Brawley said the new study, by showing which DCIS patients were at highest risk, would help enormously in defining who might benefit from treatment. It could not show that the high-risk women - young, black or with tumours with ominous molecular markers - were helped by treatment because there were too few of them, and pretty much every one of them was treated. But Brawley said he would like to see clinical trials that addressed that question, as well as whether the rest of the women with DCIS, 80 per cent of them, would be fine without treatment or with anti-estrogen drugs like tamoxifen or raloxifene that can reduce overall breast cancer risk.
But if DCIS is actually a risk factor for invasive cancer, rather than a precursor, it might be possible to help women reduce their risk, perhaps with hormonal or immunological therapies to change the breast environment, making it less hospitable to cancer cells, Esserman said.
"As we learn more, that gives us the courage to try something different," she said.
The stakes in this debate are high. Karuna Jaggar, executive director of Breast Cancer Action, an education and activist organization, said women tended not to appreciate the harms of overtreatment and often overestimated their risk of dying of cancer, making them react with terror.