1. A breast cancer detection system for detecting an architectural distortion of the mammary gland included in an X-ray image of the breast, the breast cancer detection system comprising: a processor programmed to: calculate a normal architecture model of the mammary gland that scatters in a fan shape from the nipple toward the greater pectoral muscle with respect to the X-ray image of the breast; extract orientations of linear components of an image texture of a region that form a shape of a shadow in the X-ray image of the breast; and compare the orientation of the mammary gland in the calculated normal architecture model and the extracted orientation with respect to a region of interest in the X-ray image of the breast including architectural distortion candidates of the mammary gland detected by preprocessing to: (i) calculate a feature quantity based on a difference between the orientations, and (ii) determine whether the candidates are an architectural distortion of the mammary gland based on the feature quantity.