More patients are asking for screening for cancers as public knowledge about cancers increases. For instance, media reports about the growing rate of colorectal cancer make the public aware that one in twenty Singaporeans could contract colorectal cancer in their lifetime, especially if they have risk factors. The concept of screening means testing people with no symptoms.
It is different from undergoing tests when there are already symptoms that may suggest cancer. The latter is not ‘screening’, but diagnosis of symptoms.
To be effective, screening methods must detect the cancer at a stage when intervention is useful. This could be at the pre-malignant stage, or if a cancer is already present, at an early malignant stage in which a cure is still possible.
Screening tests must be safe, simple, sensitive (not missing cancer cases) and specific (correctly distinguishing normal people from those with disease.) Ideally, it must be shown by scientific studies that screening leads to greater survival benefit.
General Methods of Screening of Digestive Tract Cancers
The screening starts with a consultation during which a good history is obtained and a thorough clinical examination is performed on the patient. Following that, some of the following tests may be offered depending on the doctor’s assessment:
Blood tests, including cancer markers:
- Hints of organ dysfunction may lead to a diagnosis of cancer, for example, abnormal liver function tests may be due to bile duct obstruction by pancreatic cancer.
- Cancer markers are not that reliable as some cancers may not express their typical cancer markers. If the markers are very high, there is cause for worry. If normal, no firm conclusion can be made and cancer cannot be totally excluded.
Faecal occult blood test:
Preferably Faecal Immunochemical Test, which is sensitive only to human haemoglobin and is more reliable. However it depends on large polyps or tumours shedding blood into the stool. Smaller lesions may not do so and in any case bleeding may be an intermittent event leading to a missed diagnosis unless the test is done repetitively.
- Gastroscopy (for oesophagus, stomach and duodenum)
- Sigmoidoscopy (viewing only the left side of the colon),
- Colonoscopy (viewing the entire colon)
- ERCP (to examine the bile and pancreatic ducts)
- Endoscopic Ultrasound (to evaluate masses by ultrasound imaging through the wall of the digestive tract)
- Plain x-ray (limited use in screening of digestive cancers)
- Barium studies (largely obsolete)
- Computerized tomography (CT) scan including CT colography (3-D reconstruction of the colon) – useful in special circumstances
- Magnetic resonance imaging (MRI) scan – of special value for visualizing bile and pancreatic ducts
- Ultrasound examination – a good first line screening modality for the liver, gallbladder, bile ducts and pancreas
- Positron emission tomography (PET) scan – more for follow up of cancer treatment than for screening
Cancer of the Colon and Rectum
Screening has been determined to be appropriate for colorectal cancer (CRC). Adenomatous polyps (benign tumours of the colon and rectum) are known to be pre-malignant conditions which can be removed when found, thus preventing cancer from developing. Screening tests have been found to achieve accurate detection of polyps or early stage CRC. There is evidence that removal of adenomatous polyps reduces incidence of CRC. It has been shown that detection of early stage CRC reduces mortality.
It has been established recently by the American College of Gastroenterology that for CRC, the most reliable method of screening is by Colonoscopy, as all the other methods have methodological issues and variable sensitivity and specificity.
Those who should be screened are:
- People at Average Risk: Asymptomatic, age 50 and over, with no other risk factors; age 45 for patients with risk factors such as obesity or heavy smoking. If normal, screening can be done at 5-10 year intervals.
- People with close relatives who have had CRC or colon polyps at a young age (<60): Screening should start at age 40 or 10 years younger than the youngest relative so diagnosed.
- People with a family history of Familial Adenomatous Polyposis (FAP), a condition of florid polyp formation at a young age with a 100% chance of developing cancer: Genetic testing and genetic counseling are important. Gene carriers or indeterminate cases should be screened yearly from puberty.
- People with a family history of Hereditary Non Polyposis Colorectal Cancer (HNPCC): Those with a family history of CRC in multiple close relatives across generations, at young age, should be screened from age 20.
- People with a history of adenomatous polyps previously removed: Screening after 3 years and if normal then at 5 yearly intervals.
- People with a history of CRC after curative surgery: Colonoscopy at one year, and if normal then 3 years then 5 years.
- People with long standing Inflammatory Bowel Disease (IBD) affecting the colon, which increases the risk of CRC development.
Symptoms of Colorectal Cancer
As mentioned earlier, if a patient has symptoms and presents to a doctor for investigations, this is not called ‘screening’. It is investigation of symptoms to discover the cause.
Unfortunately, but the time CRC shows itself by symptoms, it usually represents more advanced disease, either a larger cancer or more widespread disease or both. CRC may present with one or more of the following symptoms:
- Visible bleeding in the stools
- Change in bowel habit
- Constipation or diarrhoea or alternating constipation and diarrhoea
- Obstruction of the colon leading to inability to pass motions, pain and bloatedness – this would be a medical emergency
- Tenesmus, which is painful desire to defaecate, but with little stool passed.
- Occult bleeding/anaemia discovered during health checks for other reasons.
- Rectal pain/rectal mass
- Weight loss
Patients who have any of these symptoms should not hesitate to seek medical advice at once, because even though it may be due to CRC at a more advanced stage, it is often still treatable, and the earlier it is found and treated, the better.
Gastric or Stomach Cancer
Stomach or gastric cancer is the 4th most common cause of cancer worldwide, but it is the 2nd most common cause of cancer death. It is more common in men and in developing countries. Whereas lower stomach cancer is becoming less common, upper stomach cancer is becoming more common. Lower gastric cancer predominates in developing countries and in lower socio-economic groups, whereas upper tumors are more common in developed countries.
Men are more prone to gastric cancer. Helicobacter pylori infection causes chronic gastritis and this is believed to be a direct cause of most distal gastric cancers. Risk factors include a high salt diet, preserved foods, smoking, blood group A, and a disease called Pernicious Anaemia. There is also a degree of genetic risk, including some hereditary polyp syndromes. Reflux oesophagitis is associated with upper gastric cancers, which are in continuity with lower oesophageal cancer.
Screening of the population is generally not done except in Japan which has the highest incidence of gastric cancer in the world. When patients come for screening colonoscopy for colon cancer (which is highly recommended at the age of 45-50 for average risk individuals), it makes sense to have gastroscopy at the same time to screen both the oesophagus and stomach.
Unfortunately early gastric cancer has no symptoms. If any patient has symptoms of gastritis such as upper abdominal pain related to food, investigation by gastroscopy may be helpful. Helicobacter pylori infection if found should be treated by antibiotics. Sometimes a change in the cells called intestinal metaplasia is found. This represents a higher risk of cancer and a surveillance programme of yearly gastroscopy and biopsy may be offered. If there is a suspicion of cancer, additional tests may include endoscopic ultrasound or computerized tomographic (CT) scan. Very early gastric cancers may be removable by endoscopic means without major surgery.
There are two types of oesophageal cancer, depending on the location along the length of the oesophagus, which correspond to different cell types. Most of the length of the oesophagus is lined by squamous epithelial cells. The risk of cancer in these cells is increased by smoking, excessive alcohol consumption and perhaps damage from long term intake of high temperature food and drink, or damage due to corrosives. Some conditions such as Tylosis, Achalasia or Scleroderma also increase the risk.
In the lower oesophagus, cancers called adenocarcinomas can occur in another type of cells called columnar cells. The greatest risk of cancer of the lower oesophagus is thought to arise from chronic reflux oesophagitis. The cells of the lower oesophagus can change to a type called Barrett’s Oesophagus which has the potential to turn cancerous. Squamous cell carcinomas are becoming less common, while adenocarcinomas are becoming more common.
Screening of the general population is not generally accepted as the yield is low. However, patients with symptoms suggestive of reflux oesophagitis, such as heartburn should have gastroscopy. If Barrett’s Oesophagus is discovered, surveillance by yearly gastroscopy and biopsy may be offered. Endoscopic ultrasound may be helpful in obtaining more information on suspicious lesions. CT scans are also helpful. If very early cancer is discovered, it may sometimes be removable by endoscopic means, without major surgery.
Primary cancer of the Liver
Primary liver cancer is also called hepatocellular carcinoma and it arises mainly from livers that are already chronically scarred by disease, conditions called chronic hepatitis and liver cirrhosis. The commonest cause in our region is hepatitis B infection. Other causes are chronic hepatitis C and alcohol. Less common causes are auto-immune liver diseases and toxins. The risk of liver cancer is greater in males, those over 45 years and those with a family history of primary liver cancer.
Screening for primary liver cancer in the community is not done as the yield is very low. However, surveillance is usually done for patients who are known to have chronic liver disease, especially those with liver cirrhosis, by measuring a cancer marker called Alpha-fetoprotein (AFP) and by ultrasound scans which can detect small tumours. Confirmatory tests include CT scan or magnetic resonance (MRI) scans. If early cancer is found, treatment by liver transplant removes both the cancer and the liver disease at one go.
Pancreatic cancer is increasing in frequency and occurs predominantly in men over the age of 65. It is associated with cigarette smoking and previous stomach resection. It can arise from a scarred pancreas from chronic pancreatitis, or from some types of precancerous tumours such as adenomas or cystic tumours related to the ducts of the pancreas.
Screening of the population for pancreatic cancer is not helpful. However patients often undergo routine health screenings which may include a cancer marker for pancreatic cancer, CA125, and if this is elevated, it will precipitate a search for pancreatic cancer by means of imaging, such as by CT scan or MRI. Most cancers arise from the head of the pancreas and present earlier as they tend to cause early obstruction of the common bile duct leading to jaundice. Cancers of the body and tail of the pancreas are more insidious and are often diagnosed late.