F. Bourhaleb

Heuristic optimization of the scanning path of particle therapy beams

Quasidiscrete scanning is a delivery strategy for proton and ion beam therapy in which the beam is turned off when a slice is finished and a new energy must be set but not during the scanning between consecutive spots. Different scanning paths lead to different dose distributions due to the contribution of the unintended transit dose between spots. In this work an algorithm to optimize the scanning path for quasidiscrete scanned beams is presented. The classical simulated annealing algorithm is used. It is a heuristic algorithm frequently used in combinatorial optimization problems, which allows us to obtain nearly optimal solutions in acceptable running times. A study focused on the best choice of operational parameters on which the algorithm performance depends is presented. The convergence properties of the algorithm have been further improved by using the next-neighbor algorithm to generate the starting paths. Scanning paths for two clinical treatments have been optimized. The optimized paths are found to be shorter than the back-and-forth, top-to-bottom (zigzag) paths generally provided by the treatment planning systems. The gamma method has been applied to quantify the improvement achieved on the dose distribution. Results show a reduction of the transit dose when the optimized paths are used. The benefit is clear especially when the fluence per spot is low, as in the case of repainting. The minimization of the transit dose can potentially allow the use of higher beam intensities, thus decreasing the treatment time. The algorithm implemented for this work can optimize efficiently the scanning path of quasidiscrete scanned particle beams. Optimized scanning paths decrease the transit dose and lead to better dose distributions.

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.

Radiation damage studies of a recycling integrator VLSI chip for dosimetry and control of therapeutical beams

A VLSI chip based on a recycling integrator has been designed and built to be used as front-end readout of detectors for dosimetry and beam monitoring. The chip is suitable for measurements with both conventional radiotherapy accelerators (photon or electron beams) and with hadron accelerators (proton or light ion beams). As the chips might be located at few centimeters from the irradiation area and they are meant to be used in routine hospital practice, it is mandatory to assert their damage to both electromagnetic and neutron irradiation. We have tested a few chips on a X-ray beam and on thermal and fast neutron beams. Results of the tests are reported and an estimate of the expected lifetime of the chip for routine use is given.

The CNAO system to monitor and control hadron beams for therapy

Hadrotherapy might be the last chance option for patients with cancers growing deep in the body or surrounded by very sensitive organs. The Italian National Center of Oncological Hadrotherapy (CNAO) in Pavia is a synchrotron based center for the treatment of tumors with protons and carbon ion beams. The result of this sophisticated technique is strongly affected by the beam delivery performances. A powerful on-line system to monitor and deliver particles inside the target will be available at CNAO.

A large dynamic range charge measurement ASIC family for beam monitoring in radiotherapy applications

A family of Application Specific Integrated Circuits ( ASICs ) called TERA have been developed for the readout of pixel and strip gas detectors used in radiotherapy applications. The TERA ASICs are based on the charge balancing integration technique in order to obtain a good linearity over a dynamic range of five order of magnitude.

Online beam monitoring in the treatment of ocular pathologies at the INFN Laboratori Nazionali del Sud-Catania

A detector (MOPI) has been developed for the online monitoring of the beam at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA), where shallow tumours of the ocular region are treated with 62xa0MeV protons. At CATANA the beam is passively spread to match the tumour shape. The uniformity of the delivered dose depends on beam geometrical quantities which are checked before each treatment. However, beam instabilities might develop during the irradiation affecting the dose distribution. This paper reports on the use of the MOPI detector to measure the stability of the beam profile during the irradiation in the clinical practice. The results obtained in the treatment of 54 patients are also presented.

A treatment planning code for inverse planning and 3D optimization in hadrontherapy

The therapeutic use of protons and ions, especially carbon ions, is a new technique and a challenge to conform the dose to the target due to the energy deposition characteristics of hadron beams. An appropriate treatment planning system (TPS) is strictly necessary to take full advantage. We developed a TPS software, ANCOD++, for the evaluation of the optimal conformal dose. ANCOD++ is an analytical code using the voxel-scan technique as an active method to deliver the dose to the patient, and provides treatment plans with both proton and carbon ion beams. The iterative algorithm, coded in C++ and running on Unix/Linux platform, allows the determination of the best fluences of the individual beams to obtain an optimal physical dose distribution, delivering a maximum dose to the target volume and a minimum dose to critical structures. The TPS is supported by Monte Carlo simulations with the package GEANT3 to provide the necessary physical lookup tables and verify the optimized treatment plans. Dose verifications done by means of full Monte Carlo simulations show an overall good agreement with the treatment planning calculations. We stress the fact that the purpose of this work is the verification of the physical dose and a next work will be dedicated to the radiobiological evaluation of the equivalent biological dose.

SU‐FF‐T‐502: Biological Modeling of the Beam Delivery Line Components for Active Scan Irradiation Technique in Heavy‐Ion Radiotherapy

Purpose/Objective(s): The beam delivery line of the active scanning irradiation system for heavy‐ion radiotherapy requires a series of elements along the beam path before the patient like ripple filters and monitoring system. Usually these elements are optimized only for physical dose profiles. However, it is necessary to consider the biological effect and their impact on the dose distribution, especially in the Bragg peak region for a correct estimation of the peak spread. Materials/Methods: A full beam delivery line of the national center of oncologichadrontherapy (CNAO) is simulated with the Monte Carlo package GEANT4 to get the actual distribution in the treated volume of particles and fragments and the corresponding energies. The treated volume is simulated as well defining different tissues in the head and neck region. The evaluation of biological effects was studied using a code based on the Local Effect Model (LEM). The computational effort was performed using the distributed INFN Grid computing resources. Results: We estimate the impact on the relative biological effectiveness (RBE) and the biological dose distribution of the passive elements of the beam delivery line. The shape of the physical dose at Bragg peak is different from the corresponding biological dose. Though the transfer functions characterizing each of the element should involve a rigorous evaluation of the biological effects. Conclusions: A full characterization of the beam delivery line considering the biological impact provides a flexible tool in the treatment planning system for modeling the possible variations in the same beam line or modeling a new beam lines.

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.