Computational Dosimetry
- Fig. 1: H*(10) dose conversion coefficients calculated with GEANT4 and compared to literature values.
- Fig. 2: Bonner sphere at treatment location, to measure the neutron energy distributions the patients are exposed during therapy.
Development of planning system for neutron therapy at FRMII
At the MEDAPP facility of the FRM II research reactor, Garching, Germany, neutron therapy treatment has started in June 2007. Up to now, depth dose curves measured in a simple water phantom are being used to determine the patient’s dose. In order to include the individual geometry of the patient’s body and the different elemental composition of the patient’s organs, as well as to introduce a biological weighting of the dose deposited by neutrons, a 3D treatment planning system is necessary. We are presently using the Monte Carlo program GEANT4 to develop such a system, including a voxel phantom as a virtual patient.
As a first step, the dose calculation algorithm of GEANT4 was extensively validated for neutron transport, and the dose conversion coefficients for neutrons in the ICRU sphere calculated and reproduced (Fig. 1). Next, the neutron spectrum present at the patient treatment position was measured with a Bonner sphere spectrometer, applying gold foils as thermal neutron detectors (Fig. 2). The resulting neutron spectrum, as well as a photon spectrum given in (Breitkreutz et al. 2008, Nucl. Instr. Methods Phs. Res.A593:466-471), was implemented into GEANT4. Finally, the new ICRP voxel phantoms were implemented into GEANT4 (Fig. 3), and the 3D distribution of for example absorbed dose inside the phantom was calculated (Fig. 4).
The code was developed in a way, that any biological weighting factor can easily be included in the calculations, to obtain 3D biogically-weighted doses.
- Fig. 3: Voxel phantom implemented in GEANT4.
- Fig. 4: Absorbed dose distribution in the head of the voxel phantom, due to the mixed neutron-photon field present at the patient’s treatment location.
References:
S. Garny, V. Mares, H. Roos, F.M. Wagner, W. Rühm. Measurement of neutron spectra and neutron doses at the Munich FRMII therapy beam with Bonner spheres. Radiation Measurements (2010), doi: 10.1016/j.radmeas.2010.08.015.
S. Garny, V. Mares, W. Rühm. Response functions of a Bonner sphere spectrometer calculated with GEANT4. Nuclear Instruments and Methods in Physics Research A 604 (2009), 612-617.
S. Garny, W. Rühm, F. M. Wagner and H. G. Paretzke. Neutron Therapy at the FRMII - Calculation of Dose inside a Voxel Phantom. O. Dössel and W.C. Schlegel (Eds.): WC 2009, IFMBE Proceedings 25/I, pp. 437-440, 2009.
S. Garny, G. Leuthold, V. Mares, H.G. Paretzke, W. Rühm. GEANT4 transport calculations for Neutrons and Photons below 15 MeV. IEEE, pp. 2392-2396, 2009.