A beam monitoring and validation system for continuous line scanning in proton therapy

Line scanning represents a faster and potentially more flexible form of pencil beam scanning than conventional step-and-shoot irradiations. It seeks to minimize dead times in beam delivery whilst preserving the possibility of modulating the dose at any point in the target volume. Our second generation proton gantry features irradiations in line scanning mode, but it still lacks a dedicated monitoring and validation system that guarantees patient safety throughout the irradiation. We report on its design and implementation in this paper. In line scanning, we steer the proton beam continuously along straight lines while adapting the speed and/or current frequently to modulate the delivered dose. We intend to prevent delivery errors that could be clinically relevant through a two-stage system: safety level 1 monitors the beam current and position every 10 μs. We demonstrate that direct readings from ionization chambers in the gantry nozzle and Hall probes in the scanner magnets provide required information on current and position, respectively. Interlocks will be raised when measured signals exceed their predefined tolerance bands. Even in case of an erroneous delivery, safety level 1 restricts hot and cold spots of the physically delivered fraction dose to  ±[Formula: see text] (±[Formula: see text] of [Formula: see text] biologically). In safety level 2-an additional, partly redundant validation step-we compare the integral line profile measured with a strip monitor in the nozzle to a forward-calculated prediction. The comparison is performed between two line applications to detect amplifying inaccuracies in speed and current modulation. This level can be regarded as an online quality assurance of the machine. Both safety levels use devices and functionalities already installed along the beamline. Hence, the presented monitoring and validation system preserves full compatibility of discrete and continuous delivery mode on a single gantry, with the possibility of switching between modes during the application of a single field.

[1]  A. Lomax,et al.  First experimental results of motion mitigation by continuous line scanning of protons , 2014, Physics in medicine and biology.

[2]  A. Brahme,et al.  Optimization of stationary and moving beam radiation therapy techniques. , 1988, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[3]  M. Phillips,et al.  Effects of respiratory motion on dose uniformity with a charged particle scanning method. , 1992, Physics in medicine and biology.

[4]  Radhe Mohan,et al.  The M. D. Anderson proton therapy system. , 2009, Medical physics.

[5]  Eros Pedroni,et al.  Pencil beam characteristics of the next-generation proton scanning gantry of PSI: design issues and initial commissioning results , 2011 .

[6]  M. Richter,et al.  Control system for cancer therapy with a heavy ion beam at GSI , 1999, 1999 IEEE Conference on Real-Time Computer Applications in Nuclear Particle and Plasma Physics. 11th IEEE NPSS Real Time Conference. Conference Record (Cat. No.99EX295).

[7]  C Bert,et al.  Motion in radiotherapy: particle therapy , 2011, Physics in medicine and biology.

[8]  Kevin W Eliceiri,et al.  NIH Image to ImageJ: 25 years of image analysis , 2012, Nature Methods.

[9]  D Meer,et al.  A study on repainting strategies for treating moderately moving targets with proton pencil beam scanning at the new Gantry 2 at PSI , 2010, Physics in medicine and biology.

[10]  Thomas Bortfeld,et al.  Beam delivery sequencing for intensity modulated proton therapy. , 2003, Physics in medicine and biology.

[11]  Roberto Sacchi,et al.  Design and characterization of the beam monitor detectors of the Italian National Center of Oncological Hadron-therapy (CNAO) , 2013 .

[12]  D C Weber,et al.  Assessing the quality of proton PBS treatment delivery using machine log files: comprehensive analysis of clinical treatments delivered at PSI Gantry 2 , 2016, Physics in medicine and biology.

[13]  Christoph Bert,et al.  Quantification of interplay effects of scanned particle beams and moving targets , 2008, Physics in medicine and biology.

[14]  D. Low,et al.  A technique for the quantitative evaluation of dose distributions. , 1998, Medical physics.

[15]  S. Psoroulas,et al.  Gantries and dose delivery systems , 2015 .

[16]  D. Convery,et al.  The generation of intensity-modulated fields for conformal radiotherapy by dynamic collimation , 1992 .

[17]  A J Lomax,et al.  Experimental verification of motion mitigation of discrete proton spot scanning by re-scanning , 2013, Physics in medicine and biology.

[18]  Martin Grossmann,et al.  More than 10 years experience of beam monitoring with the Gantry 1 spot scanning proton therapy facility at PSI. , 2009, Medical physics.

[19]  E. Pedroni,et al.  The 200-MeV proton therapy project at the Paul Scherrer Institute: conceptual design and practical realization. , 1995, Medical physics.

[20]  Eros Pedroni,et al.  Improving the precision and performance of proton pencil beam scanning , 2012 .

[21]  Yoshiyuki Iwata,et al.  Performance of the NIRS fast scanning system for heavy-ion radiotherapy. , 2010, Medical physics.

[22]  O. Actis,et al.  Precise On-line Position Measurement for Particle Therapy , 2014 .