# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 1 Introduction
In a review article, [PERSON] [48] focused on the clinical studies performed with human patients using the prototypes developed until 2014 by different research groups and gave a comparative analysis of system configuration of different prototypes for breast cancer imaging, as well as their performance parameters. In another review article about the clinical application of PA imaging, [PERSON] et al. [49] summarized the recent advances of PA imaging for breast cancer based on the results of clinical studies of various research groups. [PERSON] et al. [50] presented a review article containing the state-of-the-art PA imaging techniques, highlighting the clinical applications of PA imaging techniques for different types of cancers. Summarizing various preclinical studies related to brain, breast, lung, prostate, thyroid, and skin cancers, [PERSON] et al. [51] described the preclinical applications of PA imaging for cancer detection using endogenous and exogenous contrast agents. [PERSON] et al. [34], presented various clinical studies performed using PA imaging. In some recent review articles [52; 53], the state of the art PA imaging systems, as well as the clinical studies for breast cancer diagnosis using PA imaging, were reported. Translation of any medical imaging system into clinical practice depends on its progressive development beginning with a proof of concept stage where the system is tested on numerical phantoms. Following successful proof of concept studies, the system will be tested using phantoms mimicking human tissue and in vivo animal studies. In the final stage, the system will be evaluated in carefully-designed clinical trials involving human patients. In each stage, based on the findings of the research studies, the system is modified and fine-tuned to accommodate the requirements of its ultimate usage in a clinical environment. Our review gives a complete picture of the evolution of photoacoustic (PA) imaging for breast cancer management based on the pre-clinical and clinical studies performed by several research groups. Specifically, for deep tissue applications, endogenous contrast provided by PA imaging systems is not sufficient to detect the malignancy. To see the potential of PA imaging for deep breast imaging applications, its performance has to be evaluated using exogenous contrast agents. Animal studies were performed using various contrast agents, such as dyes and nanoparticles for breast cancer detection. Our review presents the results of these animal studies usingcontrast agents for breast cancer detection and detection of sentinel lymph nodes (SLNs). In order to understand the potential of PA imaging using contrast agents, such as dyes and nanoparticles, in detecting SLNs, a summary of different studies reporting the utility of PA imaging with contrast agents for detecting SLNs is presented. This gives the readers a complete picture of the development of PA imaging systems for breast cancer management. The recent review articles [48; 52] only focused on clinical PA imaging studies which did not include phantom studies, animal studies using endogenous and exogenous contrast agents (dyes and nanoparticles) for breast cancer detection as well as SLN detection, ex vivo studies with human breast tissue samples. To understand the potential of PA imaging, its performance needs to be compared with the conventional imaging techniques for breast cancer detection. This review presents a detailed comparative analysis of PA imaging systems with the conventional imaging systems used for breast cancer detection based on the clinical results reported by different research groups. Spectroscopic PA imaging can be used to measure the concentration of different light-absorbing elements, like oxy-hemoglobin, de-oxy hemoglobin, melanin, lipid, etc., present in soft tissue. This will enable us to estimate the oxygen saturation, total hemoglobin, lipid content, etc., in order to differentiate between different breast pathologies (normal, hyperplasia, DCIS and IBC).