Abstract: Breast cancer is the most common malignant tumor in women, and its incidence is increasing year by year. Regional lymph node metastasis status is an important prognostic indicator of breast cancer. Sentinel lymph node biopsy (SLNB) has replaced axillary lymph node dissection (ALND) as the standard method to identify regional lymph node metastasis in early-stage breast cancer patients. The current guidelines recommend blue dye and nuclear methods as the standard sentinel lymph node (SLN) tracing methods. The ideal SLN tracer should be visualized, targeted and reproducible, supported by evidence-based medical evidence, easy to use and inexpensive with minimal adverse effects. Currently, the tracers can be broadly divided into visual tracers and contrast tracers, with the former including cyanine dyes such as methylene blue, carbon nanomaterials such as carbon nanosuspensions, and mitoxantrone for tracing; the latter including nucleophiles, fluorescent tracers such as indocyanine green, magnetic tracers such as superparamagnetic iron oxide, and ultrasound contrast agents such as sulfur hexafluoride microbubbles. All these tracers have certain limitations, such as lack of targeting of methylene blue, low fluorescence penetration of indocyanine green, and radioactive contamination of nucleophiles. For this reason, many studies have been conducted on the development and improvement of new tracers for SLN of breast cancer. Photoacoustic imaging (PAI) is a new imaging technique utilizing photoacoustic effect, in which biological endogenous chromophores or exogenous contrast agents absorb pulsed light to cause local thermoelastic expansion and subsequently radiate ultrasonic waves outward, forming a biological tissue light energy absorption difference distribution map and imaging. The PAI technique can be used to evaluate the morphology and function of labeled tissues through photoacoustic contrast agents and their coupling groups. In this paper, we reviewed the progress of PAI and its contrast agent application in SLNB, focusing on the small molecule NIR dyes, metallic nanomaterials and carbon nanomaterials. At the same time, we discussed the research of new targeted fluorescent tracer formed after combining small molecule near-infrared dyes with molecular antibodies as a common ligand. At last, we presented the future perspectives in this field.

Key words: Photoacoustic imaging, Sentinel lymph node biopsy, Photoacoustic contrast agent