## 1 Introduction  
The ability to evade programmed cell death is one of the hallmarks of cancer, leading to therapeutic resistance or failure [1]. Although apoptosis is perhaps the most famous, other forms of programed cell death have been well-characterized [2]. Previous studies have demonstrated that cancer cells that are resistant to one type of cell death may be more vulnerable to another. Ferroptosis and necrotosis, for example, can be exploited to induce death in cancer cells that have successfully evaded apoptosis [3; 4; 5; 6; 7; 8]. However, the interplay between each of these programed cell death pathways and their involvement in cancer progression is complex. For example, pan-caspase inhibitors have been shown to inhibit apoptosis while simultaneously inducing necrotosis [9]. Induction of necrotosis can be antitumorigenic in some settings or promote tumor progression in others [10; 11; 12; 13; 14]. In another example, vulnerability of melanoma cells to induction of ferroptosis is dependent on the phenotypic subtype [15]. Such observations highlight the need for further investigation and suggest that the therapeutic potential of targeting distinct programmed cell death pathways might require case-by-case assessment. Methodsthat efficiently and simultaneously quantify apoptosis, ferrotposis, and necroptosis would be valuable for further exploring the clinical utility of agents that regulate cell death pathways. Apoptosis, ferrotposis and necroptosis are all active forms of cell death, meaning they comprise defined molecular pathways that drive organized termination. The key regulatory molecules that activate, inhibit, or orchestrate each process permit molecular classification of cell death pathways using a variety of methods [2]. For example, changes in expression, post-translational processing, or subcellular localization of key proteins can be monitored using immunological methods such as fluorescence microscopy, immunoblotting, flow analysis, or enzyme-linked immunoassays (ELISA). Other useful methodologies detect outer membrane permeabilization or mitochondria function and health--both of which change during the process of apoptosis [16]. While each of these approaches has distinct advantages and limitations, none permit both high-throughput classification of all three forms of cell death in real time and with single cell resolution. Digital Holographic Cytometry (DHC) is a label-free technique that allows for live cell imaging and real time morphologic assessment [17]. DHC uses a laser that is split into two beams--a reference beam and an object beam. The reference beam is undisturbed, while the object beam is directed through the specimen. When the two beams are merged, an image can be generated through interpretation of the resulting interference pattern, with pixel intensity representing optical thickness--a function of both physical thickness and optical density. Cells are identified using standard image segmentation techniques, and individual cell features (optical volume, optical thickness, perimeter length, etc.) are derived. Quantitative phase images can be captured over a period of time, permitting tracking of DHC-derived features as cells undergo change, for example, due to exposure to a therapeutic agent. DHC has previously been used to distinguish different cell states based upon morphology, including identification of pre-apoptotic and dying cells [18; 19; 20; 21; 22]. In addition to molecular changes, a distinct set of morphological changes accompanies each form of programmed cell death. Apoptosis is associated with an increase in cell thickness, plasma membrane blebbing, decrease in cell volume, and chromatin condensation [23; 24]. Ferrotposis is associated with iron-dependent membrane lipid oxidation and damage to the plasma membrane [24; 25]. Necroptosis is associated with cell swelling, an increase in cell thickness, decrease in cell area, and eventual rupture of the plasma membrane [4; 23; 24].