SOME PEOPLE WHO FINISH TREATMENT for early-stage breast cancer will go years without any signs of cancer, only to learn that the disease has returned and spread to other parts of the body. Some researchers are turning their attention to dormant cancer cells, which are not actively dividing but remain in the body after treatment, as a potential cause.

Recurrences may happen when these cells wake up years after treatment, according to Angela DeMichele, a breast medical oncologist at Penn Medicine’s Abramson Cancer Center in Philadelphia. “Going back all the way to the ’70s, even late ’60s, there were theories that there must be a dormant phase where the cells are sleeping, not dividing, but they’re still alive,” DeMichele says.

A phase II clinical trial published Sept. 2, 2025, in Nature Medicine suggests that detecting these dormant cells—and treating them—could keep cancer from coming back. The study involved 51 women who had completed breast cancer treatment but had dormant cancer cells in their bone marrow samples. The women received hydroxychloroquine, which blocks dormant cancer cells’ ability to use their own energy sources; everolimus, which blocks a protein that signals cells to grow and divide; or a combination of both drugs.

Three months after receiving the treatments, more than 80% of participants in each group had no dormant cells detected. In addition, those patients had no evidence of cancer seven years later.

Two ongoing clinical trials are pairing these drugs with other cancer treatments to test patient safety and the drugs’ effectiveness for eliminating dormant cells. The researchers are also developing a new flow cytometry-based test for detecting dormant cells in bone marrow samples. They hope the new test will be more sensitive and provide faster results than the current tests.

Physician-scientist Lewis Chodosh from Penn Medicine’s Abramson Cancer Center says the current process is time consuming and may miss dormant cancer cells. Chodosh, who, with DeMichele, was a lead author on the study, says more sensitive approaches to detect dormant cancer cells could deliver value by giving people reliable answers about their recurrence risk, especially if doctors can offer effective treatments for people with a high risk of recurrence.

“Something that patients are yearning for is some greater degree of certainty. Am I cured, or likely to be cured, or not?” Chodosh says.