Now, experts in Australia believe they may have uncovered one reason why. Scientists at the Garvan Institute of Medical Research found that certain breast-cancer cells can survive treatment by slowing themselves down dramatically instead of becoming fully dormant.

The findings, published in Nature Communications, could help explain why some cancers quietly return long after patients appear to have recovered.

The research focused on oestrogen receptor-positive breast cancer, the most common type of breast cancer worldwide and one particularly associated with late relapse.

Researchers discovered that some cancer cells essentially “hide” during treatment by dividing extremely slowly. As many cancer therapies are designed to target fast-growing cells, these slower cells can sometimes escape destruction and remain in the body for years.

Over time, they may form tiny secondary tumours known as micrometastases in organs such as the bones, liver or brain.

Although these microscopic tumours may remain undetectable for long periods, they can eventually grow large enough to trigger a dangerous relapse. The scientists say this may help explain why some patients experience recurrence even after years of clear scans and successful treatment.

The study identified a cellular signalling system known as the Rac1 pathway as one of the key mechanisms helping these slow-growing cells survive and spread.

Encouragingly, researchers found that blocking this pathway in laboratory studies helped shrink the cancer cells, raising hopes that future treatments could specifically target these hidden survivors.

Cancer researchers have increasingly focused on recurrence in recent years, particularly as survival rates continue improving and more patients live for decades after treatment.

According to organisations such as Breast Cancer Now and the American Cancer Society, recurrence risk varies widely depending on cancer type, stage and hormone receptor status. Oestrogen receptor-positive breast cancers are known for having a lower short-term relapse risk, but a longer “tail” of possible recurrence extending many years into survivorship.

This lingering uncertainty can also take an emotional toll on survivors, many of whom continue living with fears that the disease could return unexpectedly.

While the findings are still in the research stage and not yet ready for routine clinical use, scientists say understanding how these slow-growing cells survive could eventually lead to therapies designed specifically to prevent late relapse.