Purpose: This paper describes the system for the dose delivery 15 currently used at the Centro Nazionale di Adroterapia Oncologica (CNAO) for ion pencil beam scanning radiotherapy. Methods

A new hadron-therapy facility implementing an active beam scanning technique has been developed at the italian National Center of Oncological Hadron-therapy (CNAO). This paper presents the design and the characterization of the beam monitor detectors developed for the online monitoring and control of the dose delivered during a treatment at CNAO. The detectors are based on five parallel-plate transmission ionization chambers with either a single large electrode or electrodes segmented in 128 strips (strip chambers) and 32x32 pixels (pixel chamber). The detectors are arranged in two independent boxes with an active area larger than 200 x 200 mm 2 and a total water equivalent thickness along the beam path of about 0.9 mm. A custom front-end chip with 64 channels converts the integrated ionization channels without dead-time. The detectors were tested at the clinical proton beam facility of the Paul Scherrer Institut (PSI) which implements a spot scanning technique, each spot being characterized by a predefined number of protons delivered with a pencil beam in a specified point of the irradiation field. The short-term instability was measured by delivering several identical spots in a time interval of few tenths of seconds and is found to be lower than 0.3%. The non-uniformity, measured by delivering sequences of spots in different points of the detector surface, results to be lower than 1% in the single electrode chambers and lower than 1.5% in the strip and pixel chambers, reducing to less than 0.5% and 1% respectively in the restricted 100 x 100 mm 2 central area of the detector.

Purpose: Quasi-discrete scannig 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 quasi-discrete scanned beams is presented.Methods: The classical simulated annealing algorithm is used. It is a heuristic algorithm frequently used in combinatorial optimization problems, which allows to obtain nearly optimal solutions in acceptable running times.Results: 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-neighbour algorithm to generate the starting paths. Conclusions: The algorithm implemented for this work can optimize efficiently the scanning path of quasi-discrete scanned particle beams. Optimized scanning paths decrease the transit dose and lead to better dose distributions.

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Abstract. In hadron-therapy one of the most advanced methods for beam delivery is the active scanning technique which uses 37 fast scanning magnets to drive a narrow particle beam across the target. The Centro Nazionale di Adroterapia Oncologica 38(CNAO) will treat tumours with this technique. The CNAO scanning system includes two identical dipole magnets for horizontal 39 and vertical beam deflection, each one connected to a fast power supply. The dose delivery system exploits a set of monitor 40 chambers to measure the fluence and position of the beam and drives the beam during the treatment by controlling the sequence 41 of currents set by the power supplies. A test of the dynamic performance of the scanning system has been performed using a Hall 42 probe to measure the field inside the magnet and the results are presented in this paper. 43 44