Response to neoadjuvant chemotherapy observed by diffuse optical spectroscopy (DOS). (a) DOS linescan of the tumor using the combined tissue optical index (TOI) shows a clear maximum in the region of the tumor (TOI = ctHHb × ctH2O/%lipid). (b) Changes in the TOI observed post-therapy. Time point 0 was taken just prior to treatment. Note that changes are observed in the TOI of the tumor (triangles) in as little as one day post-therapy. The dynamics of these early changes may be useful in assessing functional response to a given neoadjuvant chemotherapy regimen. Error bars represent the standard deviation of the measurement.