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Through the measurement of the optical properties (absorption and scattering), diffuse optical spectroscopy allows one to estimate non-invasively the composition of biological tissues (water, lipid and collagen content) and functional blood parameters. Further, it provides information on the microscopic tissue structure. It can therefore be effectively used in vivo as an absolutely non-invasive diagnostic tool. The Department of Physics of the Politecnico di Milano has designed and built an optical mammograph that exploits diffused optics, operating with pulsed light at 7 wavelengths in the red and near infrared spectral range (635-1060 nm). The instrument was used in a clinical study on 200 subjects, in collaboration with the European Institute of Oncology: optically derived tissue composition and in particular collagen content in tissues proved to be effective both to discriminate between malignant and benign breast lesions, and to estimate the risk of breast cancer related to the density of breast tissue, which is recognized among the most important independent risk factors. Partly based on those results, “SOLUS - Smart optical and ultrasound diagnostics of breast cancer”, a European project in the H2020 Framework Program, is now working to improve the specificity of non-invasive breast cancer diagnosis by combining diffuse optics with ultrasound imaging.