective at detecting bowel cancers early, when they are easier to treat. The AIHW found that NBCSP participants with screen-detected bowel cancer were more likely to be diagnosed at a less advanced stage than those who did not participate in 2006-2010 (44% vs 24%). Furthermore, NBCSP participants with screen-detected bowel cancer had a lower risk of bowel cancer mortality compared to those who were diagnosed outside of the program (9.6% vs 23.8%). Independent studies have shown similar outcomes which is 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 for people aged 50-74 years is the potential for reducing bowel cancer mortality by 36% with a current participation rate of 40% and 59% for participation rates of 70%. An independent analysis published in 2017 estimated that 2300-3200 bowel cancer deaths would be prevented each year from 2015-2040 through the NBCSP.
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 with a positive FOBT result. This is particularly important as a high number of people with positive FOBT results are not diagnosed with cancer. A recent NBCSP monitoring report has shown that only one in every 26 people who underwent a colonoscopy to follow up a positive iFOBT result are diagnosed with a confirmed or suspected cancer, and one in 9 found to have advanced adenomas.
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 iFOBT.
Other potential screening tools
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.
Major randomised controlled trials using flexible sigmoidoscopy conducted in the US, UK, Italy and Norway were conducted following evidence from case-control studies showing a substantial reduction in bowel cancer mortality. Three of these trials showed that this tool was feasible, safe and well accepted. These studies showed a decrease in bowel cancer incidence of 18-26% and mortality of 22-30%. The population-based study conducted in Norway showed a similar reduction in incidence and mortality of bowel cancer.
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.
A cost-effectiveness analysis showed that a once-off sigmoidoscopy at 60 years was the least effective strategy with the lowest reductions in colorectal cancer incidence and mortality compared to other strategies included in the evaluation.
High cost, low acceptability and concerns about impact on colonoscopy services are significant issues for the use of sigmoidoscopy 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 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 iFOBT 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.
Screening with 10-yearly colonoscopy was not found to be cost-effective when compared with biennial iFOBT screening and other screening strategies considered in the study.
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.
Colonoscopy has been considered as a potential screening tool, 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. Moreover, the feasibility of providing colonoscopy for the circa 8 million Australians recommended for screening is also doubtful, given the high cost of the procedure, potential adverse effects, workforce limitations and other logistical issues.
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 of such tests have not been evaluated for population screening.
Faecal DNA testing sensitivity is 41-57% for detecting advanced adenomas and 93-100% for bowel cancer (stage I to III). These studies have observed a specificity of 87-91%. In 2016, the U.S. Preventive Services Task Force noted the lower specificity of faecal DNA testing in comparison with iFOB testing may result in higher numbers of false-positive results, higher likelihood of follow-up colonoscopies and experiencing an associated adverse event per screening test.
In a preliminary evaluation of plasma DNA testing, sensitivity was 7% for advanced adenoma, 29% for stage I cancer and 68% for stage II cancer; specificity was 94%. Similar sensitivity and specificity rates at 51% and 92% were reported for colorectal cancer from another assessment. The poor sensitivity of plasma DNA testing for advanced adenoma and for stage I cancer makes the test inappropriate for population screening.
Five-yearly faecal DNA testing and 2-yearly plasma DNA testing have not been shown to be cost-effective when compared with biennial iFOBT testing and were also found to be less effective in preventing bowel cancer deaths. In comparison, the good sensitivity and specificity, cost-effectiveness and availability of iFOBT make it the most effective screening tool currently available.
Role of GPs in screening
GPs are urged to encourage Australians who receive an iFOBT kit mailed to their homes to participate in the NBCSP given the evidence for mortality benefit. All Australians aged 50 to 74 will be eligible for screening every two years by 2019. See the Policy context section of this chapter for more information on the NBCSP.
GPs have an important role at critical points in the screening process. These include:
- determining the appropriateness of screening for individual patients (e.g. excluding those with significant co-morbidities, or those who have recently undergone screening or colonoscopy outside the national program);
- assessing high-risk individuals and managing them according to NHMRC guidelines;
- receiving iFOBT results where the participant has nominated a GP;
- managing participants with a positive iFOBT result, such as organising colonoscopy through the usual care pathway; and
- notifying the central registry of outcomes.
GPs may also assess individuals with symptoms that could be related to cancer and for whom diagnostic investigations (rather than screening) are required. Symptoms include:
- 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 iFOBT. One survey showed more than 90% of respondents would be “likely” or “very likely” to have an iFOBT 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..
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:
- the first category (near average risk; people with no relative diagnosed with bowel cancer and people with one first-degree and/or second-degree relative diagnosed with bowel cancer at age 55 or older) are advised to screen with biennial iFOBTs from age 50 to 74, which is the same screening as those at average risk and is the screening provided by the National Bowel Cancer Screening Program;
- the second category (moderately increased risk; people with one first-degree relative diagnosed with bowel cancer under age 55, or two first-degree relatives diagnosed with bowel cancer at any age, or a combination of the two) are advised to screen with biennial iFOBTs from age 40 to 49 and colonoscopy every five years from age 50 to 74; and
- the third category (potentially high risk; people with at least three first-degree or second-degree relatives diagnosed with bowel cancer where at least one was diagnosed under age 55, or at least three first-degree relatives diagnosed with bowel cancer at any age are advised to screen with biennial iFOBTs from age 35 to 44 and colonoscopy every five years from age 45 to 74. Patients with suspected Lynch syndrome should be managed as recommended in the High-risk familial syndromes chapter of Guidelines.
Detailed recommendations for surveillance by colonoscopy in those with past bowel cancer or adenoma are set out in the Practice Guidelines for Surveillance Colonoscopy.
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