Shannen Doherty's cancer: What does it mean to be in remission?

Actress Shannen Doherty announced in an Instagram post on Saturday (April 29) that her breast cancer is in remission. (Reuters)

Actress Shannen Doherty announced in an Instagram post on Saturday (April 29) that her breast cancer is in remission.

Doherty, who is now 46, was diagnosed with breast cancer in August 2015 and has publically documented her treatments, according to Today .

But what does it mean for cancer to go into remission?

A "complete remission" means that doctors see no evidence of cancer when they examine a patient and take scans, said Dr. Otis Brawley, the chief medical officer for the American Cancer Society.

A cancer patient is said to be in remission if, after treatment, the doctors look at the area where there was once cancer and can no longer see the cancer, Brawley told Live Science.

But when a patient is in complete remission, it doesn't mean that he or she is entirely in the clear.

Cancer patients who are in remission need periodic exams so that doctors can look to see whether the cancer has returned, an occurrence that doctors call a relapse, Brawley said. When a patient first enters remission, these "surveillance" exams are conducted every few months, but they usually become more spaced out the longer the patient goes without a relapse.

It's possible for patients to be in complete remission for several years only to have a doctor then detect cancer again in the body, Brawley said. In these cases, it's possible that the cancer was actually there all along, but was just too small for doctors to detect, he noted.

Because of this possibility, some doctors dislike using the phrase "complete remission," disagreeing with the certainty it implies. Instead, a doctor may say that a patient is "apparently in complete remission," Brawley said.

Some cancers are more likely to return than others, Brawley said. The likelihood of a relapse depends on the type of cancer and what stage it was in when it was discovered. For example, a person with stage 1 colon cancer (meaning that cancer hasn't spread beyond the colon) has a 5 percent chance of a relapse, Brawley said. But for a patient with stage 3 colon cancer (meaning that the cancer has spread to nearby lymph nodes ), the likelihood of a relapse is about 45 percent, he said.

Patients can also experience what doctors call "partial remission," Brawley said. That means that the cancer has decreased in size by 50 percent, he said. When a cancer goes into partial remission, the patient may have fewer symptoms of the disease and his or her quality of life may improve, Brawley said. But this doesn't necessarily mean that the patient will live longer than they would have if their cancer had not shrunk by this amount, he added.

Originally published on Live Science .