BACKGROUND: In women undergoing breast conserving surgery (BCS), up to 60\% can require re-excision. Our objective is to develop an optically based technology which can differentiate benign from malignant breast tissues intraoperatively through differences in tissue composition factors. METHODS: A prospective study of optical imaging of BCS margins is being performed. Optical images are transformed into tissue composition maps with parameters of total hemoglobin concentration, b-carotene concentration and scattering. The predicted outcome is then compared to the margin-level pathology. RESULTS: Fifty-five margins from 48 patients have undergone assessment. Within 34 specimens with pathologically confirmed positive margins, the ratio map of b-carotene/scattering showed the most significant difference reflecting a decrease in adipose and an increase in cell density within malignant margins (p=.002). These differences were notable in both in-situ and invasive disease. CONCLUSIONS: We present a novel optical spectral imaging device that provides a rapid, non-destructive assay of the tissue composition of breast tumor margins. (C) 2009 Elsevier Inc. All rights reserved.