Widely tunable near-infrared Raman fiber laser irradiating breast cancer cells leads to cell size reduction and increased granularity of intracellular components as a precursor to cell death

Photodynamic therapy and radiotherapy have traditionally relied on fixed wavelength sources (typically 1064 nm Nd:YAG laser), due to their ready availability. But with advances in development of wide wavelength spanning Raman fiber lasers, a greater flexibility in choice of the light source is provided. In a previous work by our group, we demonstrated the first use of a widely tunable Raman fiber laser (from 1-1.5 um) in achieving varying degrees of cytotoxicity with wavelength at fixed power and exposure for an epithelial breast cancer cell line. However, the reasons for this variation in cytotoxicity was unknown. In this work, we analyse the causes of this variation of cytotoxicity with wavelength for the same epithelial breast cancer cells irradiated in-vitro. Cell-viability is performed through a Propidium-Iodide based assay analysed through a flow cytometer. Thereafter, with the implementation of a novel-image processing algorithm on the same cytometer plots, we obtain a surprising insight into the reasons for cell death for certain wavelengths of irradiation. Specifically, we hypothesise that the effect of these wavelengths is twofold: to desiccate the water content of the cells, leading to reduction in cell size; and to lead to increased granularity or denaturation in the intra-cellular components. We make a surprising observation that cell size reduction is not as correlated with cytotoxicity as increased granularity. This paves the way for a better and insightful deployment of tunable wavelength Raman lasers in photodynamic therapy with the choice of an optimum wavelength to achieve the greatest tumoricidal effect.