ESTRO 2021 Abstract Book

S1407

ESTRO 2021

On average, the Cranial Distortion Correction Elements software allows for an improvement based on the RMSE correlation between the baseline MRI and the distorted MRI. For the corrected datasets, the RMSE ranges between 9.83 (SD 4.20) and 55.58 (SD 16.99) and average at 33.36 (SD 14.06). For the non-corrected datasets, the RMSE ranges between 7.70 (SD 0.44) and 83.14 (SD 13.48) and average at 44.18 (SD 25.93). As a control, we measured the RSME between the original MRI and the corrected dataset averaging for all slices to 14.01 (SD 6.27) to evaluate the error baseline linked to the correlation between image modalities. Conclusion The results of the cranial distortion correction elements software show an improvement increasing in correlation with the noise and intensity non-uniformity levels. This provides to the stereotactic treatments an added robustness and reliability related to the accuracy of the MR images and independent from any distortion level and, per extension, to the target definition and patient positioning. This novel approach to evaluate distortion correction can be used with other modalities and to compare different methods to improve the image datasets accuracy for stereotactic treatments. PO-1682 Towards a clinical helium ion imaging system L. Volz 1 , T. Vichtl 2 , C. Collins-Fekete 3 , J. Seco 2 1 GSI Helmholtzzentrum for Heavy Ion Research, Biophysics, Darmstadt, Germany; 2 German Cancer Research Center, Biomedical Physics in Radiation Oncology, Heidelberg, Germany; 3 University College London, Department of Medical Physics and Biomedical Engineering, London, United Kingdom Purpose or Objective Single-event particle imaging is a promising low dose candidate for particle therapy image guidance. Helium ion imaging might offer an ideal balance between noise, spatial resolution and dose. Here, we investigate different energy detector designs in their performance for helium ion imaging towards a clinical prototype. Materials and Methods Three principle designs are considered: multistage scintillator detectors (MSD), binary range telescope (BRT), and time-of-flight (TOF) detector systems. The metrics for comparison are WEPL noise and accuracy. For each design, a dynamic WEPL range of 26cm (head sized object) and 200 MeV/u initial energy are assumed. Each detector are modeled in TOPAS Monte Carlo simulations. WEPL Noise/Accuracy : Noise and systematic errors in reconstructed water equivalent path length (WEPL) are first derived from theoretical principles for the different detectors. For MSD, the impact of the thickness/number of stages was studied as function of energy resolution. dE-E filtering : We assess the capacity of each detector design to perform as a dE-E telescope to filter secondaries. For the TOF detector, we implemented a dE-TOF design, including a leading 2.54cm dE scintillator. For a qualitative comparison, helium ion radiographs of a pediatric head phantom were simulated. Results WEPL Noise/Accuracy: A MSD composed of 10 longitudinally segmented stages offers a WEPL noise close to the helium ion range straggling, with minimal dependence on the stage intrinsic energy resolution. However, MSD suffer from systematic uncertainties in the energy to WEPL conversion at stage interfaces (Figure 1b). On the other hand, a BRT composed of 3mm slabs offered similar noise without the intrinsic noise artefact (Figure 1a), at the cost of complexity in design. For a TOF design, primary particle loss was reduced by up to a factor 2, due to the reduced material budget. Still, to achieve a dose-to-noise ratio comparable to other designs at a realistic TOF plane distance (<50cm), a yet unachievable time resolution (<10ps) is needed (Figure 1c). dE-E filtering: The dE-E filter efficiency was not significantly different between a 5-stage or a 10-stage MSD. A BRT detector requires a leading thin calorimeter to enable dE-E filtering. Filtering secondary fragments with the dE-TOF was less effective due to lower energy variation in the dE stage for higher energy particles. For the time resolution achieved with recent TOF prototypes (64ps), the dE-TOF acquired radiography of the head yielded significantly increased noise compared to other designs (Figure 2).