Irradiation of pigmented melanoma cells with high intensity pulsed radiation generates acoustic waves and kills cells.

Photokilling of pigmented mouse melanoma cells (B-16) was investigated using pulsed high intensity visible radiation. Melanin acts as an endogenous chromophore, and 694 nm radiation with 40 nsec pulse duration and 0.5-3 X 10(7)w/cm2 intensity causes cell death. Irradiation of non-pigmented human melanoma cells (U1) or human squamous carcinoma cells (FaDu) under similar conditions did not kill the cells. Also, irradiation of B-16 cells with 300 microsec laser pulses (10(3)W/cm2) or with continuous wave (CW) radiation (10(-3)W/cm2) did not kill the cells. These data indicate that pigmented cell killing is due to absorption of radiation by melanin and that the pulsewidth and intensity of radiation play important roles in cell killing. The generation of acoustic waves due to absorption of the pulsed radiation by pigmented cells and by isolated melanosomes was demonstrated at 532 and 625 nm and 8.5 nsec pulse duration (10(7)-10(8) W/cm2); the amplitudes of the acoustic signals were approximately 2.5-3.0-fold higher at 532 nm compared with 625 nm, and they increased with increasing fluence. In contrast, irradiation of U1 or FaDu cells with comparable fluences and intensities did not generate acoustic waves. A possible correlation between the generation of photoacoustic waves and pigment cell death is proposed. Since the thermal relaxation time of melanosomes is 0.5-1.0 microsec, the mechanism proposed is that thermal confinement of high intensity, short-pulse visible radiation generates acoustic waves by thermal expansion, leading to mechanical damage to the cells.