Kate Middleton Is in Cancer Remission. It Doesn’t Always Mean the Illness Is Cured.

Princess Catherine, wife of Prince William, reported on Tuesday that her cancer was in remission. But what does it mean to be in remission from cancer?

Last March, she revealed that doctors had discovered her cancer unexpectedly when she had abdominal surgery. She has not revealed the type of cancer she has, nor how advanced it was when it was discovered.

But she did say she had chemotherapy, which she said had been completed in September. She told the British news agency PA Media that she had a port, a small device that is implanted under the skin and attached to a catheter that goes into a large vein. It allows medicines like chemotherapy drugs to be delivered directly to veins in the chest, avoiding needle sticks.

Catherine told PA Media that chemotherapy was “really tough.”

“It is a relief to now be in remission and I remain focused on recovery,” she wrote on Instagram.

Her announcement “certainly is good news and is reassuring,” said Dr. Kimmie Ng, associate chief of the division of gastrointestinal oncology at the Dana-Farber Cancer Institute in Boston.

But cancer experts like Dr. Ng say that the meaning of remission in a patient can vary.

In general, when doctors and patients talk about remission, they mean there is no evidence of cancer in blood tests or scans.

The problem is that a complete remission does not mean the cancer is gone. Even when a cancer is “cured” — defined as no evidence of cancer for five years — it may not be vanquished.

That makes life emotionally difficult for patients, who have to have frequent visits with oncologists for physical exams, blood tests and imaging.

“It’s really scary,” Dr. Ng said. “The amount of uncertainty is very very hard,” she added.

But that ongoing surveillance is necessary, despite the toll it takes on patients.

“Different cancers have different propensities of returning or not returning,” said Dr. Elena Ratner, a gynecologic oncologist at the Yale Cancer Center.

As many as 75 to 80 percent of ovarian cancers, she noted, can come back in an average of 14 to 16 months after a remission, depending on the stage the cancer had reached when it was found and on the cancer’s biology.

“Once the cancer returns, it becomes a chronic disease,” Dr. Ratner said. She tells her patients: “You will live with this cancer. You will be on and off chemotherapy for the rest of your life.”

Dr. Ratner’s gynecological cancer patients have to come back every three months for CT scans to keep an eye out for evidence that the cancer has returned.

“The women live CT scan to CT scan,” she said. “They say that for two and a half months, they have a wonderful life, but then, in time for the next CT scan, the fear returns.”

“It costs them — it costs them a lot,” she said.

“It’s awful, yet I am amazed every day by their strength,” she said of her patients.