PURPOSE: In a pilot study, we introduce fast handheld Multi-Spectral Optoacoustic Tomography (MSOT) of the breast at 28 wavelengths, aiming to identify high-resolution optoacoustic (photoacoustic) patterns of breast cancer and non-cancerous breast tissue. EXPERIMENTAL DESIGN: We imaged 10 female patients aged 48-81 years with malignant non-specific breast cancer or invasive lobular carcinoma. Three healthy volunteers aged 31-36 years were also imaged. Fast-MSOT was based on unique single-frame-per-pulse (SFPP) image acquisition employed to improve the accuracy of spectral differentiation over using a small number of wavelengths. Breast tissue was illuminated at the 700 - 970 nm spectral range over 0.56 seconds total scan time. MSOT data were guided by ultrasonography and X-ray mammography or MRI. RESULTS: The extended spectral range allowed the computation of oxygenated hemoglobin (HBO2), deoxygenated hemoglobin (HB), total blood volume (TBV), lipid and water contributions, allowing first insights into in-vivo high-resolution breast tissue MSOT cancer patterns. TBV and Hb/HBO2 images resolved marked differences between cancer and control tissue, manifested as a vessel rich tumor periphery with highly heterogeneous spatial appearance compared to healthy tissue. We observe significant TBV variations between different tumors and between tumors over healthy tissues. Water and fat lipid layers appear disrupted in cancer vs. healthy tissue; however offer weaker contrast compared to TBV images. CONCLUSION: In contrast to optical methods, MSOT resolves physiological cancer features with high resolution and revealed patterns not offered by other radiological modalities. The new features relate to personalized and precision medicine potential.