Reproductive death of cancer cells induced by femtosecond laser pulses.

PURPOSE: High intensity femtosecond (1 fs = 10(-15) s) laser pulses may, via multi-photon processes, cause reproductive cell death at wavelengths that otherwise are harmless. We study the efficacy of inducing reproductive death of cancer cells by ultraviolet (UV), visible (VIS) and near infrared (IR) femtosecond laser pulses.
MATERIALS AND METHODS: Human squamous carcinoma cervical cancer cells are irradiated by femtosecond laser pulses at 800 nanometers (nm), 400 nm, 266 nm and 200 nm. The reproductive death is assessed by colony forming assay. The contribution from multi-photon processes is evaluated by comparing the cell reproduction subsequent to irradiation by collimated (low intensity) and focused (high intensity), pulsed laser beams with identical fluences.
RESULTS: Suitable femtosecond pulses are capable of arresting cell reproduction at all the tested wavelengths. Irradiation at 266 nm is far more efficient than the other wavelengths, both in terms of the fluence and the absorbed dose needed to induce reproductive cell death. The collimated 800 nm beam is unable to induce reproductive cell death even at a fluence of 230 Joule/square centimeters (J/cm2). However, focused 800 nm pulses with much higher intensities, but lower fluences efficiently arrest cell reproduction, thus highlighting the dramatic effect of multi-photon processes. At the intensities used in the present work focusing the 400 nm beam improves its efficacy by an order of magnitude, whereas focusing the 266 nm beam does not improve its efficacy.
CONCLUSION: Femtosecond pulses at 200, 266, 400 and 800 nm induce reproductive cell death if the intensity is sufficiently high. Multi-photon processes can improve the efficacy substantially and even result in reproductive cell death at wavelengths, where single-photon processes are harmless.