Half of colorectal cancers hide until screening finds them
Cancer of the colon or rectum can invade or spread while producing no warning at all. At least half of cases are asymptomatic until a routine check. That is why most countries screen average-risk adults on a schedule—and why blood in the stool or thinning stools still demand attention.
Colorectal cancer starts in the lining of the colon or rectum, the lower stretch of the large intestine, when cells begin dividing without restraint and can push into nearby tissue or seed elsewhere in the body. Its warning signs depend on where the tumour sits: worsening constipation, blood in the stool, stools that grow thinner, lost appetite and weight, and nausea. In people over 50, rectal bleeding or anaemia count as high-risk symptoms, while weight loss or a change in bowel habit mainly worries doctors when bleeding comes with it.
Genes matter less than many assume. Between 75 and 95% of cases arise in people with little or no inherited risk. The listed risk factors are older age, diets heavy in fat, sugar, alcohol, red and processed meat, obesity, smoking and inactivity, with lack of exercise tied to roughly one case in ten. Alcohol is the dietary factor with the strongest support, and more than one drink a day raises risk. Whole grains and calcium point the other way. Having two or more parents or siblings with the disease doubles or triples the odds, and that group makes up about a fifth of all cases.
Inside the cell, the usual first step is a mutation that turns up the Wnt signalling pathway. More damage has to follow, often in the TP53 gene whose protein normally orders faulty cells to die; TGF-beta is disabled in at least half of these tumours. Most deaths come from cancer that has spread.
A sample taken during sigmoidoscopy or colonoscopy confirms the diagnosis, and imaging then shows how far it has gone. Surgery can cure tumours still inside the bowel wall; widespread disease is usually managed for comfort. Because screening lowers deaths, most countries recommend it for average-risk adults.
Source: Colorectal cancer
