New NHMRC guidelines put age and family history up front and centre in determining who should be screened for bowel cancer with colonoscopy and who needs iFOBT.
It has been known for some time that family history can influence the risk of developing bowel cancer, Australia’s second most common cause of cancer death. But it is also known that specific, identified genetic mutations causing conditions such as Lynch syndrome or familial adenomatous polyposis are rare, accounting for less than 5% of all bowel cancers diagnosed. At most, the researchers say, this only explains half of the reasons why family history is a risk factor for bowel cancer.
“The remainder of the observed increases in familial risk could be due in part to mutations in yet to be discovered colorectal cancer susceptibility genes, polygenic factors such as single-nucleotide polymorphisms, or dietary and other lifestyle factors shared by family members,” the guideline authors said in the Medical Journal of Australia.
Therefore, the researchers, led by Professor Mark Jenkins, director of the Centre for Epidemiology and Biostatistics, in the University of Melbourne’s School of Population and Global Health, analysed all the available cohort studies to determine the risk of developing colorectal cancer based on age and family history. They categorised cohorts into one of three levels of risk and this determined at what age screening would be worthwhile starting and which screening method was most appropriate. The screening guidelines exclude people with a known or suspected cancer-causing genetic syndrome, as these people require much more intensive screening and should be managed in a family cancer clinic.
The majority of Australians (90%) fall into the lowest risk category, category 1, which puts their risk at age 40 of developing colorectal cancer in the next 10 years at about 0.25% (one in 400). As with most other cancers age is a risk factor, so it is unsurprising that at age 50 the risk of developing this cancer has risen to 0.9%.
Screening for this category 1 group should be the two-yearly iFOBT test that is currently available via the National Bowel Screening program for adults between the ages of 50 and 74 years. Interestingly, people aged 75 and older still develop bowel cancer but there have been no studies to determine the cost-effectiveness or benefit vs risk analysis of screening in this age group which is why the program and the guideline recommendations stop at 74 years.
One of the differences in these new guidelines, a revision from the previous ones published back in 2005, is that people with a first degree relative who has had or has a bowel cancer at age 55 or older are still considered at average risk (category 1).
However, people with this history might consider starting the iFOBT screening at a younger age (45 years), the guideline authors suggest.
Category 2 includes people with a moderately increased risk of developing colorectal cancer, 3-6 times higher than average. This will mean having a first degree relative diagnosed with a bowel cancer before the age of 55 or having two first degree relatives who developed bowel cancer at any age (or one first degree and two second degree relatives).
Category 2 people are recommended to have iFOBT every two years for the decade between ages 40 and 50 and then switch to five yearly colonoscopies until the age of 75.
Finally, the high risk, category 3 is for all those patients without a genetic syndrome whose family history is even stronger than those people in category 2. Their risk is between 7-10 times higher than average.
This includes people with at least three first-degree relatives who have been diagnosed with colorectal cancer at any age or people who have multiple relatives with the cancer including at least one diagnosed before aged 55.
These high-risk people need to start screening earlier, with the guidelines recommending iFOBT every two years starting at age 35 and continuing for 10 years and then having a colonoscopy every five years between the ages of 45 and 75.
Of note is that the revised guidelines have deleted the reference in the previous guidelines to starting screening 10 years before the earliest age colorectal cancer was diagnosed in a first degree relative.
“There have been no studies conducted to determine the utility of beginning screening 10 years before the earliest diagnosis in the family, which was a recommendation in the 2005 guidelines and, therefore, it is not included in these guidelines,” they said.
The new guidelines aim not only to more strongly define risk based on the latest evidence, but also to determine the most appropriate screening method based on that risk, taking into consideration cost-effectiveness and rationalisation of available services, in particular, colonoscopies.
Jenkins MA, Ouakrim DA, Boussioutas A, Hopper JL, Ee HC, Emery JD, et al. Revised Australian national guidelines for colorectal cancer screening: family history. Med J Aust. 2018 Oct 29.
doi: 10.5694/mja18.00142. [epub ahead of print]