Catherine, Princess of Wales, has announced she has now completed a course of preventive chemotherapy.

The news comes nine months after the princess first revealed she was being treated for an unspecified form of cancer.

In the new video message released by Kensington Palace, Princess Catherine says she’s focused on doing what she can to stay “cancer-free”. She acknowledges her cancer journey is not over and the “path to recovery and healing is long”.

While we don’t know the details of the princess’s cancer or treatment, it raises some questions about how we declare someone fully clear of the disease. So what does being – and staying – “cancer-free” mean?

What’s the difference between being cancer-free and in remission?

Medically, “cancer-free” means two things. First, it means no cancer cells are able to be detected in a patient’s body using the available testing methods. Second, there is no cancer left in the patient.

These might sound basically the same. But this second aspect of “cancer-free” can be complicated, as it’s essentially impossible to be sure no cancer cells have survived a treatment.

It only takes a few surviving cells for the cancer to grow back. But these cells may not be detectable via testing, and can lie dormant for some time. The possibility of some cells still surviving means it is more accurate to say a patient is “in remission”, rather than “cancer-free”.

Remission means there is no detectable cancer left. Once a patient has been in remission for a certain period of time, they are often considered to be fully “cancer-free”.

Princess Catherine was not necessarily speaking in the strict medical sense. Nonetheless, she is clearly signalling a promising step in her recovery.

What happens during remission?

During remission, patients will usually undergo surveillance testing to make sure their cancer hasn’t returned. Detection tests can vary greatly depending on both the patient and their cancer type.

Many tests involve simply looking at different organs to see if there are cancer cells present, but at varying levels of complexity.

Some cancers can be detected with the naked eye, such as skin cancers. In other cases, technology is needed: colonoscopies for colorectal cancers, X-ray mammograms for breast cancers, or CT scans for lung cancers. There are also molecular tests, which test for the presence of cancer cells using protein or DNA from blood or tissue samples.

For most patients, testing will continue for years at regular intervals. Surveillance testing ensures any returning cancer is caught early, giving patients the best chance of successful treatment.

Remaining in remission for five years can be a huge milestone in a patient’s cancer journey. For most types of cancer, the chances of cancer returning drop significantly after five years of remission. After this point, surveillance testing may be performed less frequently, as the patients might be deemed to be at a lower risk of their cancer returning.

Measuring survival rates

Because it is very difficult to tell when a cancer is “cured”, clinicians may instead refer to a “five-year survival rate”. This measures how likely a cancer patient is to be alive five years after their diagnosis.

For example, data shows the five-year survival rate for bowel cancer among Australian women (of all ages) is around 70%. That means if you had 100 patients with bowel cancer, after five years you would expect 70 to still be alive and 30 to have succumbed to the disease.

These statistics can’t tell us much about individual cases. But comparing five-year survival rates between large groups of patients after different cancer treatments can help clinicians make the often complex decisions about how best to treat their patients.

The likelihood of cancer coming back, or recurring, is influenced by many factors which can vary over time. For instance, approximately 30% of people with lung cancer develop a recurrent disease, even after treatment. On the other hand, breast cancer recurrence within two years of the initial diagnosis is approximately 15%. Within five years it drops to 10%. After ten, it falls below 2%.

These are generalisations though – recurrence rates can vary greatly depending on things such as what kind of cancer the patient has, how advanced it is, and whether it has spread.

Staying cancer-free

Princess Catherine says her focus now is to “stay cancer-free”. What might this involve?

How a cancer develops and whether it recurs can be influenced by things we can’t control, such as age, ethnicity, gender, genetics and hormones.

However, there are sometimes environmental factors we can control. That includes things like exposure to UV radiation from the sun, or inhaling carcinogens like tobacco.

Lifestyle factors also play a role. Poor diet and nutrition, a lack of exercise and excessive alcohol consumption can all contribute to cancer development.

Research estimates more than half of all cancers could potentially be prevented through regular screening and maintaining a healthy lifestyle (not to mention preventing other chronic conditions such as heart disease and diabetes).

Recommendations to reduce cancer risk are the same for everyone, not just those who’ve had treatment like Princess Catherine. They include not smoking, eating a nutritious and balanced diet, exercising regularly, cutting down on alcohol and staying sun smart.

Amali Cooray, PhD Candidate in Genetic Engineering and Cancer, WEHI (Walter and Eliza Hall Institute of Medical Research) ; John (Eddie) La Marca, Senior Research Officer, Blood Cells and Blood Cancer, WEHI (Walter and Eliza Hall Institute of Medical Research) , and Sarah Diepstraten, Senior Research Officer, Blood Cells and Blood Cancer Division, WEHI (Walter and Eliza Hall Institute of Medical Research)

This article is republished from The Conversation under a Creative Commons license. Read the original article.