SINGAPORE - Catherine, Princess of Wales, revealed on March 22 that she is undergoing preventive chemotherapy following a cancer diagnosis. The cancer was discovered after the Princess, the former Kate Middleton, underwent abdominal surgery in January. Details of the cancer have not been disclosed, but the 42-year-old reportedly started treatment in late February. So what exactly is preventive chemotherapy and can chemotherapy really prevent cancer?
Dr Wong Seng Weng, medical director and consultant medical oncologist at The Cancer Centre, answers these questions and more.
Q: What is preventive chemotherapy?
A: It is chemotherapy that is administered to prevent a cancer relapse, often after surgery is done to remove the cancer. It is called adjuvant chemotherapy. This form of chemotherapy is given with the intention to permanently cure a cancer in the early stage, rather than to merely keep the cancer under control. It has to be administered before a relapse occurs.
A watch-and-wait approach to the handling of early stage cancer at significant risk of relapse is not workable because cancer is generally incurable at the point of relapse, and any attempt to treat it with chemotherapy then only stalls it to buy time for the patient.
There is only one bite of the cherry, and that is at the point of initial diagnosis, not at the point of relapse.
Q: If a cancer is diagnosed as being in the early stage, how can it relapse if it has been completely removed by surgery?
A: When doctors draw the conclusion that a cancer has not spread from one part of the body to another, the conclusion is limited by what current medical technology is able to pick up. In other words, the label “early stage” cancer is a qualified diagnosis and does not exclude cancer spread that is below the threshold of detection of today’s technology.
By and large, even the best of modern medical imaging techniques searching the patient’s body for evidence of cancer spread, such as the commonly used CT (computed tomography) scan and PET (positron emission tomography)-CT scan, can only confidently pick up an area of cancer spread of least 1cm in diameter.
A cancer growth measuring just 1cm contains a billion cancer cells, much like the population of a big country such as China or India. If a population of cancer cells, say, similar to the population of Singapore of about five million cells, has spread to a distant body site, no technology today can reliably pick that up. By the time cancer relapse is picked up on a CT scan, there would be billions of cancer cells all over the body and the window for a permanent cure would have closed irreversibly.
Q: How do doctors decide who among cancer patients should receive preventive chemotherapy?
A: Patients who do not have undetected cancer cells in their bodies following surgery do sometimes undergo chemotherapy to prevent the relapse of a cancer not destined to recur. They may genuinely not have traces of cancer cells in their bodies.
The decision on preventive chemotherapy is based on the calculated risk of relapse versus the risk of side effects. To put it bluntly, it is based on “best guess”. Of course, doctors do not base the decision on the toss of a coin but make use of past data on the risk of relapse in a similar situation aided by tools, such as cancer genetic analysis, to make an educated guess.
If the potential benefits of preventive chemotherapy in improving the probability of a cure outweigh the risks of side effects, treatment proceeds. Otherwise, chemotherapy would be omitted.
Q: Can preventive chemotherapy completely stop a cancer from relapsing?
A: Cancer patients with a minute number of cancer cells in the body can still potentially suffer a relapse, even after completing preventive chemotherapy, as some cancer cells can be resistant to treatment and chemotherapy may fail to mop them up.
Q: If a patient rejects the recommendation to undergo preventive chemotherapy, is he or she destined to relapse?
A: A patient who does not harbour residual cancer cells in the body, even though the risk of having such cells is estimated to be high, will not suffer a relapse if he or she skips preventive chemotherapy.
Hence, even with the help of preventive chemotherapy, the eventual outcome of cancer treatment still involves a roll of the dice. Fate still plays a part. What the doctor does with preventive chemotherapy is to, metaphorically, weigh the dice such that the outcome is more likely to be in the patient’s favour. A tragic fate can sometimes be cheated.
Q: If preventive chemotherapy is able to help one avoid cancer recurrence, does this mean that advanced cancer, too, can be permanently cured?
A: Unfortunately, no. In early stage cancer, when the residual number of cancer cells is small, the probability of the presence of a clone of “super” cancer cells powerful enough to resist the assault of chemotherapy is low. Total eradication is potentially doable.
In advanced cancer, treatment starts when the cancer cell population is large. Hence, the probability of a “super” clone is high. This “super” clone can be suppressed, but not eradicated.
Cancer cells in the body are not completely identical; there is an element of heterogeneity. As an analogy, the chance of finding a Nobel laureate in a country with a small population is low, and the probability of finding such an individual in a large country is way higher.
Q: Can preventive chemotherapy be given before the first occurrence of cancer to forestall this dreaded disease? After all, prevention is better than cure.
A: No. Cancer is not an inborn condition. The tale that we are all born with cancer cells in our bodies is a myth. However, we do harbour the tendency to form cancer cells, especially later in life with advancing age. The probability of this occurring may be higher in some people who have inherited defective genes that lead to the phenomenon of hereditary cancer syndromes plaguing their families.
Chemotherapy cannot be used to destroy a cancer that has yet to form. Cancer cannot be forestalled with chemotherapy. We cannot kill an unborn enemy.
