It arises in the glands lining the large bowel (colon), often within a previously benign polyp (or adenoma).
This is why it is called an adenocarcinoma. Whilst for most people with colon cancer this occurs as a new event, in 5% of cases it is inherited. Patients with a strong family history (one or more direct relatives diagnosed at a young age) should see their GP who may refer them to a specialist genetic service.
Symptoms that should alert you to see your GP include bleeding from the back passage, a change in bowel habits towards a looser stool, a lump or pain in your tummy, an inability to completely empty your rectum when opening your bowels, weight loss, pain in your rectum, and a low blood level (anaemia) which can lead to tiredness or breathlessness. Cancer of the large bowel can result in a blockages (bowel obstruction) resulting in griping abdominal pains, bloating and constipation.
Who is at increased risk?
Medical conditions that increase the risk of colon cancer include having ulcerative colitis or Crohn's disease for many years. Diabetes and obesity may also increase your risk but we do not yet understand exactly why. Finally, having had a previous colon cancer, cancers of the womb, testicle or lymphoma place you at higher risk.