T Lin

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.

Uncertainties in IMRT dosimetry.

PURPOSE The purpose of this study is to investigate some characteristics of the beam delivery system and their effects on the dose distribution of intensity-modulated radiation therapy (IMRT) and the results of patient-specific IMRT quality assurance (QA). These characteristics include the accelerator source distribution and multileaf collimator (MLC) geometry. METHODS Monte Carlo dose calculations based on intensity maps that were built from the actual deliverable IMRT leaf sequences with and without considering the characteristics of the beam delivery system were performed in this study using in-house Monte Carlo software. The effect of the resolution of the intensity maps on the dose distribution was investigated first. The mean dose of the treatment target and the voxel doses in the high dose region of seven IMRT plans generated by different treatment planning systems for Varian 21EX and Siemens Primus linear accelerators were used for comparison and evaluation. RESULTS The results show that a 0.2×0.2mm2 or smaller pixel size is needed for the intensity maps for accurate IMRT dose calculation. The extrafocal source, MLC leaf thickness, leakage, tongue-and-groove structure, and the effective leaf offset can affect the mean dose by up to 1.5%, 4.5%, 5.6%, 5.3%, and 7.8%, respectively, when these factors are considered separately. They also cause significant uncertainties to the voxel dose with standard deviations up to 2.5%, 0.7%, 2.1%, 1.3%, and 5%, respectively. The overall effect on the mean dose is up to 8% and the standard deviation of the voxel dose uncertainty is up to 6.4% when all the effects are included. The maximum standard deviation is reduced to 4.6% if the voxel size of the dose calculation matrix is increased from 0.04 to 0.3cm3 to make it comparable with the sensitive volume of the ionization chamber used for IMRT QA measurements. CONCLUSIONS It can be concluded that the characteristics of the beam delivery system are the major contributors to the uncertainty of measurement-based IMRT QA because most of them are not fully considered in the currently available treatment planning systems.

Dosimetric investigation of high dose rate, gated IMRT

Increasing the dose rate offers time saving for IMRT delivery but the dosimetric accuracy is a concern, especially in the case of treating a moving target. The objective of this work is to determine the effect of dose rate associated with organ motion and gated treatment using step-and-shoot IMRT delivery. Both measurements and analytical simulation on clinical plans are performed to study the dosimetric differences between high dose rate and low dose rate gated IMRT step-and-shoot delivery. Various sites of IMRT plans for liver, lung, pancreas, and breast cancers were delivered to a custom-made motorized phantom, which simulated sinusoidal movement. Repeated measurements were taken for gated and nongated delivery with different gating settings and three dose rates, 100, 500, and 1000 MU/min using ion chambers and extended dose range films. For the study of the residual motion effect for individual segment dose and composite dose of IMRT plans, our measurements with 30%-60% phase gating and without gating for various dose rates were compared. A small but clinically acceptable difference in delivered dose was observed between 1000, 500, and 100 MU/min at 30%-60% phase gating. A simulation is presented, which can be used for predicting dose profiles for patient cases in the presence of motion and gating to confirm that IMRT step-and-shoot delivery with gating for 1000 MU/min are not much different from 500 MU/min. Based on the authors sample plan analyses, our preliminary results suggest that using 1000 MU/Min dose rate is dosimetrically accurate and efficient for IMRT treatment delivery with gating. Nonetheless, for the concern of patient care and safety, a patient specific QA should be performed as usual for IMRT plans for high dose rate deliveries.

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.

TU‐C‐AUD‐04: Laser‐Proton Acceleration for Radiation Therapy

Purpose: Rapid developments in laser technology have facilitated proton (light ion) acceleration using laser‐induced plasmas. In this work, we investigate an experimental system for laser‐accelerated proton therapy.Method and Materials: Our system consists of a commercial 150 TW laser, custom‐made laser‐pulse compression and target chambers, particle selection and beam collimating devices, dosimetry monitoring systems and shielding constructions. We have performed initial laser‐proton acceleration experiments with thin aluminum foils as target materials. The maximum protonenergy was measured using CR‐39 film and a Thomson parabola ion analyzer. We have performed particle‐in‐cell simulations to investigate the optimal laser parameters and target configurations to facilitate laser‐proton acceleration and dosimetric studies. Results: The primary particles resulting from the laser‐target interaction are protons and electrons. Our particle in cell simulation predicted protons of up to 300 MeV and electrons of 20 MeV for a laser intensity of 1021 W/cm2. The maximum number was 1011 and 1012 per pulse for protons and electron, respectively. Our initial testing with a 1018 W/cm2 laser intensity (at 10 TW) produced up to 1 MeV protons with a broad energy spectrum. Conclusion: We have developed an experimental laser‐proton accelerator for radiation therapy applications. Initial experimental studies have demonstrated proton acceleration at low laser power levels. Further studies with laser intensities up to 2 × 1020 W/cm2 are being conducted with different target materials and configurations.

SU-FF-T-146: Pulsed Reduced Dose-Rate Intensity Modulated Radiotherapy (IMRT) Delivery for Use in the High Dose Re-Irradiation Setting

Purpose: Pulsed reduced dose‐rateradiotherapy (PRDR) in the re‐irradiation setting has been reported in the literature. In an effort to reduce normal tissue toxicities a series of 0.2Gy pulses separated by 3 minute intervals are delivered for a time‐averaged dose rate of approximately 0.06Gy/min. We have combined PRDR with IMRT for increased conformality and normal tissue sparing. The purpose of this work was to explore the efficacy of this pairing. Method and Materials: The case presented represents a 31 year old patient with recurrent pancreatic cancer, previously treated with 3DCRT to 50.4Gy. Re‐irradiation of a 153cc PTV was planned to deliver 50Gy in 25 fractions via a 14 field, non‐coplanar IMRT plan on a Varian Trilogy, resulting in a total dose of 100.4Gy to parts of the target volume. The IMRT plan was delivered through 385 segments and 565 MU via the step‐and‐shoot method. MU linearity and a fluence map profile comparison were evaluated for both high and low dose rate settings. Due to the large number of low MU segments the treatment was delivered at 100MU/min to promote beam stabilization. A time‐averaged dose rate of approximately 0.06Gy/min was arrived at by inserting a 2 minute and 23 second time interval between the initiation of delivery for each beam. Results: The percentage of segments delivered using fractional MU between 1–2, 2–3, and 3–4 were 95.6%, 3.4% and 0.3%, respectively. Measured absolute dose and spatial distribution agreed to within 0.1% and within 3%/3mm DTA, respectively. Conclusions: A minimum of 10 beam directions are used to ensure relatively smooth intensity maps, adequate normal tissue sparing and that no aspect of the target volume is un‐irradiated for 2 consecutive pulses. With normal tissue sparing being the dominant endpoint for PRDR, IMRT is shown to provide an optimal and accurate delivery mechanism.

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.

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.

SU‐FF‐T‐244: Impact of the Isocenter Shift as a Function of Couch and Gantry Angles On the Stereotactic Radiosurgery (SRS) Dose

Purpose: The most important component of the pre‐treatment QA for radiosurgery is the verification of the target position in the beam. There are some generally accepted rules for the alignment test, e.g., the positioning differences should be within 1 mm or better. However, the impact on delivered dose of the shift in different directions during gantry and couch rotation may not be the same. Detailed investigations are desired to find out the relationship between the shift functions and the final dose distributions. Method and materials: In this study, the impact on the delivered dose was evaluated by Monte Carlo simulations using an EGS4‐based code MCSIM. The code was modified for arc therapy so that it can be used to do patient dose calculation for any given arc range and couch angle. A two‐step investigation has been carried out in this research. First, several assumed meaningful shift functions of gantry and couch movement were implemented into the Monte Carlo simulation to find out the dose impact from each component. Then actual shift functions based on measurements were used to evaluate the dose change due to the isocenter uncertainties for a real machine. A SRS plan for a braintumor (9 arcs with a 10 mm cone) was used in these simulations. Results and conclusions: Based on the results from the assumed shift functions and the measured shift functions, we found that the isodose line shift is generally less than 0.5 mm on our Trilogy and Primart, which is much smaller than the isocenter uncertainties. Also big differences were mainly found in the high dose region (>90% of the maximum dose). The isodose line shift at the typical dose prescription 2457_4level, e.g., 70% or 80%, has been reduced to 0.2∼0.3 mm, which is comparable to the imaginguncertainties.