Development of a new prompt gamma ray detection system for full 3D proton range verification

Abstract

Purpose or Objective The ability to determine proton range in 3D is well suited for spot-scanning systems and for detecting non-uniform anatomical changes. In range verification via prompt gamma (PG) detection range is determined through the reconstruction of the origin of PG-rays emitted from nuclear de-excitations following proton bombardment. We report the first results of a new PG-based 3D range verification technique. Material and Methods Our technique utilises the 2.741-6.128 MeV (p, 16O) PG rays, emitted in cascade. Within the limitation of spectroscopy detector/electronic systems, these rays are effectively emitted simultaneously in time and position. When protons impinge tissues several 2.741-6.128 MeV PGray couples are produced. Their detection, coupled with a reconstruction algorithm, allows the identification of the common emission point. The maximum intensity of the PGray distribution is located few millimeters prior to the Bragg peak. For a beam passing through tissues with constant oxygen concentration, the beam range can be determined from the emission points of all detected couples. The detection system is comprised of 16 LaBr3(Ce) detectors, in a symmetrical design. To determine the PGray PGray emission positions, the information recorded by each detector is fed into an in-house built reconstruction algorithm. Results The position reconstruction capability of the system was investigated by means of Geant4 simulations. The spectrometer, with a 8 cm internal radius, has been modelled with realistic energy/temporal resolution. Figure 1 shows the detector/algorithm performance when a clinical 180 MeV beam impinges a water phantom. An excellent agreement is observed between the algorithmreconstructed PG-rays emission positions (blue curve), the phantom-scored PG-rays emission positions (red curve) and the dose distribution (black curve). The PG-ray origin is determined from the width of the detected peak with a σ of 4.17 mm. Subsequently a 175 and a 177.5 MeV proton beam were shot on the same phantom. The phantom is modelled so that the spectrometer centre coincides with the Bragg peak for a 180 MeV beam; this translates into a range undershoot of 5 and 10 mm. Figure 2 depicts the algorithm-reconstructed PG-rays emission positions for the 175 (purple curve) and 177.5 MeV (green curve) beams. The same plot for a 180 MeV beam is shown for comparison (blue curve). The σ is 4.31 and 5.47 mm for a 5 and 10 mm undershoot, respectively. Conclusion A new technique for proton range verification was developed; it allows to determine the range of a 180 MeV beam to within 4.17 mm. With a 10 mm undershoot the range was still reconstructed to within 6 mm. These uncertainties are lower than the ones fed into robust planning or the margins used in proton treatment planning. Next stage is to test the technique for a beam passing through different tissues. Developments are ongoing to construct a spectrometer prototype. The ultimate goal is a clinically compliant system for on-line, real-time range verification.