1 Afdeling for Medicinsk Fysik, Faculty of Health Sciences, Aarhus University, Aarhus University2 Department of Experimental Clinical Oncology, Faculty of Health Sciences, Aarhus University, Aarhus University3 Department of Physics and Astronomy, Faculty of Science, Aarhus University, Aarhus University4 Department of Physics and Astronomy, Science and Technology, Aarhus University5 Department of Clinical Medicine - Department of Medical Physics, Department of Clinical Medicine, Health, Aarhus University6 German Cancer Research Center Heidelberg, Germany and Heidelberg Ion Therapy Center, Germany7 Department of Physics and Astronomy, Science and Technology, Aarhus University8 Department of Clinical Medicine - Department of Medical Physics, Department of Clinical Medicine, Health, Aarhus University
1 Background In clinical practice the quantity dose to water (Dw ) is used as a reference standard for dosimeters and treatment planning systems. Treatment planning systems usually rely on analytical representation of the particle beam, which are normally expressed as dose with respect to water. The dose to medium (Dm ) may however differ from Dw , due to the different particle spectrum and stopping power found herein. Monte Carlo particle transport codes are capable of directly calculating dose to medium (Dm ), and was for instance recently investigated by Paganetti 2009 for various proton treatment plans. Here, we quantisize the effect of dose to water vs. dose to medium for a series of typical target materials found in medical physics. 2 Material and Methods The Monte Carlo code FLUKA [Battistioni et al. 2007] is used to simulate the particle fluence spectrum in a series of target materials exposed to carbon ion beams. The scored track-length fluence spectrum Φi for a given particle i at the energy E, is multiplied with the mass stopping power for target material for calculating Dm . Similarly, Dw is calculated by multiplying the same fluence spectrum with the mass stopping power for water. This represents the case that our “detector” is an infinitesimal small non-perturbing entity made of water, where charged particle equilibrium can be assumed following the Bragg-Gray cavity theory. Dw and Dm are calculated for typical materials such as bone, brain, lung and soft-tissues using the PSTAR, ASTAR stopping power routines available at NIST1 and MSTAR2 provided by H. Paul et al. 3 Results For a pristine carbon ion beam we encountered a maximum deviation between Dw and Dm up to 8% for bone. In addition we investigate spread out Bragg peak configurations which dilutes the effect, and other tissue like materials. However, ideally further comparisons between Dw and Dm have to be done for real treatment plans using CT-scans in order to quantisize the clinical consequences.