Princess Kate has been diagnosed with cancer and is undergoing "a course of preventive chemotherapy treatment," Kensington Palace announced Friday.

It’s unclear what type of cancer she has, but Kate said she underwent “major abdominal surgery” in January. Her medical team thought her condition was non-cancerous at the time, but additional tests after the operation revealed that cancer was present. 

“My medical team therefore advised that I should undergo a course of preventive chemotherapy and I am now in the early stages of that treatment,” Kate said. 

At face value, “preventive chemotherapy” sounds like chemotherapy treatment that can prevent the growth of cancer, which is technically accurate, but it’s more complex than that — and doesn’t necessarily apply to healthy people who are not considered high risk for developing the disease or don't have an existing cancer diagnosis.

Here’s what to know. 

What is 'preventive chemotherapy'?

Chemotherapy is a drug treatment that stops or slows the growth of cancer cells.

In Kate’s case, “preventive chemotherapy” — better known as adjuvant therapy — refers to follow-up treatment that doctors sometimes recommend to kill microscopic cancer cells that can’t be detected and may not have been destroyed during the first round of treatment, thus reducing the chances that a cancer comes back, according to Dr. Mikkael Sekeres, chief of hematology at the Sylvester Comprehensive Cancer Center, which is part of the University of Miami Health System.

Adjuvant therapy also works by removing signals in the body, such as hormones, that encourage a specific type of cancer to grow, Sekeres said. 

This type of follow-up treatment is most often used to improve quality and length of life in people with colon, breast and lung cancer, but it’s being studied to treat various other types as well, Sekeres said. 

How well it works depends on the type of cancer, the stage of the cancer and the general health status of the person who has it. 

Why the phrase 'preventive chemotherapy' can be confusing

“Preventive chemotherapy” can be a confusing phrase because chemotherapy currently isn’t used to prevent cancer in people who are not high risk or have not been diagnosed with and treated for the disease.

In a broader sense, there is such a thing as “preventive treatment” to lower the chances that a person develops cancer, which could include a range of lifestyle changes such as stopping smoking and limiting alcohol consumption, Sekeres said. 

Preventive treatment could also include more drastic steps like the surgical removal of one or both breasts called a prophylactic mastectomy to decrease the chances of developing breast cancer, for example. This procedure is recommended for people considered high risk for the disease.

What is chemoprevention? 

Chemoprevention is sometimes recommended for people who face a high risk for developing cancer in the future, including those with family cancer syndrome, a strong family history of cancer or a precancerous condition. 

Chemoprevention is the use of drugs or other natural or synthetic substances to help lower a person’s risk of developing cancer in the first place or keep it from recurring, according to the National Cancer Institute (NCI). It’s not used to treat existing cancer, like chemotherapy. 

Tamoxifen and raloxifene, for example, are drugs that have been shown to reduce the risk of breast cancer in high risk women; studies have found that finasteride, commonly used to treat hair loss in men, may lower the risk of prostate cancer, according to the NCI.

Aspirin, meanwhile, has been studied as chemoprevention for different cancer types, but results mostly show that it does not prevent the disease, the NCI says, although taking it for long periods may prevent colorectal cancer in some people.