Treatment-induced apoptosis of cancer cells is one goal of cancer therapy. Interestingly, more heat is generated by mitochondria during apoptosis, especially the uncoupled apoptotic state,1,2 compared to the resting state. In this case study, we explore these thermal effects by longitudinally measuring temperature variations in a breast lesion of a pathological complete responder during neoadjuvant chemotherapy (NAC). Diffuse Optical Spectroscopic Imaging (DOSI) was employed to derive absolute deep tissue temperature using subtle spectral features of the water peak at 975 nm.3 A significant temperature increase was observed in time windows during the anthracycline and cyclophosphamide (AC) regimen but not in the paclitaxel and bevacizumab regimen. Hemoglobin concentration changes generally did not follow temperature, suggesting the measured temperature increases were likely due to mitochondrial uncoupling rather than a direct vascular effect. A simultaneous increase of tissue oxygen saturation with temperature was observed, suggesting that oxidative stress also contributes to apoptosis. Although preliminary, this study indicates longitudinal DOSI tissue temperature monitoring provides information that can improve our understanding of the mechanisms of tissue response during NAC.