Monte Carlo computation of dose deposited by carbon ions in radiation therapy

Heavy charged particles interact with matter predominantly through inelastic collisions with atomic electrons. Slower particles give more energy to the electrons in comparison with faster particles; therefore, the delivered dose increases while the particle energy decreases. The point at which the particles deposit most of their energy is called the Bragg peak. The presence of a Bragg peak makes heavy charged particles very useful to treat deep-seated tumors. By varying the energy of a charged particle beam, radiation oncologists can spread this peak to match the contours of tumors or other targets. The advantages of heavy ion therapy over the conventional photon, and proton therapies are due to the better physical dose distributions achievable, tumor-conform treatment (1), and the radiobiological characteristics of heavy ions. Carbon ion beams appear to have the most optimal characteristics in physical and biological efficiencies comparing with other heavy ions. On the other hand, the relative biological effectiveness (RBE) of carbon ions widely varies as a function of depth in a medium, whereas it is similar to the RBE of neutrons only around the Bragg peak. Beams of high-velocity carbon ions provide an excellent physical depth-dose profile with an increased RBE (2) in the target volume. Millimeter precision at any depth is another additional advantage of carbon ion beam therapy of deep-seated tumors. Hence, radiation therapy with carbon ion beam is recommended when an advanced physical precision is of great importance, such as treating a solid tumor close to sensitive organs, as well as tumors in the head and neck region (3). At present, for therapeutic purposes, carbon ions are accelerated up to 430 MeV/u (4). The study of ion trajectories in tissue is essential in the fields of radiation dosimetry, health physics, radiation biology, and ion beam therapy. The precision of a Monte Carlo technique for computation of ion trajectories in matter depends mainly on the precision of the calculation of the stopping power properties of the matter (5). Stopping powers of charged particles in elements can be Background: High-velocity carbon ion beams represent the most advanced tool for radiotherapy of deep-seated tumors. Currently, the superiority of carbon ion therapy is more prominent on lung cancer or hepatomas. Materials and Methods: The data for lateral straggling and projected range of monoenergetic 290 MeV/u (3.48 GeV) carbon ions in muscle tissue were obtained from the stopping and range of ions in matter (SRIM) computer code. The data were transformed to determine the carbon ion trajectories in tissue by means of the Monte Carlo method. Consequently, the lateral dose distributions in the Bragg peak as well as the thickness of a thin discshaped tumor in the lateral direction were computed. The absorbed dose in the tissue was obtained as a function of the diameter of a carbon ion pencil beam. Results: More than 90% of the radiation dose in the lateral direction is deposited in the Bragg peak. The simulation results are in agreement with the existing data. Conclusion: It was confirmed that this method is reliable for estimation of dose deposited in human tissue by carbon ion beams. Iran. J. Radiat. Res., 2006; 4 (3): 115-120