## 4 Discussion  
Our initial clinical studies conveyed important information on breast cancer imaging using TRS. The first was that the TRS imaging system is not useful for screening during early breast cancer as the overall detection rate of 62.7% is unacceptably low, with even lower rates for T stages 0 and 1. Nonetheless, it was useful for monitoring neoadjuvant therapy in patients with advanced breast cancer, because detection rates were acceptable among patients with T-stages 2 and 3 tumors. During neoadjuvant chemotherapy monitoring, pCR tumors showed a significantly greater decrease in tHb compared to non-pCR tumors. However, when we compared the diagnostic performance of TRS to FDG-PET/CT for predicting pCR during neoadjuvant chemotherapy, DOSI showed a  
Figure 6: (**A**) Tumor tHb change during neoadjuvant endocrine therapy; (**B**) Differential response of tumor tHb change between PET responders and non-responders. lower diagnostic performance than FDG-PET/CT. Interestingly, we found that the responsiveness of ER-positive tumors and ER-negative tumors was different in patients who received cytotoxic chemotherapy. Specifically, in patients with ER-negative tumors, TNBC was an excellent predictor of pCR based on the AUC; however, it was completely unacceptable among patients with ER-positive tumors. These findings imply that tumor hemodynamics depend on breast cancer's hormonal status. Since ER-negative tumors are characterized by higher metabolic and angiogenic activities, chemotherapy monitoring using TRS could be useful in patients with ER-negative tumors, especially among TNBC patients. Thus, attention to differential response of tHb stratified by breast cancer subtypes may be essential. TRS is feasible among patients with HR-positive tumors who receive endocrine therapy. Further, while letrozole therapy (neoadjuvant endocrine therapy using aromatase inhibitor) can distinguish between metabolic responders and non-responders based on serial FDG-PET, metabolic responders had significantly lower tHb than non-responders. A recent clinical trial (JFMC34-0601) among 107 patients treated with 25 mg/day exemestane for 16 weeks, followed by a further 8 weeks depending on the clinical response, reported that eight patients with progressive disease had markedly poorer disease-free survival and overall survival compared to patients with partial response or stable disease [34]. The discrepancy in clinical results between cytotoxic chemotherapy and endocrine therapy indicates that hemodynamic response is dependent on drug pharmacology. In our trial using the antiangiogenic drug bevacizumab, tumor SO\({}_{2}\) among non-responders, but not responders, decreased dramatically after bevacizumab administration. Thus, TRS could be useful for monitoring the dynamics of the hypoxic tumor microenvironment in breast tissue. These observations may help explain the negative impact of bevacizumab, wherein the drug fails to promote vascular remodeling and destroys the tumor microenvironment in some patients. In fact, published data demonstrate that circulating angiogenic biomarker levels, such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (FGF), and Transforming Growth Factor Beta (TGF\(\beta\)), markedly increase on days 3 and 4 after infusion of bevacizumab in non-responders, but not in responders [30]. Thus, our TRS study with bevacizumab has provided physiological insight into drug-induced hypoxia and cancer progression, and may be useful for in vivo biomarker imaging in the future for assessing the effects of antiangiogenic therapy. There are several limitations to the TRS technology. First, TRS can only be used for primary tumors and the surrounding normal tissue, not on axillary nodes. Further, we cannot precisely evaluate pCR status, including axillary node status. Optical absorption is limited to the reach of the photons, which corresponds to a few centimeters of depth from the skin.