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, 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; and determine the objective tumor response for the target lesion, wherein the objective tumor response is based on the vascular tumor burden.