Doc Talk: A simple, non-invasive blood test can now guide the choice of cancer treatment

A simple, non-invasive blood test can now guide the choice of cancer treatment

Over a casual lunch with an old friend, I was given a mini-dissertation on how our blood groups should dictate the lifestyles we adopt - from diet to exercise regimens.

"I can't eat this," my friend replied as she gently pushed away a plate of flat bread. "People of AB blood group should avoid wheat."

The lunch ended and I came away with a tome of advice on the foods I should eat and the type of physical training that will bring me greater benefits based on my blood group. Apparently, being of the O blood type, I should eat more meat.

It will take a lot more to convince me that a simple blood test carries so much information on what amounts to appropriate behaviour.

A simple blood test, however, can potentially give me vital information on the behaviour of cancers that afflict my patients.

Medical science has arrived at the era of liquid biopsies, which are blood tests to check for cancer.

A liquid biopsy allows oncologists to get hold of the secrets in a cancer's genetic code.

The behaviour of a cancer is guided by its genes. The genetic code is the "software" that controls all the functions of a cancer cell.

Any alteration of this code is the trigger to the fearsome power of cancer cells: the ability to multiple rapidly, the ability to spread through the patient's body as well as the ability to resist the process of ageing and dying.


The genetic code of the cancer also determines its vulnerability and its resistance pattern. In short, the genetic code determines what works and what does not work during the course of treatment.

Since the genes reside in cancer cells, traditionally, to get at these genes, we need to get hold of a bunch of cancer cells - a process known as a biopsy - to extract the genes for testing in the laboratory.

This process of cancer-cell collection is often invasive in nature, requiring either a surgical operation or the insertion of a fairly large needle into the body, guided by medical imaging devices.

Depending on the location of the cancer, such invasive biopsy often carries a significant risk of complications. Bleeding and serious injury to the organ being biopsied may occur.

During the course of cancer treatment, the genes of the tumour may undergo more mutations and evolution with time.

It is sometimes necessary to assess the cancer cell's genetic profile repeatedly to track these genetic changes so as to modify the treatment to maintain control over the cancer.

Even if the risk of complications from a single invasive biopsy is not big, the cumulative risk of repeated biopsies will eventually stack up.

The challenge to oncologists is, hence, how can one catch the tiger's cub without entering the tiger's lair ?

The answer, metaphorically, is written in blood. As it turns out, when some cancer cells die and break up, their genetic material is released and often seeps into the blood circulation.

The building block of these genes is DNA. These cancer genes that enter the blood circulation are termed "circulating tumour DNA".

Scientists have perfected the technique of extracting such circulating tumour DNA from the patients' blood and interrogating such genetic material to obtain vital "intelligence" on the cancer's behaviour. This is then used to guide selection of the treatment that will yield the highest probability of success.

The availability of this liquid biopsy gave one of my patients the chance to wriggle out of a dilemma.

Mr Tan, a man in his 50s who is being treated for advanced lung cancer, found that his cancer had developed resistance to the targeted anti-cancer drug, gefitinib, that he had been receiving for more than a year.

Quite rightly, his doctor advised him to go for a repeat lung biopsy to determine the genetic evolution of his cancer.

He was very reluctant to do so as the first lung biopsy he underwent more than a year ago was met with the complication of an air-leak from the biopsy site. That led to a period of prolonged hospitalisation.

Having heard his concerns, I advised him to go for a liquid biopsy.

In the past, if the patient refused to do a tissue biopsy, his doctor would have to rely on his instincts to find the right treatment.

Thankfully, the new cancer gene mutation was successfully identified from the circulating tumour DNA in his blood.

I switched his treatment to a newer generation targeted therapy - osimertinib - and brought his cancer back into remission.

A simple blood test had made the choice of treatment a superior scientific process rather than one made by a doctor's instincts.

It is why liquid biopsies are taking the cancer world by storm. There is no need to do an invasive tissue test to check for cancer. Furthermore, the quicker the detection, the earlier the treatment can start.

Nonetheless, the next time I visit my favorite steakhouse, it will still be guided by my instincts, rather than my blood test, as my dear friend suggested.

• Dr Wong Seng Weng is the medical director and consultant medical oncologist at The Cancer Centre.

A version of this article appeared in the print edition of The Straits Times on October 29, 2018, with the headline 'Catch a tiger's cub outside the den'. Subscribe