The phrase “hide and seek” likely brings to mind laughing children playing a harmless game. But it’s not fun and games when tumor cells hide. Tumor cells that have left the organ where they formed to hide elsewhere in the body can eventually emerge to produce metastatic disease. Killing these cells once they’ve hidden is extremely difficult. The ability of escaped tumor cells to tuck themselves away poses a life-long risk of cancer recurrence for women with breast cancer and for patients with most other cancers. “Very early during tumor progression, cancer cells leave the breast and [can] travel to the lymph nodes, bone marrow, liver, lungs, or brain,” said Cyrus Ghajar, Ph.D., of the Fred Hutchinson Cancer Research Center in Seattle. These disseminated cells can remain inactive (that is, dormant or quiescent) for months, years, or even decades. “In many cases, the original cancer itself is not the main problem—metastasis is,” said Nancy Boudreau, Ph.D., chief of the Tumor Metastasis Branch in NCI's Division of Cancer Biology. “And these dormant metastatic cells are a bit like a time bomb; you just don’t know when they’re going to re-emerge again and start to grow.” If cancer cells could be killed before they emerge from dormancy, the potential exists to stop metastatic tumors from ever forming. But most existing cancer treatments are thought to only target dividing cells, not dormant ones. In a study published January 21 in Nature Cell Biology, a research team led by Dr. Ghajar showed in mice that, by disrupting the relationship between breast cancer cells that had spread to the bone and the normal cells surrounding them, they could make the hidden cancer cells sensitive to treatment. This was true whether the cancer cells were dividing or dormant. “This is the first study to show that disseminated tumor cells, when they’re quiescent, can be sensitized to chemotherapy,” said Dr. Ghajar.