V Anferov

Clinical characterization of a proton beam continuous uniform scanning system with dose layer stacking

A proton beam delivery system on a gantry with continuous uniform scanning and dose layer stacking at the Midwest Proton Radiotherapy Institute has been commissioned and accepted for clinical use. This paper was motivated by a lack of guidance on the testing and characterization for clinical uniform scanning systems. As such, it describes how these tasks were performed with a uniform scanning beam delivery system. This paper reports the methods used and important dosimetric characteristics of radiation fields produced by the system. The commissioning data include the transverse and longitudinal dose distributions, penumbra, and absolute dose values. Using a 208 MeV cyclotrons proton beam, the system provides field sizes up to 20 and 30 cm in diameter for proton ranges in water up to 27 and 20 cm, respectively. The dose layer stacking method allows for the flexible construction of spread-out Bragg peaks with uniform modulation of up to 15 cm in water, at typical dose rates of 1-3 Gy/min. For measuring relative dose distributions, multielement ion chamber arrays, small-volume ion chambers, and radiographic films were employed. Measurements during the clinical commissioning of the system have shown that the lateral and longitudinal dose uniformity of 2.5% or better can be achieved for all clinically important field sizes and ranges. The measured transverse penumbra widths offer a slight improvement in comparison to those achieved with a double scattering beam spreading technique at the facility. Absolute dose measurements were done using calibrated ion chambers, thermoluminescent and alanine detectors. Dose intercomparisons conducted using various types of detectors traceable to a national standards laboratory indicate that the measured dosimetry data agree with each other within 5%.

Combined X-Y scanning magnet for conformal proton radiation therapy.

Light-ion beams have several features that make them very effective in radiation therapy applications. These include favorable depth dose distribution, finite penetration range, and high radiobiological efficiency. Moreover, magnetic scanning methods allow one to spread an ion beam to an exact image of a complex tumor shape. The ion scanning system usually consists of two magnets, each scanning horizontal and vertical directions independently. This paper discusses the design for a novel combined X-Y beam scanning magnet which is under development for the Midwest Proton Radiotherapy Institute.

Influence of beam efficiency through the patient-specific collimator on secondary neutron dose equivalent in double scattering and uniform scanning modes of proton therapy

PURPOSEnConventional proton therapy facilities use double scattering nozzles, which are optimized for delivery of a few fixed field sizes. Similarly, uniform scanning nozzles are commissioned for a limited number of field sizes. However, cases invariably occur where the treatment field is significantly different from these fixed field sizes. The purpose of this work was to determine the impact of the radiation field conformity to the patient-specific collimator on the secondary neutron dose equivalent.nnnMETHODSnUsing a WENDI-II neutron detector, the authors experimentally investigated how the neutron dose equivalent at a particular point of interest varied with different collimator sizes, while the beam spreading was kept constant. The measurements were performed for different modes of dose delivery in proton therapy, all of which are available at the Midwest Proton Radiotherapy Institute (MPRI): Double scattering, uniform scanning delivering rectangular fields, and uniform scanning delivering circular fields. The authors also studied how the neutron dose equivalent changes when one changes the amplitudes of the scanned field for a fixed collimator size.nnnRESULTSnThe secondary neutron dose equivalent was found to decrease linearly with the collimator area for all methods of dose delivery. The relative values of the neutron dose equivalent for a collimator with a 5 cm diameter opening using 88 MeV protons were 1.0 for the double scattering field, 0.76 for rectangular uniform field, and 0.6 for the circular uniform field. Furthermore, when a single circle wobbling was optimized for delivery of a uniform field 5 cm in diameter, the secondary neutron dose equivalent was reduced by a factor of 6 compared to the double scattering nozzle. Additionally, when the collimator size was kept constant, the neutron dose equivalent at the given point of interest increased linearly with the area of the scanned proton beam.nnnCONCLUSIONSnThe results of these experiments suggest that the patient-specific collimator is a significant contributor to the secondary neutron dose equivalent to a distant organ at risk. Improving conformity of the radiation field to the patient-specific collimator can significantly reduce secondary neutron dose equivalent to the patient. Therefore, it is important to increase the number of available generic field sizes in double scattering systems as well as in uniform scanning nozzles.

Scan pattern optimization for uniform proton beam scanning

Magnetic beam scanning allows one to spread proton beam over the desired radiation field area, improving beam utilization and conformity to the target area. This article discusses generic scan forms for generating uniform circular and rectangular fields and establishes criteria that can be applied to optimize selected scan patterns. During construction of the Midwest Proton Radiotherapy Institute (MPRI), Indiana University developed a magnetically scanned beam spreading system for the 3 m long gantry nozzle. Based on the commissioning experience, criteria for optimizing the scan patterns were derived. A numerical integration model was used to perform initial optimization of the resulting dose distribution. The selected scan patterns were then experimentally validated via test irradiation of Gafchromic films. Generic spiral and linear scan forms are proposed capable of delivering uniform circular and rectangular fields in continuous scanning mode. The test irradiations performed indicate that dose uniformity is within +/- 3% for both scan forms and that penumbra of the uncollimated field can approach the radius of the pristine beam spot. A well designed uniform scanning system can have a large library of uniform circular and rectangular fields of different sizes, which would increase beam utilization and minimize out-of-field dose to the patient.

Energy degrader optimization for medical beam lines

This paper describes the optimization of a variable energy degrader design for the Midwest Proton Radiotherapy Institute (MPRI) (The Indiana University Midwest Proton Radiation Institute, in: Proceedings of the 2001 Particle Accelerator Conference, PAC-2001, Chicago, 2002, p. 645). To optimize the energy degrader design we investigate the choice of an optimal material for the degrader, the beam emittance growth in the degrader, and the matching of the degraded beam with the acceptance of a medical beam line.

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.

SU-E-T-146: Beam Energy Spread Estimate Based On Bragg Peak Measurement

Purpose: ProNova is installing and commissioning a two room proton therapy system in Knoxville, TN. Beam energy out of the 230MeV cyclotron was measured on Jan 24, 2015. Cyclotron beam was delivered into a Zebra multi layered IC detector calibrated in terms of penetration range in water. The analysis of the measured Bragg peak determines penetration range in water which can be subsequently converted into proton beam energy. We extended this analysis to obtain an estimate of the beam energy spread out of the cyclotron. Methods: Using Monte Carlo simulations we established the correlation between Bragg peak shape parameters (width at 50% and 80% dose levels, distal falloff) and penetration range for a monoenergetic proton beam. For large uniform field impinging on a small area detector, we observed linear dependence of each Bragg peak parameter on beam penetration range as shown in Figure A. Then we studied how this correlation changes when the shape of Bragg peak is distorted by the beam focusing conditions. As shown in Figure B, small field size or diverging beam cause Bragg peak deformation predominantly in the proximal region. The distal shape of the renormalized Bragg peaks stays nearly constant. This excludes usage of Bragg peak width parameters for energy spread estimates. Results: The measured Bragg peaks had an average distal falloff of 4.86mm, which corresponds to an effective range of 35.5cm for a monoenergetic beam. The 32.7cm measured penetration range is 2.8cm less. Passage of a 230MeV proton beam through a 2.8cm thick slab of water results in a ±0.56MeV energy spread. As a final check, we confirmed agreement between shapes of the measured Bragg peak and one generated by Monte-Carlo code for proton beam with 0.56 MeV energy spread. Conclusion: Proton beam energy spread can be estimated using Bragg peak analysis.

TH‐B‐203‐01: Uniform Scanning and Energy Stacking with Proton Beams

Patients have received treatments with protonbeams using many different beam delivery techniques. The technique of uniform scanning with energy stacking has not often been discussed. The first part of this course will provide an introduction to the technique including terminology. Topics to be covered include: scanning modes and patterns; history of electron and light ion scanning beam use in the clinic; energy stacking methods; energy/range considerations; and radiobiology of scanned beams. Advantages and disadvantages of the technique will also be discussed along with a brief review of scanning electron beam incidents to underscore the importance of safety in the use of the technique. The second part of the course will provide a description of several potential hazards and some example mitigations for those hazards such as monitoring of the scanning magnet operation; monitoring of the delivered lateral fluence distribution; use of downstream MU detectors; MU rate checks; monitoring of the ratios of signals between MU detectors; and beam energy checks. The third part of the course will provide some practical aspects for using the technique. Optimizing the scan pattern to obtain a laterally uniform dose distribution and sharp penumbra includes consideration of the density of scan lines and the beam overscan distance beyond the collimator edges. Optimization of the weights of the individual energy stack layers to obtain a uniform depth dose distribution over the target includes consideration of the width of the peak of the non‐modulated depth dose distribution. Other parameters for which the user must be cognizant include the effective and virtual source distances for different magnet configurations and the minimum MU per layer which is a function of the MU rate and scanning frequency. The use of multi‐element detectors to efficiently measure dose distributions in the depth and lateral directions will additionally be addressed. Special considerations for quality assurance will also be discussed such as: the stability of spot size stability of scanning magnet operation; reproducibility of range modulation; and reproducibility of range shift at off‐axis positions. Learning Objectives: 1. Differentiate the uniform scanning and energy stacking technique from other beam delivery techniques. 2. Become familiar with the advantages disadvantages potential hazards and hazard mitigations associated with the technique. 3. Become familiar with methods required to implement the technique including optimization and quality assurance.

SU‐FF‐T‐462: Treatment of Retinoblastoma Using Different Proton Delivery Methods: Scattering, Rectangular Scanning, and Circular Scanning

Purpose: The high risk of radiation induced malignancy in retinoblastoma with conventional radiotherapy makes treatment with proton beams advantageous since it minimizes exposure of normal tissue in brain and orbit. Until recently, only scatteredproton beams were used. In this study we compare contaminating neutrondose from three different beam delivery methods: passive scattering, uniform scanning with a rectangular scan pattern, and uniform scanning with a circular scan pattern adjusted to the target size. In all three scenarios the same brass aperture was used. Methods: CT scans of three patients with retinoblastoma were selected for treatment planning. Patients were treated under anesthesia using an ocular suction cup. Two patients had bilateral, one had unilateral treatment. The plans were reproduced in a phantom using the three delivery methods described above. Dose distributions were measured with MAGIC gel placed inside the phantom with readout performed using MRI. Neutrondoses to the phantom were measured using Bubble detectors (Bubble Technology Industries™) and a SWENDI‐II (Thermo Electron Corporation™) detector. Results: In all three patients dose to the ethmoid bone and the frontal lobe was minimized by protons. When the optimized circular beam scanning pattern was used, secondary neutrondose was lower compared to both the rectangular scanning pattern and the passively scattered beam. For a 3 cm circular aperture, the secondary neutron component was 0.23 mSv/Gy for a 12×12 cm rectangular pattern. It was 80% lower for a 4cm optimized circular scan pattern and 15% higher for a 12cm diameter double scattered field. Conclusions: Delivery of proton beams using a scan pattern adjusted to the shape of the treatment field yields comparable dose distributions to conventional proton beam delivery but has the advantage of delivering less contaminating neutrondose to the patient.

SU‐FF‐T‐459: The Effect of Proton Therapy Beam Scanning Pattern Size On Secondary Neutron Dose Equivalent

Purpose: One way to deliver a proton therapy beam is to actively scan a uniform dose across a treatment field using a fast scanning magnet. In this study, we examine how proton therapy beam scanning patterns affect the scatteredneutron dose equivalent in an active scanning proton therapy beam delivery nozzle. Method and Materials: The nominal scanning pattern size is a 324 cm2 rectangular pattern which projects a flat proton field of 12 × 12 cm at isocenter for a 12 cm snout size. However, it is possible to reduce the size of this scanning pattern for field sizes smaller than 12 cm and maintain beam flatness while minimizing the scatteredneutron dose equivalent. Neutron measurements were taken using a SWENDI‐II (Thermo Electron Corporation™) neutron detector for a treatment field defined by apertures of 10 cm and 5 cm in diameter using protons of approximately 160 MeV. In each case, the field modulation in depth was 10 cm. A Scanditronix Matrixx™ panel was used to determine field flatness. Results: The maximum neutron dose equivalent was measured to be 0.65 mSv/Gy for the 10 cm field and 0.97 mSv/Gy for the 5 cm field 40 cm laterally from isocenter. By minimizing the rectangular scanning pattern, it was possible to reduce the secondary neutron dose equivalent by approximately 20% and 60% for the 10 cm and 5 cm diameter field sizes respectively and maintain acceptable treatment parameters. Conclusion: The efficiency of a proton treatment beam through the final patient collimator has a large influence on the creation of unwanted secondary neutrons. Increasing this efficiency using different scanning patterns minimizes neutron dose equivalent and maintains acceptable treatment parameters for the therapeuticproton beam.