One of the main concerns about chronic viral hepatitis B and C is the development of cirrhosis and liver cancer. In fact, cirrhosis from any cause is a risk for liver cancer, but viral hepatitis is the main cause in Singapore.

Liver cancer can be suspected based on blood test. Alpha-foetoprotein (AFP) is a protein that may be produced in large quantities when liver cancer develops. Hence, it is known as a tumour marker. However, up to 30% of liver cancers do not have elevation of AFP. More importantly, AFP may not be elevated in early small liver cancers, i.e. those that have high potential for cure by surgery.

Therefore, it is vital to have a more sensitive way of diagnosing liver cancer, especially those potentially curable, early, small liver cancers. Hence the usefulness of imaging, i.e. processes that "look" at the liver to identify areas of liver cancer.

The main imaging process for liver cancer is ultrasound (US). US is a procedure that uses sound waves to study the liver. These sound waves are absolutely safe. There is no radiation at all. Its safety is attested by the fact that the same method is used in pregnancy to examine the foetus in the uterus. The US machine beams sound waves through the skin to the liver and these sound waves will bounce off the liver tissue and return to the machine to be analysed. Since liver cancer tissue is different from adjacent non-cancerous liver tissue, the sound waves reflected back to the machine are different. Areas of liver cancer will appear as defects on the ultrasound. These areas of defects are also known as space-occupying lesions (see figure).

US is used as the first line imaging method for liver cancer because it is absolutely safe. As it does not involve administration of any medication, everybody can undergo an US examination. However, it is not very specific. Conditions other than liver cancers can also cause appearance of a space-occupying lesion. Therefore another more specific and sensitive method of imaging known as computer tomography (CT) scan may be used to further study a space-occupying lesion detected on US.

CT scan, unlike US, involves radiation. It is actually a special form of X-ray examination, using computer technology. X-ray images of the body (and liver) are taken in thin "slices" and then reconstituted into cross-sectional views of the body using a computer programme. Also, unlike US, a liquid X-ray contrast needs to be taken orally before the CT scan. In addition, another special contrast will be injected into the vein during the scan examination. Hence individuals with allergy to x-ray contrast will not be able to undergo CT scan and individuals with asthma will need to take medication a few days before the CT scan to prevent the contrast precipitating an asthmatic attack. Although not totally risk-free, the CT scan is still considered as a very safe procedure with the advantages of being more accurate than US in determining the presence of liver cancer (see figure).

Besides detecting the presence of liver cancer, US and CT are also useful in ascertaining the extent of the liver cancer. This is important because the extent of cancer is an important factor that determines the final outcome of the cancer process, as well as the type of treatment available for the individual patient.

One important point to bear in mind is that imaging, be it US or CT, must always be considered together with the clinical features and other laboratory blood tests. Liver cancer cannot always be diagnosed merely on US or CT scan alone. Indeed, sometimes, in doubtful cases, we may require a repeat US or CT to be done a few months later to look for any increment in the size of the original lesion, the surest way of differentiating a benign (or harmless) from a malignant (or cancerous) lesion.