Researchers at Karolinska Institutet have developed a method for assessing the effect of tamoxifen, a common drug to prevent the relapse of breast cancer. The key lies in monitoring changes in the proportion of dense tissue, which appears white on a mammogram, during treatment. Women who show a pronounced reduction in breast density during tamoxifen treatment have a fifty per cent reduction in breast cancer mortality. This tool provides doctors with the possibility to assess whether a patient is responding to tamoxifen at an early phase of treatment.