SU‐E‐T‐709: Spatial Resolution Requirements for Fragment Identification in a Carbon Ion Spread Out Bragg Peak

Purpose: To determine spatial resolution for detectors used to identify charged fragments in a carbon ion therapy spread out Bragg peak. Methods: The Geant4 Monte Carlo toolkit was used to simulate a 2 × 2 cm2 broad beam of carbon ions incident on a 30 × 30 × 12 cm3 water phantom. The carbon ion energies ranged from 165 MeV/n to 220 MeV/n and intensities were selected in order to simulate a SOBP extending from 6 to 10 cm in depth. Scoring of secondary fragments was performed for different pixel sizes at 3 depths in water. The analysis was designed to identify situations where two or more fragments intercept a pixel coincidently. Results: Initial results show that increased depths in the phantom require smaller pixel sizes due to increases in fragment population. At a pixel size of 1 × 1 cm2, a detector pixel was intercepted by two or more particles 16%, 14% and 30% percent of the time at depths of 6, 8 and 10 cm respectively. The probability of coincident hits decreases with smaller pixel size. At a pixel size of 0.05 × 0.05 cm2 a detector pixel was intercepted by two or more particles 0.7%, 0.9%, and 2% of the time at depths of 6, 8 and 10 cm respectively. Conclusion: Accurate characterization of carbon ion beams requires the consideration of secondary charged particles that have different radiobiological effectiveness. A method for accurate identification in a clinical setting could lead to improved models for treatment planning. This work investigated the spatial resolution requirements for a detector that could be used to perform secondary fragment identification in a spread out Bragg peak. These preliminary results show that detectors with pixel size on the order of 0.5 × 0.5 mm2 may be sufficient. This work was supported through research grant NNX13AD19G for Early Stage Innovations (ESI) administered through the NASA Space Technology Research Grants Program. No conflict of interest.