gms | German Medical Science

Background: The installation of the Heidelberg Ion Beam Therapy Centre (HIT) at the University Hospital is completed and the facility is currently being commissioned. The facility is equipped with magnetic beam scanning and uses an active energy variation of the synchrotron to deliver beams of protons and carbon ions. The treatment planning system (TPS) syngo PT Planning from Siemens AG, Healthcare Sector, was first released as a medical product in December 2008 and is therefore the first CE labelled TPS for combined carbon and proton therapy planning. Treatment planning for carbon ions is based on the biological model developed and tested for about 430 patients at the German pilot project for heavy ion therapy at the Gesellschaft für Schwerionenforschung (GSI). For protons a fixed RBE value of 1.1 is used. Physical dose calculation is done using conventional pencil-beam models. The basic data for both proton and carbon ion beams were obtained from FLUKA (version 2008) Monte Carlo calculations adjusted to Bragg curves measured with a T41030 Peakfinder from PTW Freiburg and to radiographic film measurements of the beam profile.

Material and methods: The aim of a commissioning is to determine the accuracy of the dose algorithms in treatment situations of varying complexity of the treatment plans and the phantom geometry, but also to define reference values for the periodic quality assurance of the TPS. For this purpose measurements of water equivalent absorbed dose have been acquired in various phantoms and compared to the TPS prediction. For evaluation, the differences between the measurements and the predictions from the TPS were normalized to the maximum dose in the TPS dose distribution. Also the lateral dose profiles, geometrical field sizes and depth dose distributions were analysed. For the biological dose calculation for carbon ions, a benchmark against the TRiP98 code was performed in order to guarantee the equivalence of the codes in this respect.

Results: In general the measurements showed good agreement with the predictions of the TPS, fulfilling all the required acceptance criteria. Small discrepancies for small field sizes at large penetration depths are explained by the approximations of the current lateral beam modelling in depth, using only a single Gaussian distribution for proton and neglecting lateral scattering for carbon ions and are therefore predictable. The biological dose calculation was shown to be equivalent to the algorithm used for treatments at GSI.

Conclusion: In a forthcoming version, the lateral spread of carbon and proton beams will be described by a two Gaussian model. Because of the overall good performance of the system and the good results of the commissioning, we are now ready for clinical therapy planning.