Sharpening a medical debate about the costs and benefits of cancer screening, a new report estimates that the U.S. spends $4 billion a year on unnecessary medical costs due to mammograms that generate false alarms, and on treatment of certain breast tumors unlikely to cause problems. The study published Monday in the journal Health Affairs breaks the cost down as follows: $2.8 billion resulting from false-positive mammograms and another $1.2 billion attributed to breast cancer overdiagnosis. That's the treatment of tumors that grow slowly or not at all, and are unlikely to develop into life-threatening disease during a woman's lifetime.