Neutron interferometry using a single modulated phase grating.

Neutron grating interferometry provides information on phase and small-angle scatter in addition to attenuation. Previously, phase grating moiré interferometers (PGMI) with two or three phase gratings have been developed. These phase-grating systems use the moiré far-field technique to avoid the need for high-aspect absorption gratings used in Talbot-Lau interferometers (TLI) that reduce the neutron flux reaching the detector. We first demonstrate, through theory and simulations, a novel phase grating interferometer system for cold neutrons that requires a single modulated phase grating (MPG) for phase-contrast imaging, as opposed to the two or three phase gratings in previously employed PGMI systems. The theory shows the dual modulation of MPG with a large period and a smaller carrier pitch P, resulting in large fringes at the detector. The theory was compared to the full Sommerfeld-Rayleigh diffraction integral simulator. Then, we proceeded to compare the MPG system to experiments in the literature that use a two-phase-grating-based PGMI with best-case visibility of around 39%. The simulations of the MPG system show improved visibility in comparison to that of the two-phase-grating-based PGMI. An MPG with a modulation period of 300 µm, the pitch of 2 µm, and grating heights with a phase modulation of (π,0, illuminated by a monochromatic beam produces visibility of 94.2% with a comparable source-to-detector distance (SDD) as the two-phase-grating-based PGMI. Phase sensitivity, another important performance metric of the grating interferometer, was compared to values available in the literature, viz. the conventional TLI with the phase sensitivity of 4.5 × 103 for an SDD of 3.5 m and a beam wavelength of 0.44 nm. For a range of modulation periods, the MPG system provides comparable or greater theoretical maximum phase sensitivity of 4.1 × 103 to 10.0 × 103 for SDDs of up to 3.5 m. This proposed MPG system appears capable of providing high-performance PGMI that obviates the need for the alignment of two phase gratings.

A REVIEW OF ANALYTICAL MODELS OF STRAY RADIATION EXPOSURES FROM PHOTON- AND PROTON-BEAM RADIOTHERAPIES.

External-beam radiation therapy is safe, effective and widely used to treat cancer. With 5-year cancer survival for adults above 70%, increasingly research is focusing on quantifying and reducing treatment-related morbidity. Reducing exposures to healthy tissues is one strategy, which can be accomplished with advanced-technology radiotherapies, such as intensity-modulated photon therapy and proton therapy. Both of these modalities provide good conformation of the therapeutic dose to the tumor volume, but they also deliver stray radiation to the whole body that increases the risk of radiogenic second cancers. To minimize these risks, one needs to create and compare candidate treatment plans that explicitly take into account these risks. Currently, clinical practice does not include routine calculation of stray radiation exposure and, consequently, the assessment of corresponding risks is difficult. In this article, we review recent progress toward stray dose algorithms that are suitable for large-scale clinical use. In particular, we emphasize the current state of physics-based dose algorithms for intensity-modulated photon radiotherapy and proton therapy.

Evaluating the accuracy of a three-term pencil beam algorithm in heterogeneous media.

The goal of this work was to evaluate the accuracy of our in-house analytical dose calculation code against MCNPX data in heterogeneous phantoms. The analytical model utilizes a pencil beam model based on Fermi-Eyges theory to account for multiple Coulomb scattering and a least-squares fit to Monte Carlo data to account for nonelastic nuclear interactions as well as any remaining, uncharacterized scatter (the 'nuclear halo'). The model characterized dose accurately (up to 1% of maximum dose in broad fields (4 × 4 cm2 and 10 × 10 cm2) and up to 0.01% in a narrow field (0.1 × 0.1 cm2) fit to MCNPX data). The accuracy of the model was benchmarked in three types of stylized phantoms: (1) homogeneous, (2) laterally infinite slab heterogeneities, and (3) laterally finite slab heterogeneities. Results from homogeneous phantoms and laterally infinite slab heterogeneities showed high levels of accuracy (>98% of points within 2% or 0.1 cm distance-to-agreement (DTA)). However, because range straggling and secondary particle production were not included in our model, central-axis dose differences of 2-4% were observed in laterally infinite slab heterogeneities when compared to Monte Carlo dose. In the presence of laterally finite slab heterogeneities, the analytical model resulted in lower pass rates (>96% of points within 2% or 0.1 cm DTA), which was attributed to the use of the central-axis approximation.

The predicted relative risk of premature ovarian failure for three radiotherapy modalities in a girl receiving craniospinal irradiation.

In girls and young women, irradiation of the ovaries can reduce the number of viable ovarian primordial follicles, which may lead to premature ovarian failure (POF) and subsequently to sterility. One strategy to minimize this late effect is to reduce the radiation dose to the ovaries. A primary means of reducing dose is to choose a radiotherapy technique that avoids irradiating nearby normal tissue; however, the relative risk of POF (RRPOF) due to the various therapeutic options has not been assessed. This study compared the predicted RRPOF after craniospinal proton radiotherapy, conventional photon radiotherapy (CRT) and intensity-modulated photon radiotherapy (IMRT). We calculated the equivalent dose delivered to the ovaries of an 11-year-old girl from therapeutic and stray radiation. We then predicted the percentage of ovarian primordial follicles killed by radiation and used this as a measure of the RRPOF; we also calculated the ratio of the relative risk of POF (RRRPOF) among the three radiotherapies. Proton radiotherapy had a lower RRPOF than either of the other two types. We also tested the sensitivity of the RRRPOF between photon and proton therapies to the anatomic position of the ovaries, i.e., proximity to the treatment field (2 ≤ RRRPOF ≤ 10). We found that CRT and IMRT have higher risks of POF than passive-scattering proton radiotherapy (PRT) does, regardless of uncertainties in the ovarian location. Overall, PRT represents a lower RRPOF over the two other modalities.

WE-G-213CD-01: 4D Optimization for Scanned Ion Beam Tracking Therapy for Moving Tumors.

PURPOSE: To apply scanned ion radiotherapy to patients with moving tumors, motion mitigation approaches are needed. The purpose of the current study was to determine whether 4D optimized ion beam tracking therapy could reduce dose to critical structures near a moving target while maintaining target dose coverage. METHODS: A conjugate gradient minimization algorithm was developed to incorporate 4D organ motion data in the optimization of scanned ion pencil beam fluences. Treatment plans for 3D and 4D optimized carbon beam tracking were prepared using an in- house code for a sphere target volume moving in water with a proximal avoidance volume. For 1 lung cancer patient, 3D and 4D optimized carbon beam tracking treatment plans were designed to provide uniform dose coverage to a clinical target volume and minimal dose to the heart. For both the water phantom and patient case, 4D dose distributions were calculated. Differences between 3D and 4D optimized beam tracking were analyzed using dose colorplanes, dose-volume histograms, and dose-volume statistics. RESULTS: For the sphere target, 3D optimized beam tracking achieved target dose coverage of (100.0 ± 0.3)% and a mean and maximum avoidance volume dose of 26.1% and 89.4%, respectively. 4D optimized beam tracking achieved target dose coverage of (99.9 ± 0.4)% and a mean and maximum avoidance volume dose of 7.7% and 42.2%, respectively. For the lung patient, 3D optimized beam tracking achieved target dose coverage of (101.0 ± 5.9)% and a mean and maximum heart dose of 7.7%and 103.4%, respectively. 4D optimized beam tracking achieved target dose coverage of (100.0 ± 0.1)% and a mean and maximum heart dose of 8.7% and 100.3%, respectively. CONCLUSIONS: 4D optimized ion beam tracking therapy may be used to reduce the maximum dose to critical structures near a moving target, compared to 3D optimized ion beam tracking therapy. Work supported by the Rosalie B. Hite Fellowship, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.

Poster - Thur Eve - 36: Out-of-Field dose in craniospinal irradiation.

The risk of radiotherapy induced secondary cancer depends on the integral dose delivered to the patient where the dose delivered within the radiation field is accounted for, as well as dose to out-of-field organs from scattered and leakage radiation. While commercial treatment planning systems allow accurate determination of in-field dose, they are generally not capable of accurate out-of-field dose prediction. Secondary cancer risk is especially an issue in craniospinal treatments where involved patients are often children or young adults. In this work we therefore propose a mathematical model that accurately predicts out-of-field dose for patients treated by craniospinal irradiation at the American University of Beirut Medical Center. An anthropomorphic phantom was imaged, planned and treated, with thermoluminescent dosimeters inserted in the phantom at in-field and out-of-field locations. The measurements showed that our treatment planning system calculated accurately (within 2%) dose inside the field, but did not perform well at points just outside the field edge and consistently underestimated the dose at points further away from the field edge. From the out-of-field measured data, a model was developed that predicts out-of-field dose at a point in the patient based on the distance of that point to the treatment field edge. The developed model is of the double-gaussian type; it contains parameters that can be tuned to make it applicable in other centers where linac geometry and treatment techniques may differ.

SU-E-T-257: Risk of Radiogenic Second Cancer after Photon and Proton Craniospinal Irradiation.

PURPOSE: To compare proton and photon therapies in terms of the risks of second cancers for a pediatric medulloblastoma patient receiving craniospinal irradiation (CSI). METHODS: Two CSI treatment plans with 23.4 Gy or Gy (RBE) prescribed dose were computed for a 4-year-old boy withmedulloblastoma: a three-field 6-MV photon therapy plan and a four-field proton therapy plan. The primary doses for both plans were determined using a commercial treatment planning system. Stray radiation doses for proton therapy were determined from Monte Carlo simulations, and stray radiation doses for photon therapy were determined from measured data. The dose-risk model based on Biological Effects of Ionization Radiation VII report was used to estimate risk of second cancer. RESULTS: Baseline predictions of the relative risk of each organ were always less for proton CSI than for photon CSI after various follow-up years for the patient. The lifetime risks of the incidence of second cancer after proton CSI and photon CSI were 7.7% and 92%, respectively, and the ratio of lifetime risk was 0.083. Uncertainty analysis revealed the qualitative findings of this study were insensitive to any plausible changes of dose-risk models and mean neutron radiation weighting factor. CONCLUSIONS: Proton therapy confers lower predicted risk of second cancer for the pediatric medulloblastoma patient compared with photon therapy.

SU-E-T-535: Proton Dose Calculations in Homogeneous Media.

PURPOSE: To develop a pencil beam dose calculation algorithm for scanned proton beams that improves modeling of scatter events. METHODS: Our pencil beam algorithm (PBA) was developed for calculating dose from monoenergetic, parallel proton beams in homogeneous media. Fermi-Eyges theory was implemented for pencil beam transport. Elastic and nonelastic scatter effects were each modeled as a Gaussian distribution, with root mean square (RMS) widths determined from theoretical calculations and a nonlinear fit to a Monte Carlo (MC) simulated 1mm × 1mm proton beam, respectively. The PBA was commissioned using MC simulations in a flat water phantom. Resulting PBA calculations were compared with results of other models reported in the literature on the basis of differences between PBA and MC calculations of 80-20% penumbral widths. Our model was further tested by comparing PBA and MC results for oblique beams (45 degree incidence) and surface irregularities (step heights of 1 and 4 cm) for energies of 50-250 MeV and field sizes of 4cm × 4cm and 10cm × 10cm. Agreement between PBA and MC distributions was quantified by computing the percentage of points within 2% dose difference or 1mm distance to agreement. RESULTS: Our PBA improved agreement between calculated and simulated penumbral widths by an order of magnitude compared with previously reported values. For comparisons of oblique beams and surface irregularities, agreement between PBA and MC distributions was better than 99%. CONCLUSIONS: Our algorithm showed improved accuracy over other models reported in the literature in predicting the overall shape of the lateral profile through the Bragg peak. This improvement was achieved by incorporating nonelastic scatter events into our PBA. The increased modeling accuracy of our PBA, incorporated into a treatment planning system, may improve the reliability of treatment planning calculations for patient treatments. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

Intercomparision of Monte Carlo Radiation Transport Codes MCNPX, GEANT4, and FLUKA for Simulating Proton Radiotherapy of the Eye.

Monte Carlo simulations of an ocular treatment beam-line consisting of a nozzle and a water phantom were carried out using MCNPX, GEANT4, and FLUKA to compare the dosimetric accuracy and the simulation efficiency of the codes. Simulated central axis percent depth-dose profiles and cross-field dose profiles were compared with experimentally measured data for the comparison. Simulation speed was evaluated by comparing the number of proton histories simulated per second using each code. The results indicate that all the Monte Carlo transport codes calculate sufficiently accurate proton dose distributions in the eye and that the FLUKA transport code has the highest simulation efficiency.

Contribution to Neutron Fluence and Neutron Absorbed Dose from Double Scattering Proton Therapy System Components.

Proton therapy offers low integral dose and good tumor comformality in many deep-seated tumors. However, secondary particles generated during proton therapy, such as neutrons, are a concern, especially for passive scattering systems. In this type of system, the proton beam interacts with several components of the treatment nozzle that lie along the delivery path and can produce secondary neutrons. Neutron production along the beam's central axis in a double scattering passive system was examined using Monte Carlo simulations. Neutron fluence and energy distribution were determined downstream of the nozzle's major components at different radial distances from the central axis. In addition, the neutron absorbed dose per primary proton around the nozzle was investigated. Neutron fluence was highest immediately downstream of the range modulator wheel (RMW) but decreased as distance from the RMW increased. The nozzle's final collimator and snout also contributed to the production of high-energy neutrons. In fact, for the smallest treatment volume simulated, the neutron absorbed dose per proton at isocenter increased by a factor of 20 due to the snout presence when compared with a nozzle without a snout. The presented results can be used to design more effective local shielding components inside the treatment nozzle as well as to better understand the treatment room shielding requirements.

Assessment of the accuracy of an MCNPX-based Monte Carlo simulation model for predicting three-dimensional absorbed dose distributions.

In recent years, the Monte Carlo method has been used in a large number of research studies in radiation therapy. For applications such as treatment planning, it is essential to validate the dosimetric accuracy of the Monte Carlo simulations in heterogeneous media. The AAPM Report no 105 addresses issues concerning clinical implementation of Monte Carlo based treatment planning for photon and electron beams, however for proton-therapy planning, such guidance is not yet available. Here we present the results of our validation of the Monte Carlo model of the double scattering system used at our Proton Therapy Center in Houston. In this study, we compared Monte Carlo simulated depth doses and lateral profiles to measured data for a magnitude of beam parameters. We varied simulated proton energies and widths of the spread-out Bragg peaks, and compared them to measurements obtained during the commissioning phase of the Proton Therapy Center in Houston. Of 191 simulated data sets, 189 agreed with measured data sets to within 3% of the maximum dose difference and within 3 mm of the maximum range or penumbra size difference. The two simulated data sets that did not agree with the measured data sets were in the distal falloff of the measured dose distribution, where large dose gradients potentially produce large differences on the basis of minute changes in the beam steering. Hence, the Monte Carlo models of medium- and large-size double scattering proton-therapy nozzles were valid for proton beams in the 100 MeV-250 MeV interval.

Neutron shielding calculations in a proton therapy facility based on Monte Carlo simulations and analytical models: criterion for selecting the method of choice.

Proton therapy facilities are shielded to limit the amount of secondary radiation to which patients, occupational workers and members of the general public are exposed. The most commonly applied shielding design methods for proton therapy facilities comprise semi-empirical and analytical methods to estimate the neutron dose equivalent. This study compares the results of these methods with a detailed simulation of a proton therapy facility by using the Monte Carlo technique. A comparison of neutron dose equivalent values predicted by the various methods reveals the superior accuracy of the Monte Carlo predictions in locations where the calculations converge. However, the reliability of the overall shielding design increases if simulation results, for which solutions have not converged, e.g. owing to too few particle histories, can be excluded, and deterministic models are being used at these locations. Criteria to accept or reject Monte Carlo calculations in such complex structures are not well understood. An optimum rejection criterion would allow all converging solutions of Monte Carlo simulation to be taken into account, and reject all solutions with uncertainties larger than the design safety margins. In this study, the optimum rejection criterion of 10% was found. The mean ratio was 26, 62% of all receptor locations showed a ratio between 0.9 and 10, and 92% were between 1 and 100.

Patient neutron dose equivalent exposures outside of the proton therapy treatment field.

A large fraction of dose to healthy tissue located outside of the treatment field during proton therapy is attributable to neutrons produced in the beam-delivery apparatus. In this work, the neutron dose equivalent (H) per therapeutic proton absorbed dose (D) was estimated for typical treatment conditions as a function of range modulation width, angle with respect to the incident proton beam, and the distance from the isocentre at the Harvard Cyclotron Laboratory's (Cambridge, MA) passively spread treatment field using Monte Carlo simulations. For a beam with 16 cm penetration (depth) and a 5 x 5 cm2 lateral field size at the patient location along the incident beam direction at 100 cm from the isocentre, the predicted H/D values are 0.35 and 0.60 mSv Gy(-1) from the simulations and measurements, respectively. At all locations, the predicted H/D values are within a factor of 2 and 3 of the measured result for no modulation and 8.2 cm of modulation, respectively.

Virtual commissioning of a treatment planning system for proton therapy of ocular cancers.

The virtual commissioning of a treatment planning system (TPS) for ocular proton beam therapy was performed using Monte Carlo (MC) simulations and a model of a double-scattering ocular treatment nozzle. The simulations produced both the input data required by the TPS and the dose distributions to validate the analytical predictions from the TPS. An MC simulation of a typical ocular melanoma treatment was compared with the TPS predictions, revealing generally good agreement in the absorbed dose distribution. However, in the depth-dose profiles, differences >5% existed in the proximal region of all validation cases considered. Comparison of the radiation coverage at or above the 90% dose level, showed that MC calculated coverage was 82% and 68% of the coverage calculated by the TPS in two planes intersecting the tumour.

Neutron radiation area monitoring system for proton therapy facilities.

A neutron radiation area monitoring system has been developed for proton accelerator facilities dedicated to cancer therapy. The system comprises commercial measurement equipment, computer hardware and a suite of software applications that were developed specifically for use in a medical accelerator environment. The system is designed to record and display the neutron dose-equivalent readings from 16 to 24 locations (depending on the size of the proton therapy centre) throughout the facility. Additional software applications provide for convenient data analysis, plotting, radiation protection reporting, and system maintenance and administration tasks. The system performs with a mean time between failures of >6 months. Required data storage capabilities and application execution times are met with inexpensive off-the-shelf computer hardware.

Therapeutic step and shoot proton beam spot-scanning with a multi-leaf collimator: a Monte Carlo study.

In step and shoot spot-scanning, a small-diameter proton beam is magnetically swept and varied in energy in order to cover the tumour. Initial estimates of the beam size indicate that additional collimating hardware will be needed for lower energy proton beams in order to achieve a clinically acceptable lateral dose falloff at the edge of the proton beam. In this report, we present dosimetric data from Monte Carlo simulations with a model of a simple multileaf collimator which indicate that such a device may be used to improve the lateral dose falloff. The dosimetric quantities relevant to the clinical usefulness of the device are studied, including lateral penumbra, leaf transmission and scalloping effect. Multileaf collimation is compared with a differential spot-weighting technique of sharpening the lateral dose falloff.

Design tools for proton therapy nozzles based on the double-scattering foil technique.

Proton therapy has been increasing over the past several years, with several new treatment facilities being built in Europe, Japan and the United States. In this work, analytical and Monte Carlo tools were combined to model the passively scattered neurosurgery treatment beamline of the Harvard Cyclotron Laboratory (Cambridge, MA). The predicted three-dimensional dose distributions agree with actual measurements to within 0.1 mm for all quantities considered in central-axis depth-dose curve and to within 2.1 mm for all quantities considered in the absorbed dose cross-field profile. The predicted neutron dose equivalent per therapeutic absorbed dose, H/D, was calculated at various locations representing clinically significant anatomical sites. Under typical treatment conditions, the average ratio of predicted-to-measured H/D is 1.8 in the gonadal region (50 cm from isocentre) and 3.4 in the thyroid region (21 cm from isocentre). The global ratio of predicted-to-measured H/D is 2.6.

106Ru/106Rh plaque and proton radiotherapy for ocular melanoma: a comparative dosimetric study.

The objective of this study was to perform comparative dosimetric studies of both 106Ru/106Rh plaque brachytherapy and external beam proton therapy proposed for ocular treatments at the University of Texas M. D. Anderson Cancer Center, Houston, TX, USA. These modalities were also compared with traditional 125I plaque brachytherapy. Using a standardised eye model with a representative ocular melanoma tumour, the relative dose distributions within the tumour and surrounding tissue were calculated using the Monte Carlo code MCNPX. Published absorbed dose distributions benchmarked the Monte Carlo models. Results indicate that the proton beam provided superior dose uniformity within the tumour volume, whereas the dose distribution from 106Ru/106Rh was more heterogeneous. Relative to 125I COMS plaque, both 106Ru/106Rh and protons have shown more confined dose distributions to the tumour volume in this situation, thus sparing other critical ocular structures. For protons, it has been shown that only doses lower than the maximum dose are delivered outside the tumour volume. Depending on the clinical situation, this may aid in the sparing of critical structures located in the sclera and optic disc boundary. The Monte Carlo model's statistical uncertainties of the mean dose estimates for the 106Ru/106Rh plaque and proton beam were 3 and 2.5%, respectively.

Dosimetry for ocular proton beam therapy at the Harvard Cyclotron Laboratory based on the ICRU Report 59.

The Massachusetts General Hospital, the Harvard Cyclotron Laboratory (HCL), and the Massachusetts Eye and Ear Infirmary have treated almost 3000 patients with ocular disease using high-energy external-beam proton radiation therapy since 1975. The absorbed dose standard for ocular proton therapy beams at HCL was based on a fluence measurement with a Faraday cup (FC). A majority of proton therapy centers worldwide, however, use an absorbed dose standard that is based on an ionization chamber (IC) technique. The ion chamber calibration is deduced from a measurement in a reference 60Co photon field together with a calculated correction factor that takes into account differences in a chamber's response in 60Co and proton fields. In this work, we implemented an ionization chamber-based absolute dosimetry system for the HCL ocular beamline based on the recommendations given in Report 59 by the International Commission on Radiation Units and Measurements. Comparative measurements revealed that the FC system yields an absorbed dose to water value that is 1.1% higher than was obtained with the IC system. That difference is small compared with the experimental uncertainties and is clinically insignificant. In June of 1998, we adopted the IC-based method as our standard practice for the ocular beam.