Verification procedure for isocentric alignment of proton beams

Abstract

Abstract:  A technique is presented for verifying whether proton beams were being delivered within the required coincidence with the gantry mechanical isocenter. Our test method employed a proton beam that was collimated by a circular aperture at its central axis and then was intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for the couch position assured a precise positioning of the steel sphere at the mechanical isocenter of the gantry.  A film-based radiation dosimetry technique, chosen for the good spatial resolution it achieves, recorded the proton dose distribution for optical image analysis. The comparison of the optical image showing a circular high dose region surrounding a lower dose area, corresponding to the proton beam absorption by the steel sphere, provided the measure of the beam alignment with the mechanical isocenter. We found the self-developing Gafchromic EBT film and commercial EPSON 10000 XL flatbed scanner to be accurate and efficient tools. The positions of the gantry’s mechanical and proton beam’s isocenters as recorded on film were clearly identifiable within the 0.17 mm/pixel-scanning resolution used for routine alignment testing. The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22 ± 0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.