A radiotherapy system has: a placing unit placing a subject an imaging unit performing an imaging of the subject a region setting unit setting a required region of first image data obtained by the imaging unit performing the imaging of the subject and setting a corresponding required region of second image data obtained by, before the imaging, performing a pre-imaging of the subject a histogram generating unit generating a dose-volume histogram of the required region of the first image data and generating a dose-volume histogram of the required region of the second image data a difference computing unit computing a difference between the dose-volume histogram of the required region of the first image data and the dose-volume histogram of the required region of the second image data and an outputting unit, if it is determined that the difference is greater than a threshold value, outputting the determination to an outside.
