M. Donetti

The CNAO dose delivery system for modulated scanning ion beam radiotherapy

PURPOSEnThis paper describes the system for the dose delivery currently used at the Centro Nazionale di Adroterapia Oncologica (CNAO) for ion beam modulated scanning radiotherapy.nnnMETHODSnCNAO 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.nnnRESULTSnThe 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.nnnCONCLUSIONSnThe 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.

PERFORMANCES OF A VLSI WIDE DYNAMIC RANGE CURRENT-TO-FREQUENCY CONVERTER FOR STRIP IONIZATION CHAMBERS

Abstract In this paper we report on the design and test of a 14-channel VLSI chip to perform the current to frequency conversion for parallel plate strip ionization chambers. The chambers measure the intensity and the geometrical characteristics of a therapeutical beam.

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.

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.

A VLSI analog pipeline read-out for electrode segmented ionization chambers ☆

Abstract We report on the design and test of a 32-channel VLSI chip based on the analog pipeline memory concept. The charge from a strip of a ionization chamber, is stored as a function of time in a switched capacitor array. The cell reading can be done in parallel with the writing.

SU‐FF‐T‐354: Penumbra Measurement with the Use of a 2D Pixel Ionization Chamber

Purpose: To attest the accuracy of the penumbra measurement with the use of a 2D pixel ionization chamber.Method and Materials: Penumbra measurements were carried out for several clinical cases, treated with Intensity Modulated Radiation Therapy, with a pixel ionization chamber and with radiographicfilm as reference. The pixel chamber consists of a matrix 1020 detectors, each 4.5 mm diameter and 5 mm height at a pitch of 7.62 mm. Each of the 1020 ionization chambers was read out independently with no dead time. Standard radiographicfilms have been used, scanned with a pitch of 0.16 mm and 0.40 mm. Both detectors were irradiated with 6 MV photon beams, using a linac equipped with a 120‐leaves multileaf collimator. Measured dose profiles were fitted with a “Fermi‐like” function: f(x) = 1 1 + e a(x−x 0 ) with x 0 and a as free parameters; the value for the penumbra could then be derived from the fit. The two sets of data were compared. Results: The data for the penumbra measured with the pixel chamber and the film for several IMRT fields show a strong linear correlation. This made it possible to correct the penumbra values obtained with the pixel ionization chamber to obtain a precise measurement even in a range well below the pixel dimensions. Conclusion: The 2D pixel ionization chamber has been shown to be able to measure the penumbra down to ∼ 2mm with an accuracy of ± 0.6 mm. This device can then efficiently replace radiographicfilm in some QA procedures with a significative gain in time as it gives results in real‐time and data are immediately available. Conflict of Interest: Research sponsored by MAESTRO, European project on Methods and Advanced Equipment for Simulation and Treatment in Radiation Oncology.

SU‐FF‐T‐336: Characterization of the Proton Beam of the Centre De Protontherapie D'Orsay with An On Line 2D Parallel Plate Ionization Chamber

Purpose: To monitor and to evaluate on line the parameters of the proton beam at the Centre de Protontherapie d Orsay (CPO) in France. Method and Materials: A parallel plate ionization chamber has been developed in the University and INFN of Torino in collaboration with the CPO; the detector has a sensitive area of 160 × 160 mm2, with the anode segmented in 1024 square pixels arranged in a matrix of 32 × 32; each pixel has an area of 5 × 5 mm2 and a pitch of 5.1 mm. The detector has a maximum rate transfer of 5 MHz and the reading of the entire chamber can be done at a frequency of some kHz without dead time. At the Centre de Protontherapie d Orsay (CPO) a synchrocyclotron is used for the treatment of eye melanoma since September 1991 and for braintumors since 1993. The dose delivery system is passive scattering and the proton energy delivered by the synchro‐cyclotron is 200 MeV which allows the treatment of lesions up to 22 cm deep in water. The pixel chamber has been placed along the CPO beam line to monitor the beam shape and to measure the stability and reproducibility of the delivery system. Results: Background measurements have been made to evaluate the pixel chamber noise; a procedure for the calibration of the detector has been beam applied that makes use of film measurements. Profiles and 2D dose distributions have been studied to see the uniformity of the delivery system and the reproducibility of the measures has been tested. Conclusion: A pixel chamber developed in Torino has been placed on the proton CPO beam line. This detector allows a fast, accurate and non‐intrusive 2D diagnosis of the beam both on‐ and off ‐line.