Breast cancer affects one out of eight women. Ultrasound (US) plays an important role in the diagnostic workflow, especially during the biopsy phase, in which tissue is extracted from the lesion for further analysis. The extension from 2D to 3D US acquisitions has multiple benefits including enhanced lesion localization and improved registration with MRI data. Current commercial 3D US systems lack the ability to preserve the breast's original shape. Robotic US scanners follow tailored trajectories and produce high quality volumes by accurate localization of 2D slices captured with a conventional linear probe. Current methods require a patient specific model to plan the scanning trajectory. In this study we investigate how to change the direction of the scanning trajectory based on US feedback, such that no patient specific model is required to perform a scan. In our method, the scanning trajectory is kept tangent to the breast based on confidence maps of the US images and an estimation of current radius of curvature of the surface. We evaluated our approach on a realistic breast phantom. The robot revolves around the breast without prior knowledge of its shape. In ten scans, the RMS error between the probe's scanning plane and the breast's surface normal is 12.6° out-of-plane, and 4.3° in-plane. A 3D US reconstruction shows the acquired data. This is a step forward to fully autonomous, high quality robotic US volume acquisitions.