Women with high breast density have an increased risk of breast cancer. At the same time is mammography less effective in finding an existing cancer in these dense breasts. Mammography creates a 2D image of a 3D structure which has the potential to result in superimposing tissue, hiding lesions of interest. Tomosynthesis is an upcoming method for 3D screening with the potential of improving the effectiveness of finding breast cancer. The 3D view could resolve the problem with superimposing tissue. Unfortunately, tomosynthesis potentially has increased side effects of screening, such as increased radiation dose, increased recalls for women without cancer and an increased expense of the screening program. This project will analyse if tomosynthesis would be beneficial for some women of the population, theoretically those with dense breast and more superimposing tissue, while some women would benefit from continuing with mammography. It will be a comparison of the effectiveness of tomosynthesis and mammography according to breast density, utilising the material from the Malmö Breast Tomosynthesis Screening Trial.