Kate Middleton says she's completed her chemotherapy treatment — and is 'doing what I can to stay cancer-free'

Catherine, Princess of Wales, says she has completed her chemotherapy after being diagnosed with an undisclosed form of cancer following abdominal surgery. "As the summer comes to an end, I cannot tell you what a relief it is to have finally completed my chemotherapy treatment," the royal says in a new YouTube video posted by the Prince and Princess of Wales account.

In the video — filmed last month in Norfolk, England, a Kensington Palace spokesperson tells Sky News — Kate, 42, is seen enjoying the countryside with husband Prince William and their three children, Prince George, Princess Charlotte and Prince Louis. She also shares her reflections on fighting cancer.

"The last nine months have been incredibly tough for us as a family," says Kate, who publicly announced her cancer diagnosis in March. "Life as you know it can change in an instant and we have had to find a way to navigate the stormy waters and road unknown.

"The cancer journey is complex, scary and unpredictable for everyone, especially those closest to you. With humility, it also brings you face to face with your own vulnerabilities in a way you have never considered before, and with that, a new perspective on everything. This time has above all reminded William and me to reflect and be grateful for the simple yet important things in life, which so many of us often take for granted. Of simply loving and being loved."

The princess went on to share some details about what her life will look like now that she's no longer undergoing treatment, which had largely kept her away from official royal duties.

"Doing what I can to stay cancer-free is now my focus," she says. "Although I have finished chemotherapy, my path to healing and full recovery is long and I must continue to take each day as it comes. I am however looking forward to being back at work and undertaking a few more public engagements in the coming months when I can.

"Despite all that has gone before, I enter this new phase of recovery with a renewed sense of hope and appreciation of life," she continues. "William and I are so grateful for the support we have received and have drawn great strength from all those who are helping us at this time. Everyone's kindness, empathy and compassion has been truly humbling. To all those who are continuing their own cancer journey — I remain with you, side by side, hand in hand. Out of darkness, can come light, so let that light shine bright."

This is the first health update since Kate shared in June that she was "not out of the woods yet" and would be in cancer treatment for a few more months. "I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days," the royal wrote in a June 14 Instagram post, shortly before attending the Trooping the Colour event that month. "On those bad days you feel weak, tired and you have to give in to your body resting. But on the good days, when you feel stronger, you want to make the most of feeling well."

"On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home," she added. "I am learning how to be patient, especially with uncertainty. Taking each day as it comes, listening to my body, and allowing myself to take this much needed time to heal."

In March, it was revealed that the princess had undergone major abdominal surgery in London, which was deemed successful. "However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment," she said in a video message.

Here's what to know about preventative chemotherapy.

What is preventative chemotherapy?

"Preventative chemotherapy isn't so much a technical term — it's a lay term — and it's more akin to adjuvant treatment, meaning 'additional,'" Dr. Ginger Gardner, a gynecological oncologist at Memorial Sloan Kettering Cancer Center and chair of the Foundation for Women's Cancer, tells Yahoo Life. "That generic term might be applied to a situation in which a tumor was removed and yet [doctors want to] provide some treatment coverage" to prevent the cancer from coming back. It could also be used to prevent recurrence after a tumor has been destroyed by radiation therapy.

When someone is prescribed preventative chemotherapy, this typically suggests that there was no visible cancer left behind after surgical removal or radiation treatment and no evidence that tumors had spread to other parts of the body — in other words, that the cancer was not metastatic.

However, even if a cancerous tumor has been completely removed or destroyed, cancer cells can break off from primary tumors and travel to other parts of the body, and cells too small for doctors to see may still linger. A course of preventative chemotherapy — which may come in the form of pills or an IV — might be deployed to "reduce the risk of any microscopic cells returning or delaying their opportunity to do so," Gardner says.

Chemotherapy isn't the only way to do this, but it is one of the most effective and commonly used. Other preventative treatments include hormone therapies that can cut off the fuel supply to tumors that are driven by hormones (such as certain types of breast cancer), radiation therapy, immunotherapy and targeted therapies, which block nonhormonal drivers of cancer.

What are the side effects of preventative chemotherapy?

Preventative chemotherapy is not necessarily given at a lower dose than a typical course used to treat existing cancer. So the potential side effects are the same, although they vary depending on the specific type of cancer the treatment targets and the medication used. In general, side effects can include:

• Nausea

• Fatigue

• Hair loss

• Nerve pain

• Diarrhea or constipation

• Low red blood cell count (also known as anemia)

• Brain fog (often referred to as "chemo brain")

• Skin changes

• Nail changes

• Mouth sores

How long do people have to be on preventative chemotherapy?

According to Mayo Clinic, treatment may last only a few weeks or continue as long as 10 years. However, adds Gardner, "cancer is not just one disease," meaning that the type, duration and dose of chemotherapy is carefully chosen based on the type of cancer, its stage and its biology, as well as what's best for the patient's quality of life.

Whether and how long someone undergoes preventative chemotherapy and how effective the therapy will be depends on the type of cancer they're being treated for. The treatment has been effective for preventing recurrences of both breast and colon cancer, for example. Kate said in March that she was "in the early stages" of treatment.

The effectiveness of chemotherapy when used this way also depends on the stage of the cancer, whether the disease is hormone-dependent and whether it has spread to any lymph nodes. In some cases and for certain cancers, doctors don't recommend preventative chemotherapy at all.

"If there's enough benefit of adjuvant therapy to retain durable remission, then it's worth some side effects," Gardner says. "But if the disease's biology means that someone is not going to benefit from this type of therapy, then it's not worth it and it's in that balance that we make decisions about adjuvant treatment."

In the Princess of Wales's case, "whatever this is, the fact that she took proactive measures about something that was supposedly benign and, from what she said, has achieved disease clearance and is taking proactive steps [to prevent recurrence] and being thoughtful about her children, that's important," Gardner says.