A New Potential Method for Proton Radiography

This work describes a proof of principle of a new proton radiography technique later called as dynamic mode. The principle is based on the acquisition of several transmission radiographies with different proton beams instead of one. Thus, using a dedicated reconstruction algorithm, an image of the object with an optimal image quality can be reconstructed. This study was performed at the medical cyclotron MEDICYC (Centre Antoine Lacassagne) with a 62.5 MeV native beam. A small object consisting of a Poly(methyl methacrylate) (PMMA) cube of 22 mm side, with 5 cylindrical air inserts with diameter ranging from 1 to 5 mm, was used. Radiochromic EBT3 films were used as detectors to carry out projections of the object. Monte Carlo N-Particle Transport Code eXtended (MCNPX) v2.5 simulations were also realized for comparisons. For different radiographies approaches, those commonly used and documented in literature and the new proposed one, good agreement was found between MCNPX simulations and film measurements. This new imaging system allowed to significantly improve the image quality index in terms of spatial and contrast resolutions. Air inserts of relatively small sizes (2 and 3 mm) are clearly visible while they were not perceptible when the maximum energy was used. The larger inserts (4 and 5 mm) also have an estimated PMMA equivalent thickness much more faithful to reality. The preliminary results obtained with this innovative acquisition technique thus offer a significant added value. The acquisition system is relatively simple: a 2D dose detector perpendicular to the beam axis. By replacing the passive film detector with an active one, such as a matrix of semiconductors, a routine clinical use could be proposed. This kind of proton imaging device is of growing interest since it could improve the accuracy of proton radiation therapy treatment planning and the alignment of the patient respect to the proton beam.