1. A computer system for determining an objective tumor response to an anti-cancer therapy using cross-sectional images, the computer system comprising: one or more processors; and one or more computer readable hardware storage devices, wherein the one or more computer readable hardware storage devices comprise computer executable instructions that when executed by at least one of the one or more processors cause the computer system to perform at least the following: receive a cross-sectional image generated at one or more of an x-ray computed tomography scanner, a computed tomography perfusion imaging system, a positron emission tomography system, a single-photon emission computed tomography system, or a magnetic resonance imaging scanner, wherein the cross-sectional image comprises a cross-sectional slice of digital medical image data; identify a set of pixels within the cross-sectional image, the set of pixels corresponding to a target lesion; analyze the set of pixels with an image processing module, wherein for the set of pixels, the image processing module performs the following: identify a lower threshold of pixel intensities within a total range of pixel intensities; identify an upper threshold of pixel intensities within the total range of pixel intensities; and define a subset of pixels from the set of pixels that correspond to vascularized tumor, wherein the subset of pixels comprises pixels having pixel intensities greater than or equal to the lower threshold and lower than or equal to the upper threshold; derive a vascular tumor burden for the set of pixels; receive one or more additional cross-sectional images, wherein the one or more additional cross-sectional images comprise one or more cross-sectional slices of the digital medical image data: identify an additional set of pixels within each of the one or more additional cross-sectional images corresponding to the target lesion; analyze the additional set of pixels with the image processing module, wherein for the additional set of pixels, the image processing module performs the following: identify the lower threshold of pixel intensities within the total range of pixel intensities; identify the upper threshold of pixel intensities within the total range of pixel intensities; and define an additional subset of pixels from the additional set of pixels within each of the one or more additional cross-sectional images that correspond to vascularized tumor, wherein the additional subset of pixels comprises pixels having pixel intensities greater than or equal to the lower threshold and lower than or equal to the upper threshold; derive a vascular tumor burden, wherein deriving the vascular tumor burden comprises determining a volume of the subset of pixels and the additional subsets of pixels; and determine the objective tumor response for the target lesion, wherein the objective tumor response is based on the vascular tumor burden.