Colorectal Cancer
What is Colorectal Cancer & How is it treated?

Overview
The colon, is about one and a half metres long and connects the small intestine to the rectum and anus.
Its main role is to store faecal matter and transport it to the rectum, from which it is expelled.
However the colon also absorbs water, short chain fatty acids and electrolytes such as sodium, potassium and chloride. The rectum's role is to store faeces.
Food is transported through the colon by successive wave-like muscular contractions in a process called peristalsis.
This occurs due to the presence of two different layers of muscle that are present; an inner circular layer and an outer longitudinal layer. These are antagonistic (when one contracts, the other relaxes).
When the circular layer of muscle contracts, the lumen (inner cavity or space) becomes narrower and longer. This pushes the food forward. When the longitudinal layer contracts, the lumen becomes wider and shorter to allow food to enter.
Colon Cancer affects 5% of the population. From the age of 40, the risk doubles each decade. The average age of diagnosis is 67 years. 2-6% of cases present with multiple foci of colon cancer. The incidence in western countries is high.

Symptoms and signs
Symptoms of colorectal cancer are often vague and have many potential causes. However any of the following symptoms may warrant further investigation to rule out colorectal cancer.
Symptoms such as fatigue and weakness have several potential causes. However it can be caused by anaemia due to colorectal cancer and depending on the presence of other risk factors, a physician may decide further investigation is appropriate.
Rectal bleeding may manifest as bright red blood or dark faeces with a distinctive abnormal odour. Rectal bleeding may not be apparent at all and may only be detected using a slide-test called a faecal occult blood test.
Other symptoms can include altered bowel habit, such as diarrhoea or constipation or increased frequency of bowel motion. A change in consistency or shape of the stools may also occur. Abdominal cramps may also be a sign of bowel cancer.
Cancer of the distal colon (lower left side) tends to cause long, thin faeces. This is because they tend to grow in an ring-shaped (annular) formation and thus narrow or constrict the colon. This may result in obstruction.
The proximal colon (right side) is wider and cancer does not grow in the same fashion, meaning that stool shape and consistency are not affected as in left-sided colon cancer.
Cancer can also grow in other patterns such as ulcerating lesions and plaques.
All forms of cancer eventually penetrate the bowel wall over the course of time and may appear as firm masses of the serosal surface.
Some tumours produce a substance called mucin which facilitates its spread and worsens the outcome.

Risk Factors
Diets high in animal fat and red meat, low in fibre increase the risk of colorectal cancer. The risk is also higher in those whose family members have developed colorectal cancer
Genetic syndromes such as Peutz-Jeghers syndrome, Lynch (HNPCC) syndrome, Gardner's and Turcot's syndrome all predispose to development of colon cancer.
Familial Adenomatous Polyposis (FAP) syndrome is a genetic syndrome in which there is an irregularity on the long arm of chromosome 5. 100% of those with the mutation develop cancer if left untreated. This syndrome can be detected at an early age in colonoscopy by visualising the development of polyps on the colon mucosa.
Other high risk groups include those who have previously had an abnormal growth (35-50% chance of developing more). Patients suffering from ulcerative colitis are also at a higher risk. Ulcerative colitis patients represent less than 1% of colon cancer patients, but their cancer may be very aggressive.
Factors which help to lower the risk include antioxidants (which neutralise free radicals), and medications such as hormone replace ment therapy. Certain non-steroidal anti-inflammatory drugs (NSAIDs) are also believed to lower the risk.

Screening

Screening for colorectal cancer may be performed in the following ways:

Digital rectal examination
For this examination the patient lies on his or her side. The physician inserts a finger into the anus and assesses the muscle tone of the sphincter.
Then the physician advances his or her finger and palpates the inside of the rectum.
The physician then withdraws the finger and checks the glove for blood by performing a faecal occult blood test. Up to 10% of colorectal cancer cases may be
detected this way.
This is a slide which changes colour when faeces containing blood are smeared onto it. This test is limited but may prove useful in order to decide whether further investigations are indicated.

Colonoscopy
This procedure involves advancing a long flexible instrument (endoscope) into the rectum and colon. The endoscope has a light and a camera at the end which transmits images of the lining of the colon onto a screen, allowing abnormalities to be detected. Tissue samples (biopsies) may be taken and certain polyps may be removed during the procedures.

Sigmoidoscopy
Sigmoidoscopy is a similar procedure to colonoscopy. However the physician looks directly into the sigmoidoscope because it does not transmit images onto a screen. Sigmoidoscopy is limited as an examination as it does not permit examination of the entire colon.

CT Colonography and X-ray with Barium Enema
CT colonography or 'virtual colonoscopy' is an alternative method of examining the lining of the colon and rectum by performing a scan of the colon and rectum. The images produced are manipulated or reformatted to allow examination of the lining of those organs.
This is a less invasive procedure than colonoscopy and patients rarely require sedation. However, unlike colonoscopy, biopsies may not be obtained.

X-rays with barium contrast
This is simply an x-ray in which the rectum and colon are filled with a substance called barium. This allows the lining of the colon to be directly visualised, facilitating diagnosis.

Blood Tests
Blood tests may be performed to check for anaemia (a low red blood cell count). Anaemia has many causes, however it may (in certain age groups) warrant further investigation.

Metastasis
If cancer has been diagnosed, certain tests such as MRI or CT scans may be performed to check for invasion or metastases.
The most common sites of metastasis include local lymph nodes, liver, lungs and bones.
There are several means by which cancer may spread, for example:
Haematogenous spread (through blood)
Lymphogenous (through lymphatic drainage)
Direct extension

Treatment
Treatment depends on the subtype of cancer, its location in the colon or rectum and the stage.
It always requires surgical resection of the tumour and its lymphatic drainage.
Resection of metastatic disease and chemotherapy may be added depending on the stage. Radiotherapy may be performed in cases of rectal cancer.
In treatment of colon cancer, a substance called carcinogen embryonic antigen (CEA) is monitored to follow the progression of the disease and efficacy of treatment.