# Polarimetric Detection of Chemotherapy-Induced Cancer Cell Death
## 1 Introduction  
Polarimetry has been shown to be a very useful approach in problems that involve light-matter interaction. This is supported by its many successful applications in a variety of fields, like astronomy, agriculture, weather radar, environmental science, etc. [1; 2; 3]. More recently, imaging polarimetry has become a field of increasing activity in medicine and biology because of its non-destructive nature and its potential to identify local properties in propagating media, something particularly important in the context of biomedical diagnosis [4]. Specifically, polarized light has been used to perform several studies over different tumor tissues [5] from colon [6; 7], cervix [8; 9], or thyroid to skin [10; 11; 12]. Other studies perform polarimetry and microscopy on both cancer and healthy blood samples, finding differences in physical magnitudes, like retardance or depolarization [13]. Regarding the diattenuation, a recent study [14] showed that this parameter is able to reveal different brain tissue properties. There are also studies that focus their attention on the analysis of the scattered light by suspensions of cancer and normal cells [15; 16] since it is well known that both types of cells present differences in their refraction index [17; 18]. In summary, when interacting with matter, the polarimetric properties of light are affected in a way that is related to the optical properties of the material. These changes can be analyzed with the Mueller matrix **MM**[19], which fully characterizes the optical changes induced by a given sample. Polarimetric magnitudes should be eventually related to the biological properties of cells and the corresponding tissues so that knowing this information could be useful in order to understand their structure or at least identify some of the processes that take place in them. However, between the raw polarimetric data given by **MM** values and any other that might be of medical use, there is a step in which physically-interpretable magnitudes must play an important role. This physical mediation is commonly done through matrix transformation operations. In other words, by means of a proper decomposition, the optical properties of each part of the system can be described in terms of parameters with a physical interpretation. Connecting them with medical properties is something that experience and observation should provide with time. Different analysis methods are widely used in the bibliography to extract the information provided by **MM**. The two most commonly-used are Polar Decomposition (MMPD) [20] and Mueller Matrix Transformation (MMT) [21]. A third one is Differential Decomposition (MMDD) [22]. Each of them produces a set of magnitudes per image point, therefore transforming **MM** images into other \"physical\" images that contain the optical properties and behavior or structural characteristics of the sample under study. Most of the work done in the area of biology focuses the attention on biological tissue imaging [5]. Regarding biological cells, previous works showed that some processes occurring in cells may alter their optical properties and therefore be potentially detectable under polarimetric observation. For instance, it can be a very handy tool for the identification and discrimination of cancerous cells [23; 24]. Other works also employ Mueller matrix imaging for discrimination and classification of microalgae and bacteria [25; 26; 27]. We suggest that polarimetry could also be useful for the identification of processes that promote changes inside the cell, like cell death. We performed transmittance imaging polarimetry over samples of two different leukemia cell lines, U937 and THP1. Cells were treated with a cytotoxic drug, commonly used in chemotherapy, that induces cell death. **MM** were obtained for both control (non-treated) and treated cells. Then, different decompositions of the matrices were done in order to find sensitive polarimetric magnitudes that allow assessing the differences between control and treated cells.