A new study about the genetics of breast cancer may help doctors decide how aggressively to treat a perplexing form of the disease that often never spreads beyond the milk ducts. It's called ductal carcinoma in situ, or DCIS. Sometimes it can spread and turn deadly, but often it can remain in place harmlessly for many years. Because of the uncertainty, doctors often do not know whether to attack it aggressively, with a mastectomy, or just wait and watch. A study in Wednesday's Journal of the American Medical Association says experts have linked two genetic mutations to DCIS which appear to increase the risk of invasive breast cancer. Researchers say the presence of the mutations could help doctors decide the most appropriate treatment.