## 1 Introduction  
Breast cancer is one of the most common causes of death worldwide. Screening and recent advances in adjuvant therapy for primary breast cancer have reduced its mortality rate by approximately 10-16% in Japan [1]. As data suggest that one in fourteen Japanese women will develop breast cancer in their lifetimes, screening, diagnosis, and adjuvant therapy are essential strategies. X-ray mammography is the current gold standard screening modality, and ultrasonography is considered an adjunct therapy to mammography in premenopausal women [2]. Diffuse optical spectroscopic imaging (DOSI) is an emerging modality with potential applications in oncology. DOSI uses near-infrared light, in the 600-1000 nm wavelength range, to measure the concentrations of oxy-hemoglobin (O\({}_{2}\)Hb), deoxy-hemoglobin (HHb), water (water), lipid (lipid), and oxygen saturation (SO\({}_{2}\)) in breast tissue [3]. DOSI is a non-invasive, non-ionizing, cost-effective modality that also provides functional quantification of tumor angiogenesis, hypoxia, edema, and adipose tissue [4; 5; 6].  
However, DOSI in breast cancer surveillance is not expected to provide precise anatomical information, but only to contribute towards providing evidence of tumor physiological activity. This makes DOSI an adjunct tool to conventional modalities for the early detection of breast cancer.  
Neoadjuvant chemotherapy (NAC) before surgery is the standard procedure for patients with advanced breast cancer [7]. Clinical studies have shown that disease-free survival after NAC is equal to that after adjuvant chemotherapy, and the advantage of NAC is that a greater number of patients undergo breast-conserving surgery after tumor shrinkage. Patients with pathological complete response (pCR) after NAC experience longer survival compared to those with non-pCR, and prediction of pCR before NAC or early during chemotherapy can improve patient outcomes [8]. Additionally, as more effective therapies in patients with low response to chemotherapy are needed, functional imaging technologies such as positron emission tomography (PET) and magnetic resonance imaging (MRI) play a definite role in determining whether to continue, change, or abandon treatment [9]. DOSI has substantial advantages during longitudinal monitoring as it captures physiological changes in breast tumors, especially in the vasculature and the microenvironment.  
Many optical imaging methods for diagnosis and treatment monitoring have been reported [10; 11; 12; 13], and we have established a two-dimensional optical imaging system with time-resolved spectroscopy (TRS) for breast cancer surveillance, as previously reported [14]. TRS is a technique that measures the time of flight, in addition to the light intensity, at the boundary of a medium. This method uses a short light pulse as a light source, and measures the time point spread function of the light passing through the tissue. Although TRS has the disadvantages of being complicated, expensive and having a long measuring time, the short pulsed light used in the time domain contains all the frequency components, and the measurement is stabilized compared to the measurement using the limited frequency component of the frequency domain. In this report, we present findings obtained from three clinical studies and discuss the usefulness and limitations of TRS in breast cancer treatment.