In this report we present the technical features of Gantry 2, the new second generation scanning system of PSI. On the basis of the experience and success with the first prototype, Gantry 1, built in the 90s for introducing pencil beam scanning and IMPT into the field of proton therapy, we have recently implemented a new system capable of offering much faster repainted conformal scanning for being able to treat moving targets with scanning under image guidance, the next challenge in the field of proton therapy.The new technical developments are conducted in parallel to the ongoing basic commissioning of Gantry 2, which should go into operation with usual discrete spot scanning for treating static targets in 2013.The innovative layout of Gantry 2 and the integration in the treatment area of the basic equipment for image guidance are presented. Noteworthy are the sliding CT within reach of the patient table and the unique new Beam's-Eye-View X-ray fluoroscopy system for taking images in the beam direction synchronized with the proton beam delivery.The first preliminary results with the development of much faster scanning modes look very encouraging. We can change the beam energy with the beam line within 80 ms for typical 0.5 cm range steps. We can deliver whole fluence-shaped energy layers within a time of the order of 100 ms. Dose lines are painted by changing the velocity of the scan magnets. The instantaneous dose rate of the pencil beam can be varied dynamically as well. The dose is precisely controlled with a feedback loop connecting the main gantry beam monitor with a vertical deflector plate at the ion source.These new fast scanning modes should be used for providing scanning with repainting, gating and tracking for treating moving targets. The goal is to develop pencil beam scanning as a universal beam delivery solution capable of treating optimally all possible clinical indications for proton therapy. Scanning could then completely replace the old beam delivery methods based on passive scattering from the market.The long term projects of Gantry 2 should represent the new contributions of PSI to the proton therapy field in the next 5-10 years, by providing direct translational cancer research from the physics laboratory into industry and clinics.

The MedAustron project realizes a synchrotron based accelerator facility in Austria for cancer treatment with protons and carbon ions, currently in the construction phase. In order to allow full patient treatment flexibility, one of the four treatment rooms will be equipped with a proton gantry. While its hardware design is a copy of the PSI Gantry 2, different constraints on the beam optics must be accounted for as MedAustron uses a synchrotron as particle accelerator and a rotator to match the beam optics into the rotated coordinate system, as compared to the cyclotron of the PSI PROSCAN facility. This paper presents the current status of the hardware design and procurement and a review of the design characteristics of the PSI Gantry 2 for the MedAustron case. In particular the stability of the beam parameters during beam scanning over the treatment scan area is investigated in detail. To achieve utmost parallel active scanning performance, the magnet design parameters (edge angles, corrector quadrupole, tapered dipole) have been optimized for PSI Gantry 2. Equivalent studies are undertaken for the MedAustron requirements and constraints in this paper.

PURPOSE:Proton radiography and tomography was investigated since the early 1970s because of its low radiation dose, high density resolution and ability to image directly proton stopping power. However, spatial resolution is still a limiting factor and as a consequence experimental methods and image reconstruction should be optimized to improve position resolution.METHODS:Spatial resolution of proton radiography and tomography is given by multiple Coloumb scattering (MCS) of the protons in the patient. In this paper we employ an improved MCS model to study the impact of various proton tomographic set-ups on the spatial resolution, such as different combinations of entrance and exit coordinate and angle measurements, respectively, initial particle energy and angular confusion of the incident proton field.RESULTS:It was found that best spatial resolution is obtained by measuring in addition to the entrance and exit coordinates also the entrance and exit angles. However, by applying partial backprojection and by using a perfect proton fan beam a sufficient spatial resolution can be achieved with less experimental complexity (measuring only exit angles). It was also shown that it is essential to use the most probable proton trajectory to improve spatial resolution. A simple straight line connection for image reconstruction results in a spatial resolution which is not clinically sufficient. The percentage deterioration of spatial resolution due to the angular confusion of the incident proton field is less than the phase space in mrad. A clinically realistic proton beam with 10 mrad angular confusion results in a less than 10% loss of spatial resolution.CONCLUSIONS:Clinically sufficient spatial resolution can be either achieved with a full measurement of entrance and exit coordinates and angles, but also by using a fan beam with small angular confusion and an exit angle measurement. It is necessary to use the most probable proton path for image reconstruction. A simple straight line connection is in general not sufficient. Increasing proton energy improves spatial resolution of an object of constant size. This should be considered in the design of proton therapy facilities.

In this paper we report on the main design features, on the realization process and on selected first results of the initial commissioning of the new Gantry 2 of PSI for the delivery of proton therapy with new advanced pencil beam scanning techniques. We present briefly the characteristics of the new gantry system with main emphasis on the beam optics, on the characterization of the pencil beam used for scanning and on the performance of the scanning system. The idea is to give an overview of the major components of the whole system. The main long-term technical goal of the new equipment of Gantry 2 is to expand the use of pencil beam scanning to the whole spectrum of clinical indications including moving targets. We report here on the initial experience and problems encountered in the development of the system with selected preliminary results of the ongoing commissioning of Gantry 2.

This article describes the main ideas underlying the design of the new Gantry 2 (G2) system of PSI. Based on a conceptually simple but very powerful scanning technology, the new second-generation scanning system is derived from the positive experience of using the previous gantry, Gantry 1 (G1). This article will provide a general overview on the various technological options faced when realizing such a new device. The reasoning for the present system will be discussed, and alternative solutions will be described, where appropriate.

PURPOSEIn this experimental study, the authors explored the possibility to deliver the dose for proton therapy with fast uniform scanning on a gantry primarily designed for the delivery of conformal beam scanning and IMPT. The uniform scanning submode has been realized without equipment modifications by using the same small pencil beam used for conformal scanning, resulting in reduced realization costs. Uniform scanning has recently been adopted in a few proton therapy centers, as a basic beam delivery solution, and as an alternative to the use of scattering foils. The option to use such a mode to mimic scattering on a full-fledged scanning gantry could be of interest for treating some specific indications and as a possible solution for treating moving targets.METHODSUniform iso-energy dose layers were painted by fast magnetic scanning alternated with fast energy changes with the gantry beam line. The layers were stacked and repainted appropriately to produce homogeneous three-dimensional dose distributions. A collimator∕compensator was used to adjust the dose to coincide laterally∕distally with the target volume. In addition, they applied volumetric repainting, since they are confident that this will further mitigate the effects of organ motion as compared with the presently used clinical scanning solutions. With the approach presented in this paper, they can profit from the higher flexibility of the scanning system to obtain additional advantages. For instance the shape of the energy layers can be adjusted to the projected target shape in order to reduce treatment time and neutrons produced in the collimator. The shape of the proximal layers can be shrunk, according to the cross section of the target at the corresponding range. This provides variable range modulation (proximal conformity) while standard scattering only provides fixed range modulation with unnecessary 100% dose proximal to the target. The field-specific hardware for a spherical target volume was mounted on the Gantry 2 nozzle. One field with proximal field size shrinking and one without, each of 1 Gy, were delivered. The dose distributions at different depths were recorded as CCD images of a scintillating screen.RESULTSThe time to scan the volume once was about 4 s and the total delivery time was approximately 30 s. For the field with proximal conformity, dose sparing of up to 25% was measured in the region proximal to the target. A repainting capability of 48 times was achieved on the most distal layer. The proximal layers were repainted more due to the contribution of the plateau dose from the deeper layers.CONCLUSIONSThe flexibility of a fast scanning gantry with very fast energy changes can easily provide beam delivery by uniform layer stacking with a significant degree of volumetric repainting and with the benefit of a dose reduction proximal to the target volume.

Proton therapy is a widely used method of cancer treatment. Undesired secondary particles such as neutrons accompany the irradiation. Depending on the measurement position, the expected main dose contribution due to undesired secondary particles arises from neutrons with energies >20 MeV. Conventionally used Andersson and Braun-type survey instruments may underestimate the ambient dose equivalent up to a factor of 2 due to their limited response for high-energy neutrons. Therefore, it is desirable to investigate the neutron stray field in conditions comparable to therapy treatment, in particular the resulting dose to equipment placed in the treatment vault to estimate possible consequences to its operation. The irradiation of a water phantom with 200-Me V protons adequately reproduces these conditions.

We propose an innovative method for proton radiography based on nuclear emulsion film detectors, a technique in which images are obtained by measuring the position and the residual range of protons passing through the patient's body. For this purpose, nuclear emulsion films interleaved with tissue equivalent absorbers can be used to reconstruct proton tracks with very high accuracy. This is performed through a fully automated scanning procedure employing optical microscopy, routinely used in neutrino physics experiments. Proton radiography can be used in proton therapy to obtain direct information on the average tissue density for treatment planning optimization and to perform imaging with very low dose to the patient. The first prototype of a nuclear emulsion based detector has been conceived, constructed and tested with a therapeutic proton beam. The first promising experimental results have been obtained by imaging simple phantoms.

Treating moving targets using a scanning gantry for proton therapy is a promising but very challenging, not yet clinically demonstrated treatment modality. The interference of organ motion with the sequence of the beam delivery produces uncontrolled dose inhomogeneities within the target. One promising approach to overcome this difficulty is to increase the speed of scanning in order to apply the dose repeatedly (so-called repainting). To obtain sufficiently high scanning speeds a new, technologically improved gantry—Gantry 2—has been designed and is currently under construction at PSI. As there are many possible repainting strategies, the way repainting will be implemented on Gantry 2 will depend on the result of a careful analysis of the various treatment delivery strategies available. To achieve this aim, and prior to the start of experimental work with Gantry 2, simulations of dose distribution errors due to organ motion under various beam delivery strategies were investigated. The effects of motion on the dose distribution were studied for moderate motion amplitudes (5 mm) for spherical target volumes in a homogeneous medium and with homogeneous dose. In total over 200 000 dose distributions have been simulated and analyzed and selected results are discussed. From the obtained results we are confident to be able to treat moderately moving targets on Gantry 2 using repainted pencil-beam spot scanning. Continuous line scanning seems to be the most elegant solution; it provides higher repainting rates and produces superior results but is probably more difficult to realize. For larger motion amplitudes, continuous line scanning still shows good results, but we plan anyways to use a gating system for these cases, not only to reduce the inhomogeneity within the target volume but also to reduce safety margins.

The absorbed dose to water is the reference physical quantity for the energy absorbed in tissue when exposed to beams of ionizing radiation in radiotherapy. The SI unit of absorbed dose to water is the gray (Gy = 1 J/kg). Ionization chambers are used as the dosimeters of choice in the clinical environment because they show a high reproducibility and are easy to use. However, ionization chambers have to be calibrated in order to convert the measured electrical charge into absorbed dose to water. In addition, protocols require these conversion factors to be SI traceable to a primary standard of absorbed dose to water. We present experimental results where the ionization chamber used for the dosimetry for the scanned proton beam facility at PSI is compared with the direct determination of absorbed dose to water from the METAS primary standard water calorimeter. The agreement of 3.2% of the dose values measured by the two techniques are within their respective statistical uncertainties.

This article reports on a 4D-treatment planning workshop (4DTPW), held on 7-8 December 2009 at the Paul Scherrer Institut (PSI) in Villigen, Switzerland. The participants were all members of institutions actively involved in particle therapy delivery and research. The purpose of the 4DTPW was to discuss current approaches, challenges, and future research directions in 4D-treatment planning in the context of actively scanned particle radiotherapy. Key aspects were addressed in plenary sessions, in which leaders of the field summarized the state-of-the-art. Each plenary session was followed by an extensive discussion. As a result, this article presents a summary of recommendations for the treatment of mobile targets (intrafractional changes) with actively scanned particles and a list of requirements to elaborate and apply these guidelines clinically.

PURPOSEThe beam monitoring equipments developed for the first PSI spot scanning proton therapy facility, Gantry 1, have been successfully used for more than 10 years. The purpose of this article is to summarize the author's experience in the beam monitoring technique for dynamic proton scanning.METHODSThe spot dose delivery and verification use two independent beam monitoring and computer systems. In this article, the detector construction, electronic system, dosimetry, and quality assurance results are described in detail. The beam flux monitor is calibrated with a Faraday cup. The beam position monitoring is realized by measuring the magnetic fields of deflection magnets with Hall probes before applying the spot and by checking the beam position and width with an ionization strip chamber after the spot delivery.RESULTSThe results of thimble ionization chamber dosimetry measurements are reproducible (with a mean deviation of less than 1% and a standard deviation of 1%). The resolution in the beam position measurement is of the order of a tenth of a millimeter. The tolerance of the beam position delivery and monitoring during scanning is less than 1.5 mm.CONCLUSIONSThe experiences gained with the successful operation of Gantry 1 represent a unique and solid background for the development of a new system, Gantry 2, in order to perform new advanced scanning techniques.

Thirty dogs with spontaneous tumors were irradiated with proton therapy using a novel spot scanning technique to evaluate the safety and efficacy of the system, and to study the acute and late radiation reactions. Nasal tumors, soft tissue sarcomas, and miscellaneous tumors of the head were treated with a median total dose of 52.5 Gy given in 3.5 Gy fractions. Acute effects, late effects, tumor response, and outcome were analyzed. No unexpected radiation reactions were seen, however two dogs did develop in-field osteosarcoma, and one dog developed in-field bone necrosis. Complete response to therapy was seen in 40% (12/30), partial response in 47% (14/30), and no response in 13% (4/30). Median survival for all dogs was 385 days (range of 14-4583 days). Dogs with nasal cavity tumors had a median survival of 385 days (range of 131-1851 days) and dogs with soft tissue sarcomas had a median survival time of 612 days (range of 65-4588 days). Treatment outcome was similar to historical controls. This new proton spot scanning technique proved to be safe and reliable.

Spot scanning based proton radiotherapy (PT) is one of the leading technologies of proton application. It has been pioneered at Paul Scherrer Institute (PSI), is in routine clinical use since 1996 and presently PSI remains the only facility with long-term experience. This analysis reviews the clinical status of safety and efficacy of spot scanning based PT at PSI. Between April 1996 and December 2008, 421 patients ranging in age between 1 and 88 years have been treated. Patient numbers per year have slowly increased and reached 106 patients in 2008 after transition to an all-year patient treatment facility. Most frequent tumor entities treated were chordomas and chondrosarcomas of skull base or axial skeleton (50%), soft tissue (STS), and osteogenic sarcomas (OGS; 15%), and meningiomas and primary CNS tumors (ependymomas, craniopharyngiomas, and gliomas; 18%). Skull base (including head and neck and orbital origin), CNS, paraspinal locations, and pelvis were the predominant anatomic sites. Between 1996 and 2005, treatments were delivered at 2 Gy (RBE) per fraction, 4 fractions per week due to limited beam availability. Since 2007, the newly dedicated cyclotron permitted year-round treatments at 1.8–2 Gy (RBE) per fraction, 5 fractions per week. Several analyses have recently been completed. Five-year actuarial results are available on >120 patients and >280 patients have been followed for >2 years. Specifically, 5-year actuarial local control rates (LC) for 64 patients with skull base tumors were 81% for chordomas (42 patients) and 94% for chondrosarcomas (22 patients) after mean target doses of 74 and 68 Gy (RBE), respectively. Five-year LC rates for chordomas of the entire spine were overall 66% (40 patients) and 100% (19 patients) for patients without major surgical stabilization. The LC rates for meningiomas were 92% at 3 years, and for STS 74% at 4 years. High-grade late toxicities in skull base (6%) and paraspinal locations (5%), were low given the high doses employed. Fifty-one children and infants (67% requiring anesthesia) were treated between 2004 and 2007, the majority for primary CNS tumors or STS. Preliminary LC rates of 86%% at medium follow-up of 29 months were achieved. All pediatric patients were prospectively enrolled in quality of life and long-term toxicity assessment studies. Safety and efficacy of spot scanning based PT has been demonstrated at PSI in more than 400 patients, including adult and pediatric patients with various histologies at various anatomic sites. One of the future challenges will be its expansion to other indications, notably motion sensitive tumor entities.