Label-Free Imaging of Melanoma with Confocal Photothermal Microscopy: Differentiation between Malignant and Benign Tissue

Malignant melanoma (MM) is one of the most common cancers worldwide. It has a favorable prognosis only if the affected area is removed at an early stage. MM reportedly causes the large majority of skin cancer deaths despite the fact that it accounts for <2% of skin cancer cases. The incidence of MM has been increasing for >30 years and one of its most ominous characteristics is its high propensity to produce distant metastases, because it can get disseminated throughout the body through lymphatic and hematogenous spread. For this reason, early detection and treatment of MM are crucial life-saving measures. Label-free confocal photothermal (CPT) microscopy was utilized for the first time to investigate malignancy in mouse skin cells. Laser diodes (LDs) with 405 nm or 488 nm wavelengths were used as pumps, and a 638 nm LD was used as a probe for the CPT microscope. A Grey Level Cooccurrence Matrix (GLCM) for texture analysis was applied to the CPT images. Nine GLCM parameters were calculated with definite definitions for the intracellular super-resolved CPT images, and the parameters Entropy, Contrast, and Variance were found to be most suited among the nine parameters to discriminate clearly between healthy cells and malignant cells when a 405 nm pump was used. Prominence, Variance, and Shade were most suited when a pump wavelength of 488 nm was used.