N. Givehchi

Monte Carlo simulation of ripple filters designed for proton and carbon ion beams in hadrontherapy with active scanning technique

Proton and carbon ion beams have a very sharp Bragg peak. For proton beams of energies smaller than 100 MeV, fitting with a gaussian the region of the maximum of the Bragg peak, the sigma along the beam direction is smaller than 1 mm, while for carbon ion beams, the sigma derived with the same technique is smaller than 1 mm for energies up to 360 MeV. In order to use low energy proton and carbon ion beams in hadrontherapy and to achieve an acceptable homogeneity of the spread out Bragg peak (SOBP) either the peak positions along the beam have to be quite close to each other or the longitudinal peak shape needs to be broaden at least few millimeters by means of a properly designed ripple filter. With a synchrotron accelerator in conjunction with active scanning techniques the use of a ripple filter is necessary to reduce the numbers of energy switches necessary to obtain a smooth SOBP, leading also to shorter overall irradiation times. We studied the impact of the design of the ripple filter on the dose uniformity in the SOBP region by means of Monte Carlo simulations, implemented using the package Geant4. We simulated the beam delivery line supporting both proton and carbon ion beams using different energies of the beams. We compared the effect of different kind of ripple filters and their advantages.

WE‐C‐AUD B‐09: Evaluation of Radiobiological Effects of Carbon Ion Beams: Mixed Particle Fields and Fragmentation

Purpose: The purpose of this report is to investigate the effects on the Relative Biological Effectiveness (RBE) due to nuclear fragmentation during irradiation with therapeuticcarbonion beams, by using biological model calculations. Method and Materials: In order to disentangle the biological effects of the mixed particle fields, we evaluated the RBE originated by primary carbon ions and by fragments together and separately. The radiobiological efficiency of charged particles is mainly characterized by their high local ionization density which can be directly correlated to the local density of DNA damage. We developed a code based on the Local Effect Model (LEM) for the calculation of the cell survival after irradiation with carbon ions. As input we use particle distributions sampled at different depths in a water volume, for different energies of the primary ions (between 150 and 400 MeV/u), generated by Monte Carlo simulations using the package GEANT4. The computational effort was performed using the distributed INFN (Istituto Nazionale di Fisica Nucleare) Grid computing resources. Results: For carbon ions, high RBE values were found in the high‐LET region (Bragg peak), as well as in the low‐dose region, the distal region after the Bragg peak, where the contribution to the RBE is mainly due to fragments. We show that the global biological effect can be well reconstructed by a weighted combination of the alpha and beta parameters of the Linear‐Quadratic Model (LQM) evaluated separately for primary ions and fragments. The analysis provided also an estimate of the RBE sensitivity to the uncertainties present in the experimental data of fragment yield. Conclusion: Our work estimated the impact of the fragments in hadrontherapy using carbonion beams. Due to the high RBE values found, the resulting biological dose can be important in regions outside the irradiated target, in presence of organs at risk.

SU‐FF‐T‐108: Clinical Use of Strip Ionization Chamber Detector as Online Proton Beam Monitor

Purpose: In proton therapy it is important to deliver uniform dose distribution in tumor volume. The parameters which indicate the beam geometry have to be evaluated and the beam has to be controlled during radiation. For this reason a detector system has been developed for online beam monitoring at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) within a collaboration with the Istituto Nazionale di Fisica Nucleare‐ Torino (INFN‐To). Shallow tumors (32 mm maximum depth) like uveal melanomas have been treated since spring 2002 in this center. Method and Materials: The 62 MeV proton beam, extracted from LNS Superconducting Cyclotron, is delivered based on double foils scattering system. A Range shifter followed by an energy modulator is placed downstream of the scattering system to provide the Spread Out Bragg Peak (SOBP) at the tumor position. The detector has been placed upstream of the last collimator; it consists of two parallel plate strip ionization chambers segmented in vertical and horizontal orientation respectively. Each anode consists of 256 0.5 mm wide strips with 12.8 × 12.8 cm2 sensitive area. Results: The detector has been checked in different beam conditions and is currently used in clinical practice. The beam symmetry and integrated fluence are measured with this detector. The value of skewness and centre of gravity have been tested in different clinical beam settings and the ranges of allowed values have been defined. During treatment these parameters are evaluated and checked against the set limits to ensure the correct delivery of the dose.Conclusion: A strip ionization chamberdetector has been developed to be used as online beam monitor in the proton therapy beam line at LNS (Catania, Italy). The beam is monitored with frequency of the order of one Hertz and it can be stopped in case of misbehavior during treatment.