5.5.2.5　Monitoring circulating tumor cells

The high mortality rate of melanoma is attributed mainly to its high propensity for metastasis.The presence of circulating tumor cells has been suggested as an early predictor for metastatic development，motivating intensive studies for detection and isolation of circulating tumor cells in recent years.Based on photoacoustic sensing，in vivo photoacoustic flow cytometry excites selected blood vessels with short laser pulses，followed by timeresolved photoacoustic measurements by an ultrasonic transducer placed on the skin.Based on the difference in the corresponding photoacoustic signals，pure blood and blood containing melanoma cells can be distinguished.However，the detection voxel is large and the fractional photoacoustic signal changes caused by circulating tumor cells is small，which restricts the clinical application of in vivo photoacoustic flow cytometry.One solution is to tag circulating tumor cells with a magnetosensitive contrast agent and then magnetically concentrate them in the detection voxel，thus increasing the photoacoustic signal changes.However，the potential toxicity of such an exogenous contrast agent hinders its applications in humans.Thus further improvement of in vivo photoacoustic flow cytometry is most likely to focus on increasing the sensitivity of label-free photoacoustic tomography to circulating tumor cells.

In addition，applications of photoacoustic imaging in skin grafts and thermal burns have also attracted much attention.With the development of photoacoustic imaging technique，the application of photoacoustic imaging in dermatosis has expanded from simple diagnostics to theranostics.Recently，photoacoustic imaging has been utilized for melanoma resection.It is found that photoacoustic imaging can more accurately determine the edge of melanoma，which is beneficial for the complete resection of melanoma and greatly reduces the recurrence rate after surgery.In addition，dual-wavelength photoacoustic flow cytometry with nanosecond pulsed laser has been used to perform label-free imaging of melanoma cells in vivo.The imaging-guided nanosecond pulsed laser can generate thermal effect at the specific position to achieve specific killing of the melanoma cells，which is extremely important for monitoring and preventing early metastasis of melanoma.At present，the drawbacks of photoacoustic imaging in dermatosis，such as insufficient resolution and slow imaging speed，restrict the wide clinical applications.However，it is believed that photoacoustic imaging will be gradually improved and upgrade from simple diagnostics to theranostics.(https://www.daowen.com)