Martin Grossmann

Experimental characterization and physical modelling of the dose distribution of scanned proton pencil beams

In this paper we present the pencil beam dose model used for treatment planning at the PSI proton gantry, the only system presently applying proton therapy with a beam scanning technique. The scope of the paper is to give a general overview on the various components of the dose model, on the related measurements and on the practical parametrization of the results. The physical model estimates from first physical principles absolute dose normalized to the number of incident protons. The proton beam flux is measured in practice by plane-parallel ionization chambers (ICs) normalized to protons via Faraday-cup measurements. It is therefore possible to predict and deliver absolute dose directly from this model without other means. The dose predicted in this way agrees very well with the results obtained with ICs calibrated in a cobalt beam. Emphasis is given in this paper to the characterization of nuclear interaction effects, which play a significant role in the model and are the major source of uncertainty in the direct estimation of the absolute dose. Nuclear interactions attenuate the primary proton flux, they modify the shape of the depth-dose curve and produce a faint beam halo of secondary dose around the primary proton pencil beam in water. A very simple beam halo model has been developed and used at PSI to eliminate the systematic dependences of the dose observed as a function of the size of the target volume. We show typical results for the relative (using a CCD system) and absolute (using calibrated ICs) dosimetry, routinely applied for the verification of patient plans. With the dose model including the nuclear beam halo we can predict quite precisely the dose directly from treatment planning without renormalization measurements, independently of the dose, shape and size of the dose fields. This applies also to the complex non-homogeneous dose distributions required for the delivery of range-intensity-modulated proton therapy, a novel therapy technique developed at PSI.

Intensity modulated proton therapy: A clinical example

In this paper, we report on the clinical application of fully automated three-dimensional intensity modulated proton therapy, as applied to a 34-year-old patient presenting with a thoracic chordoma. Due to the anatomically challenging position of the lesion, a three-field technique was adopted in which fields incident through the lungs and heart, as well as beams directed directly at the spinal cord, could be avoided. A homogeneous target dose and sparing of the spinal cord was achieved through field patching and computer optimization of the 3D fluence of each field. Sensitivity of the resultant plan to delivery and calculational errors was determined through both the assessment of the potential effects of range and patient setup errors, and by the application of Monte Carlo dose calculation methods. Ionization chamber profile measurements and 2D dosimetry using a scintillator/CCD camera arrangement were performed to verify the calculated fields in water. Modeling of a 10% overshoot of proton range showed that the maximum dose to the spinal cord remained unchanged, but setup error analysis showed that dose homogeneity in the target volume could be sensitive to offsets in the AP direction. No significant difference between the MC and analytic dose calculations was found and the measured dosimetry for all fields was accurate to 3% for all measured points. Over the course of the treatment, a setup accuracy of +/-4 mm (2 s.d.) could be achieved, with a mean offset in the AP direction of 0.1 mm. Inhalation/exhalation CT scans indicated that organ motion in the region of the target volume was negligible. We conclude that 3D IMPT plans can be applied clinically and safely without modification to our existing delivery system. However, analysis of the calculated intensity matrices should be performed to assess the practicality, or otherwise, of the plan.

The PSI Gantry 2: a second generation proton scanning gantry

PSI is still the only location in which proton therapy is applied using a dynamic beam scanning technique on a very compact gantry. Recently, this system is also being used for the application of intensity-modulated proton therapy (IMPT). This novel technical development and the success of the proton therapy project altogether have led PSI in Year 2000 to further expand the activities in this field by launching the project PROSCAN. The first step is the installation of a dedicated commercial superconducting cyclotron of a novel type. The second step is the development of a new gantry, Gantry 2. For Gantry 2 we have chosen an iso-centric compact gantry layout. The diameter of the gantry is limited to 7.5 m, less than in other gantry systems (approximately 10-12 m). The space in the treatment room is comfortably large, and the access on a fixed floor is possible any time around the patient table. Through the availability of a faster scanning system, it will be possible to treat the target volume repeatedly in the same session. For this purpose, the dynamic control of the beam intensity at the ion source and the dynamic variation of the beam energy will be used directly for the shaping of the dose.

Initial experience of using an active beam delivery technique at PSI

SummaryAt PSI a new proton therapy facility has been assembled and commissioned. The major features of the facility are the spot scanning technique and the very compact gantry. The operation of the facility was started in 1997 and the feasibility of the spot scanning technique has been demonstrated in practice with patient treatments. In this report we discuss the usual initial difficulties encountered in the commissioning of a new technology, the very positive preliminary experience with the system and the optimistic expectations for the future. The long range goal of this project is to parallel the recent developments regarding inverse planning for photons with a similar advanced technology optimized for a proton beam.

More than 10 years experience of beam monitoring with the Gantry 1 spot scanning proton therapy facility at PSI

PURPOSEnThe 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 authors experience in the beam monitoring technique for dynamic proton scanning.nnnMETHODSnThe 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.nnnRESULTSnThe 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.nnnCONCLUSIONSnThe 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.

A NOVEL GANTRY FOR PROTON THERAPY AT THE PAUL SCHERRER INSTITUTE

PSI has gained in the last few years the unique experience of using a proton therapy system based on a beam scanning delivery technique and on a compact gantry. This knowledge is now bringing forth new initiatives. We are continuously producing significant modifications and improvements to the present system, gantry 1. The major new step is however the decision of PSI to purchase a dedicated accelerator for the medical project. In the context of the expansion of the medical project of PSI (project PROSCAN) we have also started to plan the realisation of a second proton gantry, gantry 2. In this lecture we present the main ideas for the novel gantry, which will be based on one hand on the experience with the present technology, but on the other hand should be designed as a system more open to further developments and needs. The established and the future requirements for the beam delivery on the new gantry were routed into the specification list for the dedicated accelerator.

The exchange of radiotherapy data as part of an electronic patient-referral system

PURPOSEnTo describe the implementation and use of an electronic patient-referral system as an aid to the efficient referral of patients to a remote and specialized treatment center.nnnMETHODS AND MATERIALSnA system for the exchange of radiotherapy data between different commercial planning systems and a specially developed planning system for proton therapy has been developed through the use of the PAPYRUS diagnostic image standard as an intermediate format. To ensure the cooperation of the different TPS manufacturers, the number of data sets defined for transfer has been restricted to the three core data sets of CT, VOIs, and three-dimensional dose distributions. As a complement to the exchange of data, network-wide application-sharing (video-conferencing) technologies have been adopted to provide methods for the interactive discussion and assessment of treatments plans with one or more partner clinics.nnnRESULTSnThrough the use of evaluation plans based on the exchanged data, referring clinics can accurately assess the advantages offered by proton therapy on a patient-by-patient basis, while the practicality or otherwise of the proposed treatments can simultaneously be assessed by the proton therapy center. Such a system, along with the interactive capabilities provided by video-conferencing methods, has been found to be an efficient solution to the problem of patient assessment and selection at a specialized treatment center, and is a necessary first step toward the full electronic integration of such centers with their remotely situated referral centers.

Cancer Therapy with Protons at Paul Scherrer Institut

PSI has been actively involved in particle beam radiation therapy for many years. Conventional scattering technology has been used for eye tumors since 1981 with more than 4000 patients successfully treated. In 1996, PSI initiated the spot-scanning program for the treatment of deep seated tumors of various sizes with protons up to 220 MeV in a compact gantry. In 2006 a dedicated 250 MeV cyclotron for medical applications was commissioned and patient treatments restarted in 2007. Meanwhile a new Gantry 2 is being developed. It is based on double-parallel magnetic scanning and will make use of beam intensity modulation inside the cyclotron and of fast dynamic changes of the beam energy.

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.

Intensity modulated proton therapy: A first clinical example

By the end of 1999, over 40 patients will have been treated at the Paul Scherrer Institute with protons using the spot scanning technique [1]. For each of these treatments, an optimisation process has been used to calculate the weights of the many thousands of individually applied proton pencil beams which typically make up a single treatment port [2,3]. For the majority of these treatments however, the optimisation procedure has been applied on a field-by-field basis only, and in such a way as to ensure that a homogenous dose is applied across the target volume from each individual field.