Although bowel cancer screening usually detects cancer at an earlier stage, new research indicates that when screening detects a late stage cancer patients may be more likely to survive than those whose late stage cancer is only diagnosed after developing symptoms.
“Cancer diagnosed through screening is likely to be at an earlier stage so easier to treat. But even if diagnosed at a later stage, our research shows diagnosis through screening seems to mean patients have a better chance of doing well.” – Lead author Dr Michael Gill
The study* published today (Wednesday) in the British Journal of Cancer follows earlier research from the same team that suggested patients with screen detected bowel cancers could have a better chance of beating the disease because screening generally picks up cancers at an earlier stage when treatment is more likely to be effective.
But the latest findings suggest that patients with later stage cancers also do better if their disease is picked up through screening suggesting that there may be other factors at play in addition to early diagnosis.
Scientists are now carrying out research to see if some tumours bleed less which may make them harder to spot through screening, of if later stage cancers diagnosed after a patient goes to a GP with symptoms may be more aggressive and more likely to spread than screening identified cancers.
Alternatively these results may reflect a phenomenon known as lead-time bias** where screening makes survival times look longer even if the person’s lifespan is not extended.
The national screening programme, called faecal occult blood testing (FOBT) looks for blood in stool samples allowing the disease to be detected before symptoms become apparent. It is offered to people aged 60-74 in England, Wales and Northern Ireland and 50-74 in Scotland all of whom are sent a stool testing kit every two years***.
Researchers from the Universities of Leeds and Durham**** worked with colleagues in hospitals across Newcastle and Tyneside to compare more than 300 screen-detected bowel cancers with almost 200 same stage interval cancers – these were cancers diagnosed in people who had a negative FOBT test but then developed symptoms in the two year gap before their next test.
The results showed no difference in survival rates or death rates for early stage bowel cancers whether people were diagnosed through screening or during the interval gap. But people with later stage cancers picked up through screening appeared to survive significantly longer than others with interval-diagnosed cancers.
The researchers said that lead-time bias will have had some influence but it is unclear whether this would fully explain them.
Lead study author Dr Michael Gill said: “Our research suggests that the way we diagnose some patients seems to make a difference to how well those patients do. Even taking into account differences in gender, location of tumour, underlying health and backgrounds, patients with later-stage bowel cancer were more likely to survive the disease if they were diagnosed through screening.
“Now we want to find out why. But, even without all the answers, we know that bowel cancer screening is beneficial and saves lives. Cancer diagnosed through screening is likely to be at an earlier stage so easier to treat. But even if diagnosed at a later stage, our research shows diagnosis through screening seems to mean patients have a better chance of doing well.”
Bowel cancer is the third most common cancer in the UK, with more than 41,600 people diagnosed each year, and just under 15,700 dying from the disease. When bowel cancer is found at the earliest stage, more than 90 per cent of people survive the disease for at least five years.
Jessica Kirby, Cancer Research UK’s senior health information manager, said: “We know that bowel screening saves lives, and this research explores more about why that might be – whether it’s just because screening picks up earlier stage cancers, or whether there’s also a difference in outcomes within each stage. But unfortunately we can’t yet tell how much of the survival difference was because of lead time bias, and how much was a real difference in clinical outcomes for screen-detected patients.
“Only around 58 per cent of people who are offered bowel screening in the UK complete their testing kits, and uptake is even lower in more deprived groups, so Cancer Research UK is working hard to help more people be aware of bowel screening and get around any barriers that might be stopping them from taking the test.
“Even if you have had a normal screening result, if you notice any unusual changes in your body – such as bleeding from the bottom or persistent changes in your bowel habits – you should see your doctor. It may well be something much less serious but, if it is cancer, getting it diagnosed at an early stage can really help.”
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* M.D.Gill et al; Screen-Detected Colorectal Cancers are Associated with an Improved Outcome Compared with Stage Matched Interval Cancers. British Journal of Cancer, 2014.
** Lead-time bias is a common problem when looking at survival times after screening. Screening can pick up cancers before symptoms develop, so it can make survival times look longer even if the date on which a person dies is not affected. This is why researchers use mortality rates – the number of people who died of that cancer, rather than how long they survived – to assess how effective screening might be.
***Stool-testing kits are sent every two years to men and women as follows: England – aged 60 to 74; Northern Ireland – aged 60 to 74; Scotland – aged 50 – 74; Wales – aged 60 to 74.
**** Northern Colorectal Cancer Audit Group, North East England; School of Medicine, Pharmacy and Health, University of Durham, Durham; Northern Region Endoscopy Group, North East England; Leeds Institute of Cancer and Pathology, University of Leeds; Royal Victoria Infirmary, Newcastle; South Tyneside Foundation Trust, South Tyneside; North Tyneside General Hospital, Northumbria Healthcare NHS Foundation Trust.