# Polarimetric Detection of Chemotherapy-Induced Cancer Cell Death
## 5 Conclusions  
We measured the image Mueller matrix in the transmission configuration of biological samples corresponding to the colorectal adenocarcinoma HT29 cell line and leukemia cell lines U937 and THP1. On such matrices, we performed polar decomposition, Mueller matrix transformation, and differential decomposition in order to reach several polarimetric magnitudes that could give us more insight into the role played by the cell, or by some cell components, in changing the polarization of the light that goes through it either in the living state or when it undergoes apoptosis.  
As expected, since living cells are mainly transparent with an index of refraction close to water [17], the resulting matrix was quite close to unity. The main deviations from the trivial values after **MM** analysis were found in the diattenuation (after MMPD), anisotropy, (after MMT), total dichroism, and total birefringence (after MMDD).  
In this research, a study of cell death was performed, combining a traditional method with others based on the measurement and analysis of the above-mentioned polarimetric magnitudes. U937 and THP1 leukemia cells were treated with a chemotherapy drug in order to induce cell death. Observation under the microscope showed evidence of cell degradation, but its quantification was not a straightforward process. When performing the polarimetric analysis on U937 cells, a strong correlation between diattenuation and cell death was clearly observed, even at early times after treatment. In addition, we suggest that, since the diattenuation signal was located at the boundaries of cells, the increasing of diattenuation could be related to changes in the plasma membrane geometry and in its protein structure as cells undergo apoptosis. Similar results were obtained for the THP1 cell line and other polarimetric magnitudes mentioned above.  
We conclude that diattenuation could be an objective parameter to detect cell death and useful for assessing in vitro killing efficacy of drugs under development and the follow-up of leukemia patients undergoing therapy with cytotoxic agents. Although the type of cell death detected is likely to be apoptotic because cisplatin treatment induces apoptosis, further studies will reveal whether diattenuation is able to discriminate between the different types of cell death. Moreover, since this method could be implemented in a microscope, although further work is necessary, it could result in an objective (i.e., independent of the observer) polarimetric method of analysis, complementary to the traditional techniques used for cell death quantification.  
Figure 13: Images of individual cells at different times of the treatment taken by phase contrast microscopy. The same magnification has been used in the capturing and rendering of these images.  
**Author Contributions:** Conceptualization, all authors; methodology, A.F.-P. and O.G.-S.; software, A.F.-P.; validation, all authors; formal analysis, A.F.-[PERSON]; investigation, [PERSON] and [PERSON]; resources, J.L.F.-L., F.M., and J.M.S.; data curation, [PERSON] and [PERSON]; writing, original draft preparation, [PERSON]-[PERSON]; writing, review and editing, J.M.S., F.M., J.L.F.-L., and [PERSON]; visualization, [PERSON]-P.; supervision, J.M.S., J.L.F.-L., and F.M.; project administration, J.L.F.L. and J.M.S.; funding acquisition, J.L.F.-L., F.M., and J.M.S.  
**Funding:** This work has been supported by a grant from Instituto de Investigacion Valdecilla (IDIVAL) APG/03 to [PERSON]-L. and by SODERCAN (Sociedad para el Desarrollo de Cantabria) and Research Vicerrectorate of the University of Cantabria through Project 14)U2864661. [PERSON] acknowledges the University of Cantabria for her FPUgrant.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyzes, or interpretation of the data; in the writing of the manuscript; nor in the decision to publish the results.