Exploration of melanoma metastases in mice brains using endogenous contrast photoacoustic imaging.

Photoacoustic imaging (PAI) provides real time non-invasive and contrast agent free monitoring of some endogenous compounds concentrations that provides improved insights into tissue vascularization and oxygenation which are particularly important during tumor progression. This study assessed the input of PAI for examination of melanoma brain metastases in an orthotopic mouse model and further focused on spatial analyses within the tumor tissue. Hemoglobin content appeared to be higher in tumors than in healthy brains. Spatial analyses further showed that angiogenesis was mainly at the tumor periphery. Concomitantly, while healthy brains were highly oxygenated, the tumors were hypoxic and subjected to a gradient of hypoxia from the periphery to the core. In tumor-bearing brains, spectroscopic PAI clearly revealed the presence of melanin, generating a signal 3 times higher than the background signal in healthy brains. When inserted into tissue mimicking phantoms, the photoacoustic signal of B16F10 melanin-containing cells was linearly correlated to their concentration and the detection limit was 625 cells. In vivo biological characterization of tumor models by non-invasive imaging of vasculature and tissue hypoxia represents an interesting opportunity for better understanding cancer progression; it is opening new research prospects to improve diagnostic, therapy, and early assessment of tumor treatment efficacy.