Verification of the alignment of a therapeutic radiation beam relative to its patient positioner.

An easily-used system has been developed for routine measurements of the alignment of beams used for radiation therapy. The position of a beam of circular cross section is measured with respect to a steel sphere fixed to the patient positioning table and which should coincide with the isocenter. Since measurements can be done at all gantry angles (if one is available) and with all possible orientations of the patient table, the system is particularly suited for rapid and accurate measurements of gantry and/or couch isocentricity. Because it directly measures beam-to-positioner offset, the system provides an inclusive alignment verification of the total treatment system. The system has been developed for use with proton beams, but it could equally be used for alignment checks of an x-ray beam from a linear accelerator or other source. The measuring instrument consists of a scintillation screen viewed by a CCD camera, mounted on the gantry downstream of the sphere. The steel sphere is not large enough to stop protons of all energies of interest; however, it will always modify the energy and direction of protons which intersect it, creating a region of lower intensity (a "shadow") in the light spot created by the proton beam hitting the screen. The position of the shadow with respect to the light spot is a measure of the alignment of the system. An image-analysis algorithm has been developed for an automatic determination of the position of the shadow with respect to the light spot. The specifications and theoretical analysis of the system have been derived from Monte Carlo simulations, which are validated by measurements. We have demonstrated that the device detects beam misalignments with an accuracy (1 s.d.) of 0.05 mm, which is in agreement with the expected performance. This accuracy is more than sufficient to detect the maximum allowed misalignment of +/-0.5 mm.