A normal cellular microenvironment can prevent cells with cancer-causing mutations from expanding into tumors. Some researchers have proposed a twist on this relationship, suggesting that carcinogens such as ionizing radiation induce cancer, at least in part, by altering a cell's milieu rather than by triggering genetic damage.

One of the few studies to test this hypothesis reports that radiation speeds the development of precancerous tissue into tumors in just this way (Nguyen et al. (Cancer Cell 2011;19:640–51).

Scientists implanted snippets of breast tissue lacking the tumor suppressor p53 into mice, some of which had been exposed to low doses of radiation. Within 300 days, all of the irradiated mice harbored tumors in the implanted tissues, but only about half of the nonirradiated animals did.

Radiation activated the cytokine TGF-β, which can inhibit tumors, but might also have encouraged growth by switching off antitumor genes.

The animals were prone to an aggressive form of breast cancer that lacks the estrogen receptor, suggesting that radiation's effects on the microenvironment also help determine what type of cancer results.

Additionally, the findings could explain why women who underwent radiation treatment as children are more likely to develop breast cancer, according to the researchers, led by Mary Helen Barcellos-Hoff of New York University School of Medicine.