R. van der Meer

Geant4 simulations of proton beam transport through a carbon or beryllium degrader and following a beam line

Monte Carlo simulations based on the Geant4 simulation toolkit were performed for the carbon wedge degrader used in the beam line at the Center of Proton Therapy of the Paul Scherrer Institute (PSI). The simulations are part of the beam line studies for the development and understanding of the GANTRY2 and OPTIS2 treatment facilities at PSI, but can also be applied to other beam lines. The simulated stopping power, momentum distributions at the degrader exit and beam line transmission have been compared to accurate benchmark measurements. Because the beam transport through magnetic elements is not easily modeled using Geant4a connection to the TURTLE beam line simulation program was made. After adjusting the mean ionization potential of the carbon degrader material from 78 eV to 95 eV, we found an accurate match between simulations and benchmark measurements, so that the simulation model could be validated. We found that the degrader does not completely erase the initial beam phase space even at low degraded beam energies. Using the validation results, we present a study of the usability of beryllium as a degrader material (mean ionization potential 63.7 eV). We found an improvement in the transmission of 30-45%, depending on the degraded beam energy, the higher value for the lower energies.

Improved resolution for soft X-ray monochromatization using lamellar multilayer gratings

Lamellar Multilayer Gratings (LMG) offer improved resolution for soft-x-ray (SXR) monochromatization, while maintaining a high reflection efficiency in comparison to conventional multilayer mirrors (MM). We previously used a Coupled-Waves Approach (CWA) to calculate SXR diffraction by LMGs and identified a single-order regime in which the incident wave only excites a single diffraction order. We showed that in this regime the angular width of the zeroth-order diffraction peak simply scales linearly with Γ (lamel-to-period ratio) without loss of peak reflectivity. However, the number of bi-layers must then be increased by a factor of 1/Γ. Optimal LMG resolution and reflectivity is obtained in this single-order regime, requiring grating periods of only a few hundred nm, lamel widths < 100nm and lamel heights > 1μm [1]. For the fabrication of LMGs with these dimensions, we use a novel process based on UV-NanoImprint Lithography (UV-NIL) and Bosch-type Deep Reactive Ion Etching (DRIE). Successful fabrication of LMGs with periods down to 200nm, line widths of 60nm and multilayer stack heights of 1μm is demonstrated. SXR reflectivity measurements were performed on these LMGs at the PTB beamline at the BESSYII synchrotron facility. The measurements demonstrate an improvement in resolution by a factor 3,5 compared to conventional MMs. Further analysis of the SXR reflectivity measurements is currently being performed.

Assessing the quality of proton PBS treatment delivery using machine log files: comprehensive analysis of clinical treatments delivered at PSI Gantry 2.

Pencil beam scanning (PBS) proton therapy requires the delivery of many thousand proton beams, each modulated for position, energy and monitor units, to provide a highly conformal patient treatment. The quality of the treatment is dependent on the delivery accuracy of each beam and at each fraction. In this work we describe the use of treatment log files, which are a record of the machine parameters for a given field delivery on a given fraction, to investigate the integrity of treatment delivery compared to the nominal planned dose. The dosimetry-relevant log file parameters are used to reconstruct the 3D dose distribution on the patient anatomy, using a TPS-independent dose calculation system. The analysis was performed for patients treated at Paul Scherrer Institute on Gantry 2, both for individual fields and per series (or plan), and delivery quality was assessed by determining the percentage of voxels in the log file dose distribution within  +/-  1% of the nominal dose. It was seen that, for all series delivered, the mean pass rate is 96.4%. Furthermore, this work establishes a correlation between the delivery quality of a field and the beam position accuracy. This correlation is evident for all delivered fields regardless of individual patient or plan characteristics. We have also detailed further usefulness of log file analysis within our clinical workflow. In summary, we have highlighted that the integrity of PBS treatment delivery is dependent on daily machine performance and is specifically highly correlated with the accuracy of beam position. We believe this information will be useful for driving machine performance improvements in the PBS field.