Radiation therapy apparatus, medical image processing apparatus, radiation therapy method, and program

A radiation therapy apparatus according to an embodiment of the present invention is provided with an acquisition unit, a projection position calculation unit, an element projection image generation unit, and an element projection image synthesis unit. The acquisition unit acquires conditions for X-ray imaging in a treatment stage and a three-dimensional image of a patient captured prior to the treatment stage. The projection position calculation unit calculates, on the basis of X-ray imaging conditions, a projection position when each pixel included in a three-dimensional image is projected onto a two-dimensional X-ray fluoroscopic image generated by X-ray imaging. The element projection image generation unit generates an element projection image for each pixel when each pixel included in a three-dimensional image is projected onto an X-ray fluoroscopic image. The element projection image compositing unit generates a reconstructed image by compositing the element projection image for each pixel on the basis