Purpose: To investigate quantitatively the effects of high-z materials in the path of proton beams and to determine dosimetric uncertainties not accounted by dose calculation algorithms in proton therapy.
Methods: Two-dimensional dose distributions from high energy proton beams were measured at different planes perpendicular to the beam direction at different locations. Proton beams from the MEVION-S250 machine were used to irradiate the Leeds phantom embedded in solid-water phantom at different depths ranging from 1-15 cm. The MEVION-system provided a 250 MeV double-scattering proton beam shaped with two nozzles (14 and 25 cm diameters) and various beam ranges (5-32cm) and range modulation (2-20cm). The different objects in the Leeds phantom were used as high-z heterogeneities to map the different patterns of dose deposition. The dose distributions were measured with EBT2-Gafchromic films and computed-radiography.
Results: The dose deposition from high energy proton beams showed strong interference patterns that depended on the density, position, shape and depth of high-z heterogeneities in phantom. Large dose variations were measured between the constructive and destructive dose deposition patterns upto 10%. The interference patterns varied with depth along the track of the therapeutic beam and positon of the high-z objects in the beam path. These scatter patterns occurred upstream with the energetic protons and in the Bragg-peak dose deposition region. These patterns resulted from the interference of the primary and scattered protons.
Conclusion: Dose depositions from the double scattering proton beams are affected with quantic wave nature associated with energetic protons where it produced constructive and destructive interference regions of dose deposition and lead to large variations (10%) in local dose depositions. These dose deposition interference regions are not accounted in the available dose calculation algorithms used in proton therapy. These local dose artifacts may affect strongly tumor control and radiobiological effectiveness of proton therapy.
Not Applicable / None Entered.