An apparatus for performing nanoparticle-assisted external beam radiotherapy includes an X-ray spectrometer having an optical axis, an X-ray filter, and a mobile holding structure for holding the X-ray filter and X-ray spectrometer in a first and second relative position, and for switching from the first to second relative position while simultaneously positioning a patient body part in a target region including a target point. An X-ray beam emitted from the X-ray source crosses the X-ray filter before reaching the target point. The propagation of the X-ray beam forms an angle different from 0° and 180° with the optical axis of the X-ray spectrometer. The second relative position is obtained by inverting the relative positions of the X-ray filter and X-ray spectrometer relative to the patient body part. A method of determining an X-ray dose delivered at a region of a patient body using the apparatus is provided.