## 4 Discussion
Interestingly, the cell mass in A2780 decreased quickly after exposure to blue light dose 208 mJ/cm\({}^{2}\)\(\times\) 1000 ms, whereas the cell mass in PC-3 increased. The increase of cell mass implies higher anabolic activity in the metastatic PC-3 cells, which can serve as a mechanism of survival in harmful conditions. As the number of PC-3 cells remains the same from the beginning of the experiment, cells were not undergoing cell death, as the results could suggest, but only suppress their proliferation activity. This finding supports a study from [PERSON] et al. which reports that UV light induces mitosis via the creation of protecting factors which may lead to avoiding apoptosis [46] and also [PERSON] at. el. indicate that changes in cell mass distribution may play a role in cell motility [47]. On the contrary, A2780 cells showed a significant decrease in cell mass even though the number of cells decreased only slightly. The disproportionality of these two parameters is evidence of cell death because the automatic algorithm of the MHM tracks also dead cells during the experiment as previously described in [PERSON] et al. [23]. This is following our observations where is evident that A2780 cells were killed during the first hours of the experiment (Supplementary Video S2). Since QPI has recently been used for detection and classification of cell death [16; 22; 23; 48], based on these results, we hypothesized that light dose 208 mJ/cm\({}^{2}\)\(\times\) 1000 ms induce different types of cell death in A2780 and G361 cell lines. Based on the time-lapse videos (Supplementary Videos S2 and S4) and real-time quantitative parameters shown in Figure 6, it is evident that light dose 208 mJ \(\times\) 1000 ms induces specific changes in cell dry mass, CDS, and density in A2780 and G361 cells. For the A2780 cells, all three parameters steeply decreased in the first six hours of the experiment and remained low. The data correspond with the videos, as it shows that the effect of the light dose on A2780 cellsis fast and cells are dead. The dead cells are still adherent to the surface with the disrupted plasma membrane and there is an absence of blebbing or releasing of apoptotic bodies. This may suggest the cells underwent necrosis as these parameters are characteristic for this type of cell death [49; 50]. On the other hand, G361 cells show a stable slight increase in cell dry mass and density, which decreased in the last two hours. CDS decreased after eleven hours of the experiment. The results for G361, supported by the video (Supplementary Video S4), imply that cells underwent apoptosis. These cells showed gradual cell rounding and a loss of surface contact followed by intensive membrane blebbing. It corresponds with known facts describing morphological changes during apoptosis [51]. Recently, several studies have described the characterization of cell death based on cell morphology using quantitative phase imaging [16; 22; 23; 51]. In particular, mapping of the quantitative phase changes during the time seems to be very promising in the determination of various morphological phenomena related to cell death, because the responsible cell death molecular machinery typically changes the distribution of the cell mass (cell content cleavage by caspases and loss of attachment during apoptosis connected with increased density; cell swelling during some kinds of lytic cell death--decrease in cell density) [23]. This study could also contribute to a better understanding of how light affects cells during fluorescence microscopy experiments, as the wavelength employed in our study is widely used. From our results, it is evident that cells were not damaged with the lower doses of light with a shorter time exposure. Thus, there is a need for low fluorescence doses and time exposures to achieve less destructive conditions for live samples during fluorescence microscopy. On the other hand, fluorescent microscopy always compromises between fluorescence damage to live samples and image quality.