David Meer

A study on repainting strategies for treating moderately moving targets with proton pencil beam scanning at the new Gantry 2 at PSI

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.

Improving the precision and performance of proton pencil beam scanning

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.

Proton beam monitor chamber calibration.

The first goal of this paper is to clarify the reference conditions for the reference dosimetry of clinical proton beams. A clear distinction is made between proton beam delivery systems which should be calibrated with a spread-out Bragg peak field and those that should be calibrated with a (pseudo-)monoenergetic proton beam. For the latter, this paper also compares two independent dosimetry techniques to calibrate the beam monitor chambers: absolute dosimetry (of the number of protons exiting the nozzle) with a Faraday cup and reference dosimetry (i.e. determination of the absorbed dose to water under IAEA TRS-398 reference conditions) with an ionization chamber. To compare the two techniques, Monte Carlo simulations were performed to convert dose-to-water to proton fluence. A good agreement was found between the Faraday cup technique and the reference dosimetry with a plane-parallel ionization chamber. The differences-of the order of 3%-were found to be within the uncertainty of the comparison. For cylindrical ionization chambers, however, the agreement was only possible when positioning the effective point of measurement of the chamber at the reference measurement depth-i.e. not complying with IAEA TRS-398 recommendations. In conclusion, for cylindrical ionization chambers, IAEA TRS-398 reference conditions for monoenergetic proton beams led to a systematic error in the determination of the absorbed dose to water, especially relevant for low-energy proton beams. To overcome this problem, the effective point of measurement of cylindrical ionization chambers should be taken into account when positioning the reference point of the chamber. Within the current IAEA TRS-398 recommendations, it seems advisable to use plane-parallel ionization chambers-rather than cylindrical chambers-for the reference dosimetry of pseudo-monoenergetic proton beams.

Preliminary investigations for the option to use fast uniform scanning with compensators on a gantry designed for IMPT

PURPOSE In 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. METHODS Uniform 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. RESULTS The 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. CONCLUSIONS The 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.

A novel beam optics concept in a particle therapy gantry utilizing the advantages of superconducting magnets.

PURPOSE A first order design of the beam optics of a superconducting proton therapy gantry beam is presented. The possibilities of superconducting magnets with respect to the beam optics such as strong fields, large apertures and superposition of different multipole fields have been exploited for novel concepts in a gantry. Since various techniques used in existing gantries have been used in our first design steps, some examples of the existing superconducting gantry designs are described and the necessary requirements of such a gantry are explained. METHODS The study of a gantry beam optics design is based on superconducting combined function magnets. The simulations have been performed in first order with the conventional beam transport codes. RESULTS The superposition of strong dipole and quadrupole fields generated by superconducting magnets enables the introduction of locally achromatic bending sections without increasing the gantry size. A rigorous implementation of such beam optics concepts into the proposed gantry design dramatically increases the momentum acceptance compared to gantries with normal conducting magnets. In our design this large acceptance has been exploited by the implementation of a degrader within the gantry and a potential possibility to use the same magnetic field for all energies used in a treatment, so that the superconducting magnets do not have to vary their fields during a treatment. This also enables very fast beam energy changes, which is beneficial for spreading the Bragg peak over the thickness of the tumor. CONCLUSIONS The results show an improvement of its momentum acceptance. Large momentum acceptance in the gantry creates a possibility to implement faster dose application techniques.

A comprehensive and efficient daily quality assurance for PBS proton therapy

There are several general recommendations for quality assurance (QA) measures, which have to be performed at proton therapy centres. However, almost each centre uses a different therapy system. In particular, there is no standard procedure for centres employing pencil beam scanning and each centre applies a specific QA program. Gantry 2 is an operating therapy system which was developed at PSI and relies on the most advanced technological innovations. We developed a comprehensive daily QA program in order to verify the main beam characteristics to assure the functionality of the therapy delivery system and the patient safety system. The daily QA program entails new hardware and software solutions for a highly efficient clinical operation. In this paper, we describe a dosimetric phantom used for verifying the most critical beam parameters and the software architecture developed for a fully automated QA procedure. The connection between our QA software and the database allows us to store the data collected on a daily basis and use it for trend analysis over longer periods of time. All the data presented here have been collected during a time span of over two years, since the beginning of the Gantry 2 clinical operation in 2013. Our procedure operates in a stable way and delivers the expected beam quality. The daily QA program takes only 20 min. At the same time, the comprehensive approach allows us to avoid most of the weekly and monthly QA checks and increases the clinical beam availability.

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.

The advantages and challenges of superconducting magnets in particle therapy

This paper provides an overview of the current developments in superconducting magnets for applications in proton and ion therapy. It summarizes the benefits and challenges regarding the utilization of these magnets in accelerating systems (e.g. superconducting cyclotrons) and gantries. The paper also provides examples of currently used superconducting particle therapy systems and proposed designs.

The impact of pencil beam scanning techniques on the effectiveness and efficiency of rescanning moving targets

Therapeutic pencil beams are typically scanned using one of the following three techniques: spot scanning, raster scanning or line scanning. While providing similar dose distributions to the target, these three techniques can differ significantly in their delivery time sequence. Thus, we can expect differences in effectiveness and time efficiency when trying to mitigate interplay effects using rescanning. At the Paul Scherrer Institute, we are able to irradiate treatment plans using either of the three delivery techniques. Hence, we can compare them directly with identical underlying machine parameters such as energy switching time or minimum/maximum beam current. For this purpose, we selected three different liver targets, optimized plans for spots, and converted them to equivalent raster and line scanning plans. In addition to the scanning technique, we varied the underlying motion curve, starting phase, prescription dose and rescanning strategy, which resulted in a total of 1584 4D dose calculations and 49 measurements. They indicate that rescanning becomes effective when achieving a high number of rescans for every dose element. Fixed minimum spot weights for spot and raster scanning machines often hamper this. By introducing adaptive scaling of the beam current within iso-energy layers for line scanning, we can flexibly lower the minimum weight whenever required and achieve higher rescanning capability. Averaged over all scenarios studied, volumetric rescanning is significantly more effective than layered provided the same number of rescans are applied. Fast lateral scanning contributes to the efficiency of rescanning. We observed that in any given time window, we can always perform more rescans using raster or line scanning compared to spot scanning irradiations. Thus, we conclude that line scanning represents a promising technique for rescanning by combining both effectiveness and efficiency.