M.A. Garella

The CNAO dose delivery system for modulated scanning ion beam radiotherapy

PURPOSE This paper describes the system for the dose delivery currently used at the Centro Nazionale di Adroterapia Oncologica (CNAO) for ion beam modulated scanning radiotherapy. METHODS CNAO Foundation, Istituto Nazionale di Fisica Nucleare and University of Torino have designed, built, and commissioned a dose delivery system (DDS) to monitor and guide ion beams accelerated by a dedicated synchrotron and to distribute the dose with a full 3D scanning technique. Protons and carbon ions are provided for a wide range of energies in order to cover a sizable span of treatment depths. The target volume, segmented in several layers orthogonally to the beam direction, is irradiated by thousands of pencil beams which must be steered and held to the prescribed positions until the prescribed number of particles has been delivered. For the CNAO beam lines, these operations are performed by the DDS. The main components of this system are two independent beam monitoring detectors, called BOX1 and BOX2, interfaced with two control systems performing the tasks of real-time fast and slow control, and connected to the scanning magnets and the beam chopper. As a reaction to any condition leading to a potential hazard, a DDS interlock signal is sent to the patient interlock system which immediately stops the irradiation. The essential tasks and operations performed by the DDS are described following the data flow from the treatment planning system through the end of the treatment delivery. RESULTS The ability of the DDS to guarantee a safe and accurate treatment was validated during the commissioning phase by means of checks of the charge collection efficiency, gain uniformity of the chambers, and 2D dose distribution homogeneity and stability. A high level of reliability and robustness has been proven by three years of system activity needing rarely more than regular maintenance and working with 100% uptime. Four identical and independent DDS devices have been tested showing comparable performances and are presently in use on the CNAO beam lines for clinical activity. CONCLUSIONS The dose delivery system described in this paper is one among the few worldwide existing systems to operate ion beam for modulated scanning radiotherapy. At the time of writing, it has been used to treat more than 350 patients and it has proven to guide and control the therapeutic pencil beams reaching performances well above clinical requirements. In particular, in terms of dose accuracy and stability, daily quality assurance measurements have shown dose deviations always lower than the acceptance threshold of 5% and 2.5%, respectively.

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.

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.

MATRIX: AN INNOVATIVE PIXEL IONIZATION CHAMBER FOR ON-LINE BEAM MONITORING IN HADRONTHERAPY

The control of beam position and dose delivery are key issues in the treatment of tumours using hadron beams, especially in the case of active dose distribution systems. In this framework, an innovative pixel ionization chamber has been designed and constructed for 2-D control of beam position, shape and intensity. This chamber is characterized by a 21 × 21 cm sensitive area subdivided in 1024 pixels of 6.5 × 6.5 mm. The 1024 channels are read-out by 8 boards based on 16 TERA06 chips. The sensitivity can be adjusted in the range 100800 fC. The tested minimum read-out time of the full chamber is 1 ms. After some preliminary measurements with X-rays, tests have been performed on the proton beam of the cyclotron of the Joint Research Centre in Ispra, Italy. In March 2005, MATRIX has been successfully tested on several beam lines for proton-therapy at the Loma Linda University Medical Center, US. The very positive results of these tests are presented. Figure 1: Schematic view of the MATRIX pixel ionization chamber.

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.