OVERVIEW
Colon cancer is also sometimes referred to as colorectal cancer, that combines colon and rectal carcinoma, which begins in the rectum.

The colon (large intestine) and rectum (terminal portion of large intestine adjacent to the anal canal and anal opening ) occur in the lower part of the digestive tract.

The colon, or large intestine, is the part of the digestive tract that absorbs salt and water from solid wastes. The remaining waste then travels through the rectum and exits the body through the anus.

Colon cancer is the third most common cause of cancer associated death, and in 2017, but advances in screening, diagnosis, and treatment have led to improvements in survival in the last few years.

ETIOLOGY & RISK FACTORS OF COLON CANCER
There are many risk factors of colon cancer, some modifiable and others not.

Non-modifiable risk factors
• Older age group
• History of colonic polyps (benign growths that may turn cancerous if not removed)
• Family history of bowel cancer
• Previous history of colon cancer
• Previous radiation therapy to abdomen
• History of bowel disease eg Crohn’s disease and ulcerative colitis
• Being of European, Jewish or African origin
• Having inherited cancer syndromes such as familial adenomatous polyposis (FAP) and Lynch syndrome, also known as hereditary nonpolyposis colorectal cancer (HNPCC)

Modifiable risk factors
These are risk factors that can be controlled to reduce the risk of developing colon cancer and include
• Smoking
• Heavy alcohol consumption
• Being overweight or obese
• Sedentary lifestyle
• Type diabetes
• Diet rich in red meat and processed foods
• Low fiber diet and high in saturated and trans fat

TYPES OF COLON CANCER
Depending on the nature of cells in the colon that become cancerous, colonic cancer is classified into the following types

• Adenocarcinoma (most common >95%), arising from the cells lining the mucus secreting glands
• Lymphomas – arise from the lymphoid tissue
• Carcinoids – arise from the hormone secreting cells in the intestines
• Sarcoma – arise from the connective and muscle layer of the colon

SYMPTOMS OF COLON CANCER
In the early stages, the disease may be asymptomatic or the person may have a combination of the following symptoms
• Altered bowel habits i.e. constipation and/or diarrhea
• Stool discoloration
• Frank blood mixed with stool
• Altered stool shape, such as narrowed stool
• Rectal bleeding
• Swelling and discomfort in lower left abdomen
• Bloating sensation
• Cramping abdominal pain

With advancing disease, there may be
• Constant fatigue and tiredness
• Unexplained weakness
• Generalized weakness
• Changes in stool consistency and type
• Sensation of incomplete bowel evacuation
• Vomiting

If the cancer has spread to distant organs, there may be
• Jaundice (yellow eyes and skin)
• Cough and shortness of breath
• Chronic headache
• Blurred vision
• Bone pain and pathological fracture
• Swelling of hands and feet

DIAGNOSIS OF COLON CANCER
When a person presents with any of the above complaints, a detailed history including family history, diet and bowel history will be taken followed by a thorough clinical examination including a digital rectal examination

If there is a suspicion of malignancy, tests will be done to confirm or exclude the diagnosis

Blood tests – There is no diagnostic test for colon cancer but a full blood count, liver function tests and renal function tests may indicate general health status.

Carcinoembryonic antigen – Levels of a chemical sometimes produced by colon cancers (carcinoembryonic antigen, or CEA), a tumor marker may be measured initially and monitored over time to check response to treatment

Colonoscopy – During a colonoscopy a long flexible tube with a small camera at the end is introduced into the anal opening and guided inside gently to look for abnormalities within the lower part of the large intestine and rectum. Any suspicious looking area or areas can be biopsied and sent for histopathological examination. Polyps can also be removed during colonoscopy using special tools (endoscopic mucosal resection)

Staging imaging scans – Staging investigations such as CT scans of chest abdomen and pelvis or whole body PET (positron emission tomography) scan will be done to look for extent of disease and spread. The stage of disease will help decide mode of treatment. The earliest stage is stage 1 when cancer is localized to the colon and stage 4 is the most advanced stage when it has spread to distant organs

TREATMENT OF COLON CANCER
Treatment will depend on the type of cancer, stage of disease and importantly, the age, general health condition and other concomitant diseases the patient may have
Aim of treatment is to remove the tumor and give symptomatic relief to patient such as pain control etc
The most common options for treatment of colon cancer are surgery, chemotherapy, and radiation therapy.

Surgery
Precancerous polyps that are difficult to remove by colonoscopy can be removed by minimally invasive laparoscopic surgery.

The part of the colon containing the cancer and the adjacent areas will be removed. This procedure is called a colectomy. The nearby lymph nodes are also usually removed. If the length of the remaining colon is sufficient, the remaining normal colon will be reattached to the rectum to permit passage of stool or attached to an opening (stoma) made in the abdominal wall. The fecal waste will pass into a bag, overcoming the need for the terminal portion of the colon. This procedure is termed a colostomy.

Small growths and polyps will also be removed during the surgery

Chemotherapy
Chemotherapy employs drugs to kill the cancer cells and is usually given after surgery to destroy any residual cancer and prevent recurrence. Chemotherapy can also be give prior to operation to shrink bulky tumor to enable easier removal.

Radiation
Radiation therapy employs powerful energy sources, such as x-rays and protons, to kill cancer cells. It might be used to shrink a large cancer before an operation or to relieve symptoms in large inoperable tumors. A combination of radiotherapy and chemotherapy is also an option

Palliative therapy
In advanced untreatable cancers, palliative treatment in the form of pain control and relieving bowel obstruction due to large inoperable tumor mass may be offered to the patient

Newer treatment options for advanced colon cancer include immunotherapy, and targeted therapy

COLON CANCER PREVENTION
• Eat a balanced diet rich in whole grains, fresh fruits and green vegetables
• Avoid constipation
• Maintain healthy weight and engage in regular exercise
• Avoid or reduce red meat and saturated fat intake
• Quit smoking and alcohol consumption
• Regular screening after 50 years

PROGNOSIS OF COLON CANCER
• The best outlook is if the cancer is diagnosed and treated early 92% chance of five year survival. At stage 4, this drops to 11%
• Therefore, annual screening is recommended for all persons aged 50 to 75 years, especially since symptoms may not appear until the disease has advanced.
• Screening tests include fecal occult blood test, stool DNA testing, flexible sigmoidoscopy 5 yearly and CT colonography 5 yearly

KEY FACTS ON COLON CANCER
• Colon cancer involves the large intestine and starts as polyps in the intestinal wall.
• Symptoms may not appear until late and include rectal bleeding or change in bowel habit
• Diagnosis is best made by MRI scan
• Treatment involves a combination of surgery, chemotherapy and radiotherapy
• A high-fiber, low-fat diet is recommended and regular screening can detect early colon cancer