I Veltchev

Development of a laser-driven proton accelerator for cancer therapy

Recent advances in laser technology have made proton (light ion) acceleration possible using laser-induced plasmas. In this work, we report our work for the last few years on the investigation of a new proton therapy system for radiation oncology, which employs laser-accelerated protons. If successfully developed, the new system will be compact, cost-effective, and capable of delivering energy-and intensity-modulated proton therapy (EIMPT). We have focused our research on three major aspects: (1) target design for laser-proton acceleration, (2) system design for particle/energy selection and beam collimation, and (3) dosimetric studies on the use of laser-accelerated protons for cancer therapy. We have performed particle-in-cell (PIC) simulations to investigate optimal target configurations for proton/ion acceleration. We also performed Monte Carlo simulations to study the beam characteristics and the feasibility of using such beams for cancer treatment. Since laser-accelerated protons have broad energy and angular distributions, which are not suitable for radiotherapy applications directly, we have designed a compact particle selection and beam collimating system for EIMPT beam delivery. We also proposed a new gantry design to make the whole system compact to retrofit existing linac vaults. We have compared Monte Carlo calculated dose distributions using X-ray IMRT and laser-proton EIMPT. Our results show that EIMPT using laser protons produces superior target coverage and much reduced critical structure dose and integral dose compared to X-ray IMRT.

Shielding design for a laser-accelerated proton therapy system.

In this paper, we present the shielding analysis to determine the necessary neutron and photon shielding for a laser-accelerated proton therapy system. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. A special particle selection and collimation device is needed to generate desired proton beams for energy- and intensity-modulated proton therapy. A great number of unwanted protons and even more electrons as a side-product of laser acceleration have to be stopped by collimation devices and shielding walls, posing a challenge in radiation shielding. Parameters of primary particles resulting from the laser-target interaction have been investigated by particle-in-cell simulations, which predicted energy spectra with 300 MeV maximum energy for protons and 270 MeV for electrons at a laser intensity of 2 x 10(21) W cm(-2). Monte Carlo simulations using FLUKA have been performed to design the collimators and shielding walls inside the treatment gantry, which consist of stainless steel, tungsten, polyethylene and lead. A composite primary collimator was designed to effectively reduce high-energy neutron production since their highly penetrating nature makes shielding very difficult. The necessary shielding for the treatment gantry was carefully studied to meet the criteria of head leakage <0.1% of therapeutic absorbed dose. A layer of polyethylene enclosing the whole particle selection and collimation device was used to shield neutrons and an outer layer of lead was used to reduce photon dose from neutron capture and electron bremsstrahlung. It is shown that the two-layer shielding design with 10-12 cm thick polyethylene and 4 cm thick lead can effectively absorb the unwanted particles to meet the shielding requirements.

Measurement comparison and Monte Carlo analysis for volumetric-modulated arc therapy (VMAT) delivery verification using the ArcCHECK dosimetry system

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric‐modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in‐house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10×10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10×10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (<1.2% drop) for the 2%–3% dose and 2 mm–3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3 mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation based on the sensitivity to detecting misalignments, the clinical impact of the angular dependency, and the correlation between the dose agreements in the peripheral region and the central region. This work also demonstrated the importance of carrying out a thorough validation of both the TPS and the dosimetry system prior to utilizing it for QA, and the value of having an independent dose calculation tool, such as the MC method, in clinical practice. PACS number: 87.55.Qr

Laser-to-proton energy transfer efficiency in laser–plasma interactions

It is shown that the energy of protons accelerated in laser-matter interaction experiments may be significantly increased through the process of splitting the incoming laser pulse into multiple interaction stages of equal intensity. From a thermodynamic point of view, the splitting procedure can be viewed as an effective way of increasing the efficiency of energy transfer from the laser light to protons, which peaks for processes having the least amount of entropy gain. It is predicted that it should be possible to achieve at least a 100% increase in the energy efficiency in a six-stage laser proton accelerator compared with a single laser-target interaction scheme.

Robotic radiosurgery system patient-specific QA for extracranial treatments using the planar ion chamber array and the cylindrical diode array

Robotic radiosurgery system has been increasingly employed for extracranial treatments. This work is aimed to study the feasibility of a cylindrical diode array and a planar ion chamber array for patient‐specific QA with this robotic radiosurgery system and compare their performance. Fiducial markers were implanted in both systems to enable image‐based setup. An in‐house program was developed to postprocess the movie file of the measurements and apply the beam‐by‐beam angular corrections for both systems. The impact of noncoplanar delivery was then assessed by evaluating the angles created by the incident beams with respect to the two detector arrangements and cross‐comparing the planned dose distribution to the measured ones with/without the angular corrections. The sensitivity of detecting the translational (1–3 mm) and the rotational (1°–3°) delivery errors were also evaluated for both systems. Six extracranial patient plans (PTV 7–137 cm3) were measured with these two systems and compared with the calculated doses. The plan dose distributions were calculated with ray‐tracing and the Monte Carlo (MC) method, respectively. With 0.8 by 0.8 mm2 diodes, the output factors measured with the cylindrical diode array agree better with the commissioning data. The maximum angular correction for a given beam is 8.2% for the planar ion chamber array and 2.4% for the cylindrical diode array. The two systems demonstrate a comparable sensitivity of detecting the translational targeting errors, while the cylindrical diode array is more sensitive to the rotational targeting error. The MC method is necessary for dose calculations in the cylindrical diode array phantom because the ray‐tracing algorithm fails to handle the high‐Z diodes and the acrylic phantom. For all the patient plans, the cylindrical diode array/ planar ion chamber array demonstrate 100%/>;92%(3%/3 mm) passing rates. The feasibility of using both systems for robotic radiosurgery system patient‐specific QA has been demonstrated. For gamma evaluation, 2%/2 mm criteria for cylindrical diode array and 3%/3 mm criteria for planar ion chamber array are suggested. The customized angular correction is necessary as proven by the improved passing rate, especially with the planar ion chamber array system. PACS number: 29.40.‐n

Development of Laser Accelerated Proton Beams for Radiation Therapy

Recent advances in laser technology have made proton (ion) acceleration possible using laser induced plasmas. In this presentation we will review the theoretical and experimental results of laser-proton acceleration for radiotherapy applications. We will report on our work progress in the development of a laser-proton therapy system at Fox Chase Cancer Center. The new proton therapy system is designed as a compact and cost-effective alternative to conventional accelerator based proton systems capable of delivering intensity-modulated proton therapy (IMPT). The specific aims of our research are: (1) target design for laser-proton acceleration, (2) system design for particle/energy selection and beam collimation, and (3) dosimetric studies on the use of laser-accelerated protons for cancer therapy. We have established a 150 TW laser system for preliminary experimental studies. We also patented a compact particle selection and beam collimating system for IMPT beam delivery and a new gantry design to make the whole system compact and easy to operate with adequate shielding considerations. Our Monte Carlo results show that IMPT using laser protons provided superior target coverage and much reduced critical structure dose and integral dose. IMPT is more dosimetrically advantageous than photon IMRT or conventional proton beams.

MO-FG-CAMPUS-JeP3-05: Evaluation of 4D CT-On-Rails Target Localization Methods for Free Breathing Liver Stereotactic Body Radiotherapy (SBRT)

PURPOSE Liver SBRT patients unable to tolerate breath-hold for radiotherapy are treated free-breathing with image guidance. Target localization using 3D CBCT requires extra margins to accommodate the respiratory motion. The purpose of this study is to evaluate the accuracy and reproducibility of 4D CT-on-rails in target localization for free-breathing liver SBRT. METHODS A Siemens SOMATOM CT-on-Rails 4D with Anzai Pressure Belt system was used both as the simulation and the localization CT. Fiducial marker was placed close to the center of the target prior to the simulation. Amplitude based sorting was used in the scan. Eight or sixteen phases of reconstructed CT sets (depends on breathing pattern) can be sent to Velocity to create the maximum intensity projection (MIP) image set. Target ITV and fiducial ITV were drawn based on the MIP image. In patient localization, a 4D scan was taken with the same settings as the sim scan. Images were registered to match fiducial ITVs. RESULTS Ten liver cancer patients treated for 50Gy over 5 fractions, with amplitudes of breathing motion ranging from 4.3-14.5 mm, were analyzed in this study. Results show that the Intra & inter fraction variability in liver motion amplitude significantly less than the baseline inter-fraction shifts in liver position. 90% of amplitude change is less than 3 mm. The differences in the D99 and D95 GTV dose coverage between the 4D CT-on-Rails and the CBCT plan were small (within 5%) for all the selected cases. However, the average PTV volume by using the 4D CT-on-Rails is 37% less than the CBCT PTV volume. CONCLUSION Simulation and Registration using 4D CT-on-Rails provides accurate target localization and is unaffected by larger breathing amplitudes as seen with 3D CBCT image registration. Localization with 4D CT-on-Rails can significantly reduce the PTV volume with sufficient tumor.

WE‐A‐BRB‐06: 3D In‐Patient Dose Reconstruction from the PET‐CT Imaging of Y‐90 Microspheres for Metastatic Cancer to the Liver

Purpose: The development of radioembolization with SIR‐Spheres represents a significant advance in the treatment of patients with metastatic disease to the liver. This technique has evolved to use a formula for dose calculation that relies on body surface area as the main determinant of dose. However, what is prescribed is not dose, but rather activity. It has been traditionally thought that the tracer 90Y is a pure electron emitter. However, the decay of 90Y has also a minor s+ branch to the first excited state of 90Zr, with 34 ppm branching ratio. While the positron emission is rare, it can be observed in a PET scan. Therefore the main objective of this work is to develop a new method for 3D dose reconstruction based on the PETimaging of a patient injected with SIR‐Spheres. Methods: Using Fluka Monte Carlo code, the voxel dose kernel (VDK) was calculated for 90Y source of size equal to that of the PET scanner voxel size. Subsequently, the convolution of the VDK with measured PET data was performed to recover the absorbed dose. The absolute dose calibration was done by taking the ratio of the measured positron activity to the known activity due to electrons Results: The developed model was used to reconstruct the dose distribution for two patients treated with 90Y microspheres. The 3D dose calculation was subsequently superimposed on the patients CT information to provide a complete description of the absorbed dose to the liver,tumor and adjacent structures. Conclusions: The proposed method offers significant improvement in characterization of the dose deposited by SIR‐Spheres with the hope that this may help answer some of the clinical questions concerning the influence of the dose distribution on the response rate, progression‐free or overall survival.

Applications of laser-accelerated particle beams for radiation therapy

Proton beams are more advantageous than high-energy photons and electrons for radiation therapy because of their finite penetrating range and the Bragg peak near the end of their range, which have been utilized to achieve better dose conformity to the treatment target allowing for dose escalation and/or hypofractionation to increase local tumor control, reduce normal tissue complications and/or treatment time/cost. Proton therapy employing conventional particle acceleration techniques is expensive because of the large accelerators and treatment gantries that require excessive space and shielding. Compact proton acceleration systems are being sought to improve the cost-effectiveness for proton therapy. This paper reviews the physics principles of laser-proton acceleration and the development of prototype laserproton therapy systems as a solution for widespread applications of advanced proton therapy. The system design, the major components and the special delivery techniques for energy and intensity modulation are discussed in detail for laser-accelerated proton therapy.

WE‐C‐BRB‐10: Acceleration of Protons by High‐Contrast Ultra‐Intense Laser Pulses

Purpose: To increase the proton energy generated in a laser‐plasma accelerator by pulse contrast improvement and minimization of phase distortions. Method and Materials: In a redesigned experiment the laser chain has been upgraded to a 150TWlevel and the pulse contrast has been improved by the implementation of double chirped pulse amplification (DCPA) and a cross‐polarized wave (XPW) modulation technique. This novel method of prepulse reduction has been shown to generate contrast levels of the order of 10−10‐10−11. In our new laser system stable XPWis achieved in two‐pass geometry on a single BBO crystal, making the setup compact and versatile. The laser pulse is subsequently stretched and amplified in an additional 4‐pass amplifier. High‐dynamic range third‐harmonic autocorrelator is used for pulse contrast evaluation. The wavefront distortions in the laser pulse are monitored by a 2D micro‐lens array detector. The acceleratedprotons are registered on a CR‐39 nuclear track detector behind a range filter (for energy measurement).Results: The implementation of the XPWtechnique led to an improvement of the prepulse contrast from 2×10−5 to 5×10−9. The “cleaner” laser pulse allowed us to use thinner targets ( 3.5 MeV). The wavefront distortions due to thermal effects in the amplifier crystals and astigmatism in our imaging systems are monitored and minimized (phase distortion <λ/3) in order to achieve optimum conditions for tight focusing. Conclusion:Protonacceleration in excess of 3.5 MeV has been experimentally demonstrated using a more powerful laser source with better control over the laser pulse parameters. Further experiments to optimize target parameters for higher proton energies are underway using our 150TW laser as a proof that upward scaling of the laser power in a controlled fashion can bring us into the range of therapeutically useful protons.