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Colorectal Cancer (Bowel Cancer)

Colorectal cancer, also known as bowel cancer, is a malignancy that develops in any portion of the colon or rectum. Depending on where the cancer originates from, it may also be referred to as colon or rectal cancer.

The colon and rectum are towards the end of the body’s gastrointestinal (GI) tract, which is located in the abdomen. The lower GI tract is divided into three separate areas: the small bowel, the large bowel and the anus. The small bowel receives food from the stomach and absorbs the nutrients from the food. It is comprised of three separate parts (the duodenum, jejunum and ileum). The food is then passed onto the large bowel, where water and salts are absorbed. The large bowel also consists of three parts (the caecum, colon and rectum). What is left over is turned into solid waste (faeces or stool), and is sent to the anus to be removed from the body.

Colorectal cancers develop in the large bowel. However, cancers of the small bowel and anus can rarely occur. For more information on these cancers, please refer to the Rare Cancers Australia Knowledgebase.

Colorectal cancer is more common in males, and are generally diagnosed in people over 50. However, anyone can develop this disease.

Types of Colorectal Cancer

Colorectal Cancers can be categorised based on the types of cells (such as cancerous or pre-cancerous), as well as the size, shape and type of cells affected.

Pre-Cancerous Colorectal Growths

Most bowel cancers start as benign growths – or polyps - on the wall or in the lining of the bowel. While polyps are usually harmless, they can become cancerous if they are of adenomatous origin. An adenoma (or adenomatous polyp) is a non-cancerous or benign tumour that can be considered a pre-cursor to cancer if it is not treated. 

Adenomatous polyps are often classified by their shape and size.

Tubular Adenomas

Tubular adenomas are the most common subtype of bowel polyp, and generally a small, tube or spiral shaped tumour. They generally form over many years, and may become cancerous if left untreated.

Villous Adenomas

Villous adenomas are generally larger and grow in a cauliflower shape with finger-like projections. These types of tumours are rare, and likely to become cancerous.

Tubulovillous Adenomas

Tubulovillous adenomas generally contain a mixture of tubular and villous adenoma growths. They vary in size and growth patterns, and may become cancerous if left untreated.

Hyperplastic Adenomas

Hyperplasia is defined as enlargement of an organ or structure due to excess cell production. These types of tumours are relatively common, and rarely become cancerous.

Inflammatory Adenomas

Inflammatory adenomas often occur in patients with an inflammatory bowel disease, such as Crohn’s disease or ulcerative colitis. These types of growths rarely become cancerous.

Cancerous Colorectal Growths

Cancerous colorectal growths are often categorised by the types of cells the cancer originates from.


Adenocarcinomas are the most common type of colorectal cancer. These tumours originally begin as a benign adenoma formed from glandular cells, before developing into a malignant adenocarcinoma. While adenocarcinomas can be aggressive, they can have a good prognosis if caught early. 

Rare forms of Colorectal Cancers

These forms of colorectal cancers are very rare:

  • Colorectal lymphoma (cancer originating from white blood cells in the lymphatic system). 
  • Colorectal squamous cell carcinomas (cancer arising from squamous cells lining the GI tract).
  • Colorectal neuroendocrine tumours (cancer arising from neuroendocrine cells).
  • Gastrointestinal Stromal Tumours


If colorectal cancer is detected, it will be staged and graded based on size, metastasis (whether the cancer has spread to other parts of the body) and how the cancer cells look under the microscope. Staging and grading helps your doctors determine the best treatment for you. 

Cancers can be staged using the TNM staging system: 

  • T (tumour) indicates the size and depth of the tumour. 
  • N (nodes) indicates whether the cancer has spread to nearby lymph nodes. 
  • M (metastasis) indicates whether the cancer has spread to other parts of the body. 

This system can also be used in combination with a numerical value, from stage 0-IV: 

  • Stage 0: this stage describes cancer cells in the place of origin (or ‘in situ’) that have not spread to nearby tissue. 
  • Stage I: cancer cells have begun to spread to nearby tissue. It is not deeply embedded into nearby tissue and had not spread to lymph nodes. This stage is also known as early-stage cancer.
  • Stage II: cancer cells have grown deeper into nearby tissue. Lymph nodes may or may not be affected. This is also known as localised cancer. 
  • Stage III: the cancer has become larger and has grown deeper into nearby tissue. Lymph nodes are generally affected at this stage. This is also known as localised cancer. 
  • Stage IV: the cancer has spread to other tissues and organs in the body. This is also known as advanced or metastatic cancer.

Cancers can also be graded based on the rate of growth and how likely they are to spread: 

  • Grade I: cancer cells present as slightly abnormal and are usually slow growing. This is also known as a low-grade tumour. 
  • Grade II: cancer cells present as abnormal and grow faster than grade-I tumours. This is also known as an intermediate-grade tumour.  
  • Grade III: cancer cells present as very abnormal and grow quickly. This is also known as a high-grade tumour.  

Once your tumour has been staged and graded, your doctor may recommend genetic testing, which analyses your tumour DNA and can help determine which treatment has the greatest chance of success. They will then discuss the most appropriate treatment option for you.  

Treatment for colon and rectal cancers are treated differently, and often depend on several factors, including location, stage of disease and overall health. 

Treatment options for colon cancer may include:

  • Surgery, potentially including:
    • Right or left hemicolectomy (removal of right or left side of the colon).
    • Sigmoid colectomy (removal of the sigmoid colon).
    • Total colectomy (removal of entire colon).
    • Proctocolectomy (removal of the colon and rectum).
  • Chemotherapy. 
  • Clinical trials.
  • Palliative care.

Treatment options for rectal cancer may include:

  • Surgery, potentially including:
    • High anterior resection (removal of the lower end of the colon and upper portion of the rectum).
    • Abdominoperineal resection or excision (APR or APE) (removal of the sigmoid colon, rectum and anus). 
    • Ultra-low anterior resection (removal of the lower end of the colon and all of the rectum).
    • Colonic J-pouch (an internal pouch is created from the lining of the bowel, which acts as the rectum). 
  • Chemotherapy.
  • Radiation therapy.
  • Clinical trials.
  • Palliative care.

For more information on the treatment options, please refer to the Rare Cancers Australia Treatment Options page.

Risk factors

While the cause of colorectal cancer remains unknown, the following factors may increase the likelihood of developing this disease:

  • Being over 50 years old.
  • Having colorectal polyps.
  • Having certain diseases, such as Crohn’s disease of ulcerative colitis.
  • Having a history of bowel, ovarian and/or endometrial cancer.
  • Being obese.
  • Having an unhealthy diet.
  • Excess alcohol consumption.
  • Having a history of smoking.
  • Genetic mutations.
  • Having a family history of bowel cancer.

Not everyone with these risk factors will develop the disease, and some people who have the disease may have none of these risk factors. See your general practitioner (GP) if you are concerned.

Early symptoms

In the early stages of colorectal cancer, the disease may be asymptomatic. As the cancer progresses, some of the following symptoms may appear:

  • Changes in bowel movements, potentially including:
    • Diarrhoea.
    • Constipation.
    • Feeling of incomplete bowel movement.
    • Thin bowel stools.
    • Blood in stools.
  • Rectal bleeding.
  • Abdominal pain, bloating and/or cramping.
  • Anal and/or rectal pain.
  • A lump in the anus or rectum.
  • Unexplained weight loss.
  • Unexplained fatigue.
  • Anaemia – potentially causing fatigue, weakness and/or weight loss.
  • Changes in urinary habits, such as:
    • Blood in urine.
    • Frequent urination, especially at night.
    • Changes in urine colour – becoming dark, rusty or brown colour.

Not everyone with the symptoms above will have cancer but see your general practitioner (GP) if you are concerned.


If your doctor suspects you have a colorectal cancer, they may order the following tests to confirm the diagnosis and refer you to a specialist for treatment. 

Physical examination 

Your doctor will collect your overall medical history, as well as your current symptoms. Following this, they will examine your body (more specifically, around the abdomen) to check for any abnormalities. They may also perform a digital rectal examination (DRE), which is an exam conducted by a urologist (a doctor specializing in issues pertaining to the kidneys, bladder, prostate, and male reproductive system). In this exam, the doctor will insert a finger (or ‘digit’) into your rectum to feel the anus. If it feels hard or is an odd shape, further testing may be required. 

Blood tests 

Blood tests are used to assess overall health and detect any abnormalities. Some of these tests may include: 

  • General blood test to assess overall health. 
  • Full blood count, which measure the levels of red blood cells, white blood cells and platelets. 
  • Liver function test.
  • Immunochemical faecal occult blood test (iFOBT).
  • Blood chemistry and/or blood hormone studies, which analyse the levels of certain hormones and other substances in the blood. 
  • CEA blood test.

Imaging tests 

The doctor will take images of your body using magnetic resonance imaging (MRI), a computed tomography scan (CT scan), CT colonography and/or positron emission tomography (PET scan), depending on where it is suspected the cancer is. The doctor may also look at other parts of the body and look for signs of metastasis.  


A colonoscopy is the main diagnostic test used for colorectal cancers. This procedure examines the lining of the entire large bowel, and can detect if any polyps or abnormal tissue are present. Your doctor will insert a long, flexible tube with a light attached (colonoscope) through your anus, rectum and colon while you are under an anaesthetic. Carbon dioxide or air may be pumped through the colonoscope for better visualisation of the bowel.

Flexible Sigmoidoscopy

A flexible sigmoidoscopy is similar to a colonoscopy; however, this procedure only examines the rectum and lower portion of the colon. The instrument passed through the anus is shorter, and called a sigmoidoscope. 


Once the location(s) of the cancer has been identified, the doctor will perform a biopsy to remove a section of tissue using a needle. The tissue sample will then be analysed for cancer cells. This can be done by a fine needle aspiration (FNA) or a core needle biopsy (CNB).  

Prognosis (Certain factors affect the prognosis and treatment options)

While it is not possible to predict the exact course of the disease, your doctor may be able to give you a general idea based on the rate and depth of tumour growth, susceptibility to treatment, age, overall fitness, and medical history. Generally, early-stage colorectal cancers have a better prognosis and survival rates. However, if the cancer is advanced and has spread, the prognosis may not be as good and there may be a higher risk of cancer recurrence. It is very important to discuss your individual circumstances with your doctor to better understand your prognosis.   


Some references are to overseas websites. There may be references to drugs and clinical trials that are not available here in Australia.