Methodology paper: a novel phantom setup for commissioning of scanned ion beam delivery and TPS.

BACKGROUND: Commissioning of treatment planning systems (TPS) and beam delivery for scanned light ion beams is an important quality assurance task. This requires measurement of large sets of high quality dosimetric data in anthropomorphic phantoms to benchmark the TPS and dose delivery under realistic conditions. METHOD: A novel measurement setup is described, which allows for an efficient collection of a large set of accurate dose data in complex phantom geometries. This setup allows dose measurements based on a set of 24 small volume ionization chambers calibrated in dose to water and mounted in a holder, which can be freely positioned in a water phantom with various phantoms mounted in front of the water tank. The phantoms can be scanned in a CT and a CT-based treatment planning can be performed for a direct benchmark of the dose calculation algorithm in various situations. RESULTS: The system has been used for acceptance testing in scanned light ion beam therapy at Heidelberg Ion Beam Therapy Center for scanned proton and carbon ion beams. It demonstrated to be useful to collect large amounts of high quality data for comparison with the TPS calculation using various phantom geometries. CONCLUSION: The setup is an efficient tool for commissioning and verification of treatment planning systems. It is especially suited for dynamic beam delivery, as many data points can be obtained during a single plan delivery, but can be adapted also for other dynamic therapies, like rotational IMRT.

A motorized solid-state phantom for patient-specific dose verification in ion beam radiotherapy.

For regular quality assurance and patient-specific dosimetric verification under non-horizontal gantry angles in ion beam radiotherapy, we developed and commissioned a motorized solid state phantom. The phantom is set up under the selected gantry angle and moves an array of 24 ionization chambers to the measurement position by means of three eccentrically-mounted cylinders. Hence, the phantom allows 3D dosimetry at oblique gantry angles. To achieve the high standards in dosimetry, the mechanical and dosimetric accuracy of the phantom was investigated and corrections for residual uncertainties were derived. Furthermore, the exact geometry as well as a coordinate transformation from cylindrical into Cartesian coordinates was determined. The developed phantom proved to be suitable for quality assurance and 3D-dose verifications for proton- and carbon ion treatment plans at oblique gantry angles. Comparing dose measurements with the new phantom under oblique gantry angles with those in a water phantom and horizontal beams, the dose deviations averaged over the 24 ionization chambers were within 1.5%. Integrating the phantom into the HIT treatment plan verification environment, allows the use of established workflow for verification measurements. Application of the phantom increases the safety of patient plan application at gantry beam lines.

Test of the nuclear interaction model in SHIELD-HIT and comparison to energy distributions from GEANT4.

Monte Carlo codes are widely used to simulate dose distributions in ion radiotherapy. The benchmark of the implemented physical models against experimental data plays an important role in improving the accuracy of the simulations. To estimate the accuracy of the inelastic cross sections in SHIELD-HIT, the simulated charge is compared to measured data from a Multi Layer Faraday Cup. In addition, the results are compared to GEANT4, which are already published. Furthermore, energy distributions are simulated with SHIELD-HIT07 and GEANT4.8.1. From a comparison of depth distributions and beam profiles of 100 and 200 MeV protons, we estimate the level of agreement of the two codes. Nuclear interactions predicted by SHIELD-HIT underestimate the total amount of measured charge. The energy distributions from SHIELD-HIT and GEANT4 show differences exceeding the statistical uncertainties of 2%. Due to a difference of the Bragg curve of 0.5 +/- 0.3 mm on average, the mean difference in dose is 3.5% with a maximum deviation of 7% for the simulated cases.