Development and verification of the pulsed scanned proton beam at The Svedberg Laboratory in Uppsala

Development of Continuous Line Scanning System Prototype for Proton Beam Therapy

Purpose

Taking advantage of the continuous, high-intensity beam of the cyclotron at the National Cancer Center Hospital East, we developed a continuous line scanning system (CLSS) prototype for prostate cancer in collaboration with Sumitomo Heavy Industries, Ltd (Tokyo, Japan).

Materials and Methods

The CLSS modulates dose distribution at each beam energy level by varying scanning speed while keeping the beam intensity constant through a beam-intensity control system and a rapid on/off beam-switching system. In addition, we developed a beam alignment system to improve the precision of the beam position. The scanning control system is used to control the scanning pattern and set the value of the nozzle apparatus. It also collects data for monitoring and for cyclotron parameters and transmits information to the scanning power supplies and monitor amplifiers, which serve as the measurement system, and to the nozzle-control and beam-transfer systems. The specifications of the line scanning beam were determined in performance tests. Finally, a patient-specific dosimetric measurement for prostate cancer was also performed.

Results

The beam size, position, intensity, and scanning speed of our CLSS were found to be well within clinical requirements. The CLSS produced an accurate 3-dimensional dose distribution for clinical treatment planning.

Conclusion

The performance of our new CLSS was confirmed to comply with clinical requirements. We have been employing it in prostate cancer treatments since October 23, 2015.

Commissioning of the discrete spot scanning proton beam delivery system at the University of Texas M.D. Anderson Cancer Center, Proton Therapy Center, Houston

PURPOSE

To describe a summary of the clinical commissioning of the discrete spot scanning proton beam at the Proton Therapy Center, Houston (PTC-H).

METHODS

Discrete spot scanning system is composed of a delivery system (Hitachi ProBeat), an electronic medical record (Mosaiq V 1.5), and a treatment planning system (TPS) (Eclipse V 8.1). Discrete proton pencil beams (spots) are used to deposit dose spot by spot and layer by layer for the proton distal ranges spanning from 4.0 to 30.6 g/cm2 and over a maximum scan area at the isocenter of 30 x 30 cm2. An arbitrarily chosen reference calibration condition has been selected to define the monitor units (MUs). Using radiochromic film and ion chambers, the authors have measured spot positions, the spot sizes in air, depth dose curves, and profiles for proton beams with various energies in water, and studied the linearity of the dose monitors. In addition to dosimetric measurements and TPS modeling, significant efforts were spent in testing information flow and recovery of the delivery system from treatment interruptions.

RESULTS

The main dose monitors have been adjusted such that a specific amount of charge is collected in the monitor chamber corresponding to a single MU, following the IAEA TRS 398 protocol under a specific reference condition. The dose monitor calibration method is based on the absolute dose per MU, which is equivalent to the absolute dose per particle, the approach used by other scanning beam institutions. The full width at half maximum for the spot size in air varies from approximately 1.2 cm for 221.8 MeV to 3.4 cm for 72.5 MeV. The measured versus requested 90% depth dose in water agrees to within 1 mm over ranges of 4.0-30.6 cm. The beam delivery interlocks perform as expected, guarantying the safe and accurate delivery of the planned dose.

CONCLUSIONS

The dosimetric parameters of the discrete spot scanning proton beam have been measured as part of the clinical commissioning program, and the machine is found to function in a safe manner, making it suitable for patient treatment.

Energy spectrum control for modulated proton beams

In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to +/-21% over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than +/-3%. To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.

Parametrization and application of scatter kernels for modelling scanned proton beam collimator scatter dose

Collimators are routinely used in proton radiotherapy to laterally confine the field and improve the penumbra. Collimator scatter contributes up to 15% of the local dose and is therefore important to include in treatment planning dose calculation. We present a method for reconstruction of the collimator scatter phase space based on the parametrization of pre-calculated scatter kernels. Collimator scatter distributions, generated by the Monte Carlo (MC) package GEANT4.8.2, were scored differential in direction and energy. The distributions were then parametrized so as to enable a fast reconstruction by sampling. MC calculated dose distributions in water based on the parametrized phase space were compared to full MC simulations that included the collimator in the simulation geometry, as well as to experimental data. The experiments were performed at the scanned proton beam line at the The Svedberg Laboratory (TSL) in Uppsala, Sweden. Dose calculations using the parametrization of this work and the full MC for isolated typical cases of collimator scatter were compared by means of the gamma index. The result showed that in total 96.7% (99.3%) of the voxels fulfilled the gamma 2.0%/2.0 mm (3.0%/3.0 mm) criterion. The dose distribution for a collimated field was calculated based on the phase space created by the collimator scatter model incorporated into the generation of the phase space of a scanned proton beam. Comparing these dose distributions to full MC simulations, including particle transport in the MLC, yielded that in total for 18 different collimated fields, 99.1% of the voxels satisfied the gamma 1.0%/1.0 mm criterion and no voxel exceeded the gamma 2.6%/2.6 mm criterion. The dose contribution of collimator scatter along the central axis as predicted by the model showed good agreement with experimental data.

Monte Carlo simulations of neutron spectral fluence, radiation weighting factor and ambient dose equivalent for a passively scattered proton therapy unit

Stray neutron exposures pose a potential risk for the development of secondary cancer in patients receiving proton therapy. However, the behavior of the ambient dose equivalent is not fully understood, including dependences on neutron spectral fluence, radiation weighting factor and proton treatment beam characteristics. The objective of this work, therefore, was to estimate neutron exposures resulting from the use of a passively scattered proton treatment unit. In particular, we studied the characteristics of the neutron spectral fluence, radiation weighting factor and ambient dose equivalent with Monte Carlo simulations. The neutron spectral fluence contained two pronounced peaks, one a low-energy peak with a mode around 1 MeV and one a high-energy peak that ranged from about 10 MeV up to the proton energy. The mean radiation weighting factors varied only slightly, from 8.8 to 10.3, with proton energy and location for a closed-aperture configuration. For unmodulated proton beams stopped in a closed aperture, the ambient dose equivalent from neutrons per therapeutic absorbed dose (H*(10)/D) calculated free-in-air ranged from about 0.3 mSv/Gy for a small scattered field of 100 MeV proton energy to 19 mSv/Gy for a large scattered field of 250 MeV proton energy, revealing strong dependences on proton energy and field size. Comparisons of in-air calculations with in-phantom calculations indicated that the in-air method yielded a conservative estimation of stray neutron radiation exposure for a prostate cancer patient.

A beam source model for scanned proton beams

A beam source model, i.e. a model for the initial phase space of the beam, for scanned proton beams has been developed. The beam source model is based on parameterized particle sources with characteristics found by fitting towards measured data per individual beam line. A specific aim for this beam source model is to make it applicable to the majority of the various proton beam systems currently available or under development, with the overall purpose to drive dose calculations in proton beam treatment planning. The proton beam phase space is characterized by an energy spectrum, radial and angular distributions and deflections for the non-modulated elementary pencil beam. The beam propagation through the scanning magnets is modelled by applying experimentally determined focal points for each scanning dimension. The radial and angular distribution parameters are deduced from measured two-dimensional fluence distributions of the elementary beam in air. The energy spectrum is extracted from a depth dose distribution for a fixed broad beam scan pattern measured in water. The impact of a multi-slab range shifter for energy modulation is calculated with an own Monte Carlo code taking multiple scattering, energy loss and straggling, non-elastic and elastic nuclear interactions in the slab assembly into account. Measurements for characterization and verification have been performed with the scanning proton beam system at The Svedberg Laboratory in Uppsala. Both in-air fluence patterns and dose points located in a water phantom were used. For verification, dose-in-water was calculated with the Monte Carlo code GEANT 3.21 instead of using a clinical dose engine with approximations of its own. For a set of four individual pencil beams, both with the full energy and range shifted, 96.5% (99.8%) of the tested dose points satisfied the 1%/1 mm (2%/2 mm) gamma criterion.