Proton radiography using discrete range modulation method – A Monte Carlo study

The advantage of proton therapy is the Bragg peak, but its range uncertainties can also lead to a certain dose deviation. This uncertainty can be reduced by using proton radiography to determine the water equivalent path length (WEPL). In this study, a discrete range modulation (DRM) method is introduced and its image quality compared with that of the previously reported continuous range modulation (CRM) method. MCNPX2.7.0 was used to simulate parallel proton beams with energies from 70 to 230 MeV at 2.5 MeV/step. In the DRM method, the WEPL can be related to R80 using the relationship between the proton energy and energy deposition at 80% of the fall-off (E80). For the CRM method, a dose gradient plan was created to estimate the relationship between WEPL and accumulated dose. DRM and CRM were compared using four different phantoms. For the slab, CIRS 062M head and cake phantoms, DRM had a good agreement with the theoretic WEPL: within 2 mm or 1%, and with a standard deviation of less than 0.5 mm. For the sphere phantom, DRM can achieve a GammapRG passing rate of 0.999, passing the criteria of a WEPL deviation of less than 2 mm and a distance-to-agreement of less than 3 mm. Proton radiographs made by the DRM method are more accurate and precise than those made by CRM. Two algorithms are proposed to solve the range mixing problems. DRM can also save unnecessary doses that do not contribute to imaging.