21Predicting patient responses to chemotherapy regimens is a major challenge in 22 cancer treatment. To do this requires quantitative mathematical models to predict opti-23 mal dose and frequency for a particular drug, and experimental model systems such as 24 three-dimensional organoids that accurately recapitulate the tumor microenvironment 25 and heterogeneity. However, tracking the spatial dynamics of multiple cell types in 26 three-dimensions can be a significant challenge in terms of time and throughput. Here 27we develop a two-dimensional system that allows for simple tracking of cell populations 28 via fluorescence microscopy for modeling spatial dynamics in tumors. We first develop 29 multiple 4T1 breast cancer cell lines resistant to varying concentrations of doxorubicin, 30 and demonstrate how well mixed and spatially heterogeneous populations expand in a 31 two-dimensional colony. We subject cell populations to varied dose and frequency of 32 chemotherapy and measure colony growth radius and populations. We then build a 33 mathematical model to describe the dynamics of both chemosensitive and 34 chemoresistant populations, where we determine which number of doses can produce 35 the smallest tumor size based on parameters in the system. In the future, this system 36 can be adapted to quickly optimize dosing strategies in the setting of heterogeneous cell 37 types or patient derived cells with varied chemoresistance. 38 39 Chemotherapy dosing strategies have primarily followed a maximum tolerated dosage 41 (MTD) approach, in which patients are given high doses of chemotherapy to kill as 42 many tumor cells as possible (1)(2)(3)(4)(5). This approach is based on early assumptions that 43 tumors are composed of a homogenous, exponentially growing cell population and thus 44 maximum doses are likely to cause the highest disease eradication (6, 7). However, tu-45 mors are composed of genetically heterogeneous cells with varied chemoresistance, 46 and further diversity in chemoresistance can arise due to drug selection pressure from 47