Christian Bula

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. n nThe 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. n nThe 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. n nThe 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. n nThese 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. n nThe 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.

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

PURPOSEnIn 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.nnnMETHODSnUniform 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.nnnRESULTSnThe 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.nnnCONCLUSIONSnThe 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 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 tou2009u2009±[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.

A predictive algorithm for spot position corrections after fast energy switching in proton pencil beam scanning

PURPOSEnFast energy switching is of fundamental importance to implement motion mitigation techniques in pencil beam scanning proton therapy, allowing efficient irradiation and high patient throughput. However, depending on magnet design, when switching between different energy layers, eddy currents arise in the bending magnets yoke, damping the speed of the magnetic field change and lengthening the settling time of the magnetic field. In a proton therapy gantry, this can cause a temporary displacement of the beam trajectory and consequently an incorrect beam position in the bending direction, resulting in an unacceptable loss of position precision at isocenter. The precision can be recovered by either increasing the beam off time after an energy change (waiting until the magnetic field is fully settled) or by actively correcting for the misplacement. We studied the transient magnetic field effects at PSI Gantry 2 in order to develop a correction strategy for this beam position misplacement.nnnMETHODSnWe used position and proton range sensitive detectors (segmented strip chamber and multilayer ionization chambers respectively) to measure the difference between expected and actual proton beam position and range as a function of time. The detectors are automatically triggered, read out, and analyzed by the treatment control system. We studied the effects due to the magnets on the gantry and those upstream of the gantry separately, in order to identify which elements contribute the most to the beam position instability. We then designed a spot position algorithm to be applied with the gantry scanning magnets, to correct for the displacement observed as a function of time and achieve the PSI Gantry 2 clinical target of 1xa0mm precision at isocenter at all times, even after an energy change.nnnRESULTSnWhen switching energy layers in a field, we observed an exponentially decaying spot position displacement at isocenter. The effect increases with increasing energy difference between energy layers (ΔE). The initial residuals between expected and measured position are higher than 1xa0mm for most of the clinical cases at Gantry 2 and fall below 1xa0mm within about 1xa0s or more (depending on ΔE). We found no time dependence for the proton range, thus confirming that the displacement is purely due to a beam trajectory displacement resulting from the longer settling time of the magnetic field. A double exponential model, with two time constants and amplitudes depending on ΔE, fits the data and provides an easy model for the correction function. We implemented this correction as a spot position correction, applied by the scanning magnets during field application. After correction, the residuals were below 0.5xa0mm right after the energy change.nnnCONCLUSIONSnWe developed a spot position correction for PSI Gantry 2 which reduces the beam off time needed in current state-of-the-art gantries to settle the magnetic fields in the bending magnets. Thanks to this correction, the spot position is stable within 100xa0ms of an energy change at Gantry 2. This is low enough to make possible efficient use of motion mitigation techniques.

Generic FPGA-Based Platform for Distributed IO in Proton Therapy Patient Safety Interlock System

At the Paul Scherrer Institute (PSI) in Switzerland, cancer patients are treated with protons. Proton therapy at PSI has a long history and started in the 1980s. More than 30 years later, a new gantry has recently been installed in the existing facility. This new machine has been delivered by an industry partner. A big challenge is the integration of the vendor’s safety system into the existing PSI environment. Different interface standards and the complexity of the system made it necessary to find a technical solution connecting an industry system to the existing PSI infrastructure. A novel very flexible distributed IO system based on field-programmable gate array (FPGA) technology was developed, supporting many different IO interface standards and high-speed communication links connecting the device to a PSI standard versa module eurocard-bus input output controller. This paper summarizes the features of the hardware technology, the FPGA framework with its high-speed communication link protocol, and presents our first measurement results.