Dose Beam Profiles Research Articles

Radio-fluorogenic nanoclay gel dosimeters with reduced linear energy transfer dependence for carbon-ion beam radiotherapy.

The precise assessment of the dose distribution of high linear energy transfer (LET) radiation remains a challenge, because the signal of most dosimeters will be saturated due to the high ionization density. Such measurements are particularly important for heavy-ion beam cancer therapy. On this basis, the present work examined the high LET effect associated with three-dimensional gel dosimetry based on radiation-induced chemical reactions. The purpose of this study was to create an ion beam radio-fluorogenic gel dosimeter with a reduced effect of LET. Nanoclay radio-fluorogenic gel (NC-RFG) dosimeters were prepared, typically containing 100μM dihydrorhodamine 123 (DHR123) and 2.0wt% nanoclay together with catalytic additives promoting Fenton or Fenton-like reactions. The radiological properties of NC-RFG dosimeters having different compositions in response to a carbon-ion beam were investigated using a fluorescence gel scanner. An NC-RFG dosimeter capable of generating a fluorescence intensity distribution reflecting the carbon-ion beam dose profile was obtained. It was clarified that the reduction of the unfavorable LET dependence results from an acceleration of the reactions between DHR123 and H2 O2 , which is a molecular radiolysis product. The effects of varying the preparation conditions on the radiological properties of these gels were also examined. The optimum H2 O2 catalyst was determined to include 1mM Fe3+ ions, and the addition of 100mM pyridine was also found to increase the sensitivity. This technique allows the first-ever evaluation of the depth-dose profile of a carbon-ion beam at typical therapeutic levels of several Gy without LET effect.

Neural Representation of Linear Accelerator Beam Data from a Single Reference Dataset for Fast Commissioning and Quality Assurance

<h3>Purpose/Objective(s)</h3> We hypothesize that linear accelerator (Linac) beam properties including lateral profiles and percentage depth dose (PDD) can be modeled through a neural representation network using a single vendor-provided reference beam dataset. Such beam model can be used to predict clinical Linac beam data distributions from sparse beam measurements, which has the potential of supporting fast commissioning and routine quality assurance (QA) in the clinic. <h3>Materials/Methods</h3> A sinusoidal neural representation network that consists of multilayer perceptron and periodic activation function was used to learn a mapping between 2D coordinates (1D spatial locations x field sizes) during beam scanning and the corresponding percentage dose. The network was trained using a vendor-provided 6 MV reference beam dataset only and evaluated using clinical beam datasets that consist of ten 6MV beam profiles at 10 cm depth and seven 6MV percentage depth dose (PDD) curves collected from multiple clinics. Only beam data with 30 × 30 cm² field size were assumed available and used to update the network weights. Beam data with 10 × 10 cm² were then predicted by the network and compared to the ground truth of clinically acquired data by calculating both the relative mean absolute error and the 1D gamma passing rate with a passing criteria of 1%/1 mm. <h3>Results</h3> Compared to vendor-provided reference beam data, the network-predicted beam data showed improved consistency with the clinically acquired data. The averaged mean relative absolute error and averaged gamma passing rate between network-predicted dose profile/reference beam dose profile and the clinical measurements were 1.9%/2.5% and 95%/90% respectively. The averaged mean relative absolute error and averaged gamma passing rate between network-predicted PDD/reference beam PDD and the clinical measurements were 0.6%/0.5% and 100%/97% respectively. <h3>Conclusion</h3> Neural representation network is capable of modeling inherent Linac beam properties and predicting clinical beam data accurately from sparse beam measurements. The proposed beam model has the potential of simplifying the clinical beam scanning process with much reduced beam data collection.

Delta4 Discover transmission detector: A comprehensive characterization for in-vivo VMAT monitoring.

To investigate the dosimetric behaviour, influence on photon beam fluence and error detection capability of Delta4 Discover transmission detector. The transmission detector (TRD) was characterized on a TrueBeam linear accelerator with 6 MV beams. Linearity, reproducibility and dose rate dependence were investigated. The effect on photon beam fluence was evaluated in terms of beam profiles, percentage depth dose, transmission factor and surface dose for different open field sizes. The transmission factor of the 10x10 cm2 field was entered in the TPS's configuration and its correct use in the dose calculation was verified recalculating 17 clinical IMRT/VMAT plans. Surface dose was measured for 20 IMRT fields. The capability to detect different delivery errors was investigated evaluating dose gamma index, MLC gamma index and leaf position of 15 manually modified VMAT plans. TRD showed a linear dependence on MU. No dose rate dependence was observed. Short-term and long-term reproducibility were within 0.1% and 0.5%. The presence of the TRD did not significantly affect PDDs and profiles. The transmission factor of the 10x10 cm2 field size was 0.985 and 0.983, for FF and FFF beams respectively. The 17 recalculated plans met our clinical gamma-index passing rate, confirming the correct use of the transmission factor by the TPS. The surface dose differences for the open fields increase for shorter SSDs and greater field size. Differences in surface dose for the IMRT beams were less than 2%. Output variation ≥2%, collimator angle variations within 0.3°, gantry angle errors of 1°, jaw tracking and leaf position errors were detected. Delta4 Discover shows good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. It is also capable to detect dosimetric and geometric errors and therefore it is suitable for monitoring VMAT delivery.

On-line beam monitoring and dose profile measurements of a [formula omitted]Pb beam of 150 GeV/n with a liquid-filled ionization chamber array

In this study, the possibility of on-line monitoring of an ultra-high-energy (UHE) beam of 150 GeV/n lead (208Pb) ions using a liquid-filled ionization chamber array (OCTAVIUS 1000SRS, PTW Freiburg, Germany) is evaluated. The applicability of the array has been tested for the investigated beam, where adequate adjustments of the array have been made for measurements under very low fluxes of 102 to 104 ions / cm2/s. The results have been compared to measurements obtained using the standard beam line detectors, such as a scintillator and a delay wire chamber (DWC). Since these detectors were situated further upstream from the experimental position, measurements with these devices may not be representative of the beam characteristics at setup due to the interactions of the primary beam particles within these detectors and the subsequent transport of these particles between these detectors and the setup. The main advantage of the application of the liquid-filled array is its portability so that it can be placed directly at the position of the experimental setup to capture the beam characteristics. A comparison between the measured beam profile and independent film measurements has demonstrated its reliability for analysing the beam characteristics. In this context, the 2D array has been implemented to measure on-line not only the 2D beam profile but also the total imparting particle fluence of the UHE lead beam by using the measured dose and considering the calibration of the detector.

A Simple Approach for Experimental Characterization and Validation of Proton Pencil Beam Profiles

A precise characterization of therapeutic proton pencil beams is essential for commissioning of any treatment planning system (TPS). The dose profile characterization includes measurement of the beam lateral dose profile in the beam core and far from the beam core, in the so called low-dose envelope, and requires a sophisticated detection system with a few orders of magnitude dynamic range. We propose to use a single-quantum sensitive Minipix Timepix detector, along with an in-house designed holder to perform measurements of the pencil beam dose profile in air and in water. We validated the manufacturer calibration of the Minipix Timepix detector in proton beams of various energies and compared the deposited energy spectra to Monte Carlo (MC) simulations. The precision of the lateral dose profile measurements has been systematically validated against Krakow proton facility commissioning data and dose profile simulations performed with MC codes Gate/Geant4 and Fred. We obtained an excellent agreement between Minipix Timepix measurements and simulations demonstrating the feasibility of the system for a simple characterization and validation of proton pencil beams. The proposed approach can be implemented at any proton therapy facility to acquire experimental data needed to commission and validate analytical and MC based TPS.

Influence of heterogeneous media on Very High Energy Electron (VHEE) dose penetration and a Monte Carlo-based comparison with existing radiotherapy modalities

In this paper we investigate the dosimetric effects target inhomogeneities on VHEE (Very High Energy Electron) beam dose profiles. Dose profile sensitivity to high and low density (0.001-2.2g/cm3) embedded media in water was measured for 156 MeV VHEE beams. To the knowledge of the authors, this is the first systematic experiment on dose penetration in inhomogeneous targets at this high energy electron beams. The results of the dosimetry experiments were compared with simulation results acquired with the TOPAS/GEANT4 Monte Carlo codes and were compared with the sensitivity of photon and proton beams.Dosimetry experiments with VHEE beams at 156 MeV were performed by embedding various density inserts in a cuboid 30×30×10 cm3 water phantom. This setup was then irradiated with a narrow (σ=1.2 mm) Gaussian VHEE beams. Transverse dose profiles were recorded at various depths within the water phantom using radosensitive EBT-XD Gafchromic films. Simulations were performed using the TOPAS user wrap for the GEANT4 Monte Carlo toolkit with the measured beam and target geometry parameters from the CLEAR facility and compared with sensitivity of photon and protons beams to inhomogeneities in water phantoms.Less than 8% dose deviation was shown for all measured dose data across all recorded depths (0–20 cm). Monte Carlo simulations within the whole water phantom volume showed a 15% dose deviation. Simulation studies indicated that using proton and photon beams within a therapeutic energy range in MC simulations of this experimental setup resulted in dose change in the central plane up to 100% and 74% of the dose maximum in the central plane respectively.The dosimetry results of these experiments and TOPAS/GEANT4 simulations compare well. Dose profiles of VHEE beams were found to be relatively insensitive (<15% dose difference across the central plane) to embedded high and low density geometries compared to simulation results with therapeutic proton beams (up to 100% of dose maximum in the plane). Therefore, VHEE is robust to anatomical changes and has the potential to be a reliable mode of radiotherapy for treating tumors in highly inhomogeneous and mobile regions such as the lung.

The comparison of spatial resolution of ion chamber, diode, and EBT3 film based on the dose profile

Background: It is essential for the physicist to quantify the dose of the radiation prior and during cancer treatment to ensure proper execution of the treatment plan. The most common way to measure the dose is utilizing Farmer chamber. The limitation of Farmer chamber is that it only provides dose of a specific point, while the beam dose profile is needed for the planning system. The Gafchromic EBT3 film can provide a quantitative dose profile in a 2D space. The EBT3 film has low energy dependence, similar effective atomic number to water, and self-developing; making it an ideal candidate for dose profile measurement. Methods: Dose profiles of EBT3 was measured, analyzed and compared to semiflex chamber and EDGE detector under same conditions. The EBT3 films were placed at the center of RW3 water phantom (30×30×30 cm 3 ) under flattening filter free (FFF) Mode of Linac. The EBT3 films were calibrated by additional RW3 with a Farmer chamber under the stacked RW3 water phantom. Percentage depth dose (PDD) and Matlab were used to fit the dose curve of EBT3 and the results are presented. Results: The penumbra of the measured dose profile through EBT3 film demonstrated little differences when comparing with that of the EDGE detector and noticeable differences when compared to that of the semiflex chamber. For a field size of 2×2 cm 2 , (b=0.08, depth of 5 cm, beam off central axis 0.8 cm), the measured difference of dose profile between EBT3 and EDGE detector is –2.24%. Under the same conditions, the dose profile difference between EBT3 and semiflex chamber is 10.59%. Conclusions: This study shows that EDGE detector has the highest spatial resolution to the dose profile. EBT3 film in combination with solid phantom can quickly sample dose profile in 2D. EBT3 film can be considered to be an excellent quality assurance (QA) tool with accurate dosimetry, a good spatial resolution and tolerable dose uncertainty.

Proton beam behavior in a parallel configured MRI-proton therapy hybrid: Effects of time-varying gradient magnetic fields.

Real-time magnetic resonance (MR) guidance is of interest to various groups globally because the superior soft tissue contrast MR images offer over other x-ray-based imaging modalities. Because of the precision required in proton therapy, proton therapy treatments rely heavily on image guidance. Integrating a magnetic resonance imaging (MRI) into a proton therapy treatment is a challenge. The charged particles (protons) used in proton therapy experience magnetic forces when travelling through the MRI magnetic fields. Given that it is desired that proton beams can be delivered with submillimeter accuracy, it is important that all potential sources of beam displacement are well modeled and understood. This study investigated the behavior of monoenergetic proton beams in the presence of a simulated set of realistic three-dimensional (3D) vector magnetic gradient fields required for spatial localization during imaging. This deflecting source has not been previously investigated. Three-dimensional magnetic vector fields from a superconducting 0.5 T open bore MRI magnet model (previously developed in-house) and 3D magnetic fields from an in-house gradient coil model were applied to two types of computer simulations. In all simulations, monoenergetic proton pencil beams (from 80 to 250 MeV) were used. The initial directions of proton beams were varied. In all simulations, the orientation of the B0 field coincided with the positive z-axis in the simulation geometry. The first type of simulation is based on an analytic magnetic force equation (analytic simulations) while the second type is a full Monte Carlo (MC) simulation. The analytic simulations were limited to propagating the proton beams in vacuum but could be rapidly calculated in a desktop computer while the MC simulations were calculated in a cluster computer. The proton beam locations and dose profiles at the central plane (z = 0 cm) with or without magnetic fields were extracted and used to quantify the effect of the presence of the different magnetic fields on the proton beam. The analytic simulations agree with MC results within 0.025 mm, thus acting as the verification of MC calculations. The presence of the B0 field caused the beam to follow a helical trajectory which resulted in angular offsets of 4.9o , 3.6o , and 2.8o for the 80, 150, and 250 MeV, respectively. Magnetic field deflections caused by a rapid MRI sequence (bSSFP, with maximum gradient strength of 40 mT/m) show a pattern of distortion which remained spatially invariant in the MR's field of view. For the 80 MeV beam, this pattern shows a maximum ranged in the y direction of 1.5 mm. The presence of the B0 field during the bSSFP simulations adds the same beam rotation to the observed during the B0 only simulations. This investigation reveals that time-varying gradient magnetic fields required for image generation can cause a small spread in the proton beams used in the study which are independent of the effects arising from the B0 field. Further, studies where clinical beam kernels were convolved with this spread show that these magnetic fields are expected to have an insignificant impact on the beam's entrance dose.

Monte Carlo Simulation of Siemens Primus plus Linac for 6 and 18 MV Photon Beams.

The aim of the present study is to simulate 6 MV and 18 MV photon beam energies of a Siemens Primus Plus medical linear accelerator (Linac) and to verify the simulation by comparing the results with the measured data. The main components of the head of Siemens Primus Plus linac were simulated using MCNPX Monte Carlo (MC) code. To verify the results, experimental data of percentage depth dose (PDD) and beam dose profile for 5 × 5 cm2, 10 × 10 cm2 and 20 × 20 cm2 field sizes were measured and compared with simulation results. Moreover, gamma function was used to compare the measurement and simulation data. The results show a good agreement, within 1%, was observed between the data calculated by the simulations and those obtained by measurement for 6 MV photon beam, while it was within 2% for 18 MV photon beam, except in the build-up region for both beams. Gamma index values were less than unity in most data points for all the mentioned energies and fields. To calculate the dose in the phantom, cells were selected in different modes, one of the modes due to the lack of dose gradient and overlapping, produced better results than others produce. There was good settlement between measured and MC simulation values in this research. The simulation programs can be used for photon modes of Siemens Primus Plus linac in conditions in which it is not possible to perform experimental measurements.

Validation of Monte Carlo simulation of 6 MV photon beam produced by Varian Clinac 2100 linear accelerator using BEAMnrc code and DOSXYZnrc code

The Monte Carlo model for the photon-beam output from the Varian Clinac 2100 linear accelerator was validated to compare the calculated to measured PDD and beam dose profiles The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible. The Monte Carlo codes used in this work were the BEAMnrc code to simulate the photons beam and the DOSXYZnrc code to examinate the absorbed dose in the water phantom. We have calculated percentage depth dose (PDD) and beam profiles of the 6 MV photon beam for the 6 × 6 cm2, 10 × 10 cm2 and 15 × 15 cm2 field sizes. We have used the gamma index technique for the quantitative evaluation to compare the measured and calculated distributions. Good agreement was found between calculated PDD and beam profile compared to measured data. The comparison was evaluated using the gamma index method and the criterions were 3% for dose difference and 3 mm for distance to agreement. The gamma index acceptance rate was more than 97% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. The Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc and DOSXYZnrc codes package.

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom.

Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Two cylindrical electromagnets were used to produce a 0.207T magnetic field parallel to the central axis of a 6MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2mm γ acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field Dmax ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (>0.13T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately calculate the PDDs with these surface and buildup dose modifications in a homogeneous phantom.

Study of 6 MV Photon Beam Dose Profile Peaks, Investigation and Evaluation of the Backscattering Effects on the Beam Dose Profiles

The dose profile peaks are mainly produced by the flattening filter of the head of the medical linear accelerator due to the backscattering effects on the beam dose profiles; in this study we have investigated the backscattering effects on the beam profile peaks; the purpose was to evaluate the backscattering effects at the beam edge due to the flattening filter shape by calculating the increasing relative dose on the beam profile peaks as functions of depth along the central axis and filed size. The accent is on the photon beam dose profiles at different depths in a water phantom along central beam axis for a set of irradiation field sizes. The measures of dose profiles of the 6 MV photon beam in a water phantom, source to surface distance is 100 cm, for the set of the irradiation field sizes of 3×3 cm², 6×6 cm², 10×10 cm², 15×15 cm², and 20×20 cm² at different depths on the beam central axis, the linear accelerator is VARIAN Clinac 2100. First, we have plotted the dose profiles as function of the normalized depth to show the beam profile peaks; then we have calculated the increasing relative dose on the beam profile peaks as a function of the field size and depth along central beam axis from the measurements data. Then we have normalized the increasing relative dose on the beam profile peaks to the increasing relative dose on the beam profile peak of the standard filed size of 10×10 cm². We have plotted the results for interpretation and evaluation the backscattering effects on the beam dose profiles; the pervious study have shown that the flattening filter produced a flat profile at a depth of 10 cm and beam profile peaks; and in our study we have quantified the increasing relative dose on the beam profile peaks; then, we have shown that the variation of the normalized increasing relative dose presented the peaks at depth of 10 cm along the central beam axis, e.g. We have found that the increasing relative dose at depth of 10 cm along central beam axis for the field size of 25×25 cm² is almost 20 times of the standard field size at the same depth, but the increasing relative dose is very close for both filed sizes at a depth of 20 cm along central beam axis. Throughout this work we have use a method to evaluate the backscattering effects on the dose profiles; we have found the backscattering effects are very high at a depth 10 cm for the large irradiation field size.

Study of 6 MV Photon Beam Dose Profiles and Investigation of Jaw Motion Effects on the Beam Dose Profiles and the Dose Delivered in a Water Phantom

This study investigated the medical linear accelerator jaw motion effects on the delivered dose. The focus was to examinate the photon beam dose profiles as a function of the jaw motion and depending on the depth in the water phantom along the central beam axis and the irradiation field sizes. The measurements of dose profiles were performed for a 6 MV photon beam in a water phantom, the field sizes of 3×3 cm², 4×4 cm², 6×6 cm², 8×8 cm², 10×10 cm², 12×12 cm², 15×15 cm², 20×20 cm², 25×25 cm², 30×30 cm², and 35×35 cm² at depth of 1.5 cm, 5 cm, 10 cm, 20 cm, and 30 cm on the central beam axis, and source-to-surface distance (SSD) of 100 cm for Varian Clinac 2100 linear accelerator. The purpose of this study was to investigate the jaw motion effects on the dose profiles and subsequently on the delivered dose depending on the field size and depth in the water phantom and also the beam dose profiles’ symmetry about the central beam axis. We proceeded to calculate two parameters: the normalized width and the difference of the width between the left and right width of dose profiles as a function of the field size and depth along the central beam axis. We have shown that the motion of the jaws to define the irradiation field size have impacts on the beam dose profiles and its symmetry. the spread of the width difference was in a wave motion along the central beam axis and within a range of off-axis distance from -2 mm to 2 mm, which it should be considered in the treatment of tumors and spare the healthy cells.

SU‐F‐T‐428: An Optimization‐Based Commissioning Tool for Finite Size Pencil Beam Dose Calculations

Purpose:Finite size pencil beam (FSPB) algorithms are commonly used to pre‐calculate the beamlet dose distribution for IMRT treatment planning. FSPB commissioning, which usually requires fine tuning of the FSPB kernel parameters, is crucial to the dose calculation accuracy and hence the plan quality. Yet due to the large number of beamlets, FSPB commissioning could be very tedious. This abstract reports an optimization‐based FSPB commissioning tool we have developed in MatLab to facilitate the commissioning.Methods:A FSPB dose kernel generally contains two types of parameters: the profile parameters determining the dose kernel shape, and a 2D scaling factors accounting for the longitudinal and off‐axis corrections. The former were fitted using the penumbra of a reference broad beam's dose profile with Levenberg‐Marquardt algorithm. Since the dose distribution of a broad beam is simply a linear superposition of the dose kernel of each beamlet calculated with the fitted profile parameters and scaled using the scaling factors, these factors could be determined by solving an optimization problem which minimizes the discrepancies between the calculated dose of broad beams and the reference dose.Results:We have commissioned a FSPB algorithm for three linac photon beams (6MV, 15MV and 6MVFFF). Dose of four field sizes (6*6cm2, 10*10cm2, 15*15cm2 and 20*20cm2) were calculated and compared with the reference dose exported from Eclipse TPS system. For depth dose curves, the differences are less than 1% of maximum dose after maximum dose depth for most cases. For lateral dose profiles, the differences are less than 2% of central dose at inner‐beam regions. The differences of the output factors are within 1% for all the three beams.Conclusion:We have developed an optimization‐based commissioning tool for FSPB algorithms to facilitate the commissioning, providing sufficient accuracy of beamlet dose calculation for IMRT optimization.

SU-G-JeP2-15: Proton Beam Behavior in the Presence of Realistic Magnet Fields

Purpose: To investigate the effects of magnetic fields on proton therapy beams for integration with MRI. Methods: 3D magnetic fields from an open-bore superconducting MRI model (previously developed by our group) and 3D magnetic fields from an in-house gradient coil design were applied to various mono energetic proton pencil beam (80MeV to 250MeV) simulations. In all simulations, the z-axis of the simulation geometry coincided with the direction of the B0 field and magnet isocentre. In each simulation, the initial beam trajectory was varied. The first set of simulations performed was based on analytic magnetic force equations (analytic simulations), which could be rapidly calculated yet were limited to propagating proton beams in vacuum. The second set is full Monte Carlo (MC) simulations, which used GEANT4 MC toolkit. Metrics such as the beam position and dose profiles were extracted. Comparisons between the cases with and without magnetic fields present were made. Results: The analytic simulations served as verification checks for the MC simulations when the same simulation geometries were used. The results of the analytic simulations agreed with the MC simulations performed in vacuum. The presence of the MRI's static magnetic field causes proton pencil beams to follow a slight helical trajectory when there were some initial off-axis components. The 80MeV, 150MeV, and 250MeV proton beams rotated by 4.9o, 3.6o, and 2.8o, respectively, when they reached z=0cm. The deflections caused by gradient coils’ magnetic fields show spatially invariant patterns with a maximum range of 0.5mm at z=0cm. Conclusion: This investigation reveals that both the MRI's B0 and gradient magnetic fields can cause small but observable deflections of proton beams at energies studied. The MRI's static field caused a rotation of the beam while the gradient coils’ fields effects were spatially invariant. Dr. B Gino Fallone is a co-founder and CEO of MagnetTx Oncology Solutions (under discussions to license Alberta bi-planar linac MR for commercialization)

TH-CD-201-08: Flexible Dosimeter Bands for Whole-Body Dosimetry

Purpose: The two commonly used radiotherapy techniques are total body irradiation (TBI) and the total skin irradiation (TSI). In order to ensure the accuracy of the prescription beams, the dose received throughout the entire body must be checked using dosimetry. However, the available number of data points is limited as the dosimeters are manually placed on the patient. We developed a flexible and wearable dosimeter that can collect 1D continuous dose information around the peripheral of the patients’ body, including areas obscured from the beam path. Methods: The flexible dosimeter bands are fabricated by embedding storage phosphor powders in a thin layer of non-toxic silicone based elastomer (PDMS). An additional elastomer layer is formed on top of the phosphor layer to provide additional mechanical support for the dosimeter. Once the curing process is complete, the dosimeter is cut into multiple bands and rolled into spools prior to use. Results: The dose responses are tested using a preclinical cabinet X-ray system, where the readout is performed with a storage phosphor reader. Results show that the dose calibration factor is ∼1400 (A.U./Gy) from the beam center. Also, 1-D dose distribution experiment was performed in water phantoms, where preliminary results demonstrate that the dose in water is indeed attenuated compared to in air. Conclusion: Dose response and high-resolution 1-D dosimetry is demonstrated using the flexible dosimeters. By providing a detailed spatial description of the beam dose profile, we expect that the dosimeter bands may aid in enhancing the current existing modality in dosimetry. Since the dosimeter is flexible (can retract back to its original length), they can be comfortably worn around the patient. Potentially, multiple 1-D dose information can be stitched together and extrapolated to provide a coarse 3-D image of the dose distribution. This work was supported by funding from the Cutaneous Lymphoma Foundation under the CLARIONS grant.

Moving gantry method for electron beam dose profile measurement at extended source-to-surface distances.

A novel method has been put forward for very large electron beam profile measurement. With this method, absorbed dose profiles can be measured at any depth in a solid phantom for total skin electron therapy. Electron beam dose profiles were collected with two different methods. Profile measurements were performed at 0.2 and 1.2 cm depths with a parallel plate and a thimble chamber, respectively. 108 cm×108 cm and 45 cm×45 cm projected size electron beams were scanned by vertically moving phantom and detector at 300 cm source‐to‐surface distance with 90° and 270° gantry angles. The profiles collected this way were used as reference. Afterwards, the phantom was fixed on the central axis and the gantry was rotated with certain angular steps. After applying correction for the different source‐to‐detector distances and incidence of angle, the profiles measured in the two different setups were compared. Correction formalism has been developed. The agreement between the cross profiles taken at the depth of maximum dose with the ‘classical’ scanning and with the new moving gantry method was better than 0.5 % in the measuring range from zero to 71.9 cm. Inverse square and attenuation corrections had to be applied. The profiles measured with the parallel plate chamber agree better than 1%, except for the penumbra region, where the maximum difference is 1.5%. With the moving gantry method, very large electron field profiles can be measured at any depth in a solid phantom with high accuracy and reproducibility and with much less time per step. No special instrumentation is needed. The method can be used for commissioning of very large electron beams for computer‐assisted treatment planning, for designing beam modifiers to improve dose uniformity, and for verification of computed dose profiles.PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.jf

SU‐E‐CAMPUS‐T‐06: Initial Experience of Patient‐Specific QA Using a Pencil Beam Scanning Proton Therapy System

Purpose:To illustrate patient QA results for the first 10 patients treated at Scripps Proton Center by comparing point dose measurement using an ion chamber and in‐house developed secondary MU program, and the measurement of 2D dose distribution using an ion chamber array.Methods:At the time of writing, 10 patient plans were approved for treatment using Varian ProBeam pencil beam scanning system and Eclipse treatment planning software. We used the IBA CC04 0.04 cm3 ion chamber and PTW Unidos E electrometer for point dose measurement in a small water tank (Sun Nuclear 1D scanner). We developed independent MU check software based on measured pencil beam dose profiles for various energies. We used PTW Octavius 729 XDR array to evaluate 2D planar dose distribution. The 3D gamma at 3%/3 mm local dose was used to compare a 3D calculated dose plan with a 2D measured dose distribution using PTW Verisoft software. All fields were exported to a verification phantom plan and delivered at 0 degrees for simplicity.Results:Comparisons between the CC04 ion chamber measurement and calculated dose agree well within 1%. The PTW Octavius 729 XDR array exhibited some dose rate dependence in high dose rate pencil beam delivery. Nevertheless, the results, used as a relative measurement, passed the gamma criteria of 3%/3mm for greater than 90% of area in all patient fields. Visual inspection showed good agreement between ion chamber dose profile and the calculated plan. The in‐house secondary check for MU agreed very well with the plan dose and measurement. The results will be updated with more patients treated.Conclusion:The initial patient specific QA results are encouraging for a new pencil beam scanning only proton therapy system.

SU‐E‐T‐50: Automatic Validation of Megavoltage Beams Modeled for Clinical Use in Radiation Therapy

Purpose:To automatically validate megavoltage beams modeled in XiO™ 4.50 (Elekta, Stockholm, Sweden) and Varian Eclipse™ Treatment Planning Systems (TPS) (Varian Associates, Palo Alto, CA, USA), reducing validation time before beam‐on for clinical use.Methods:A software application that can automatically read and analyze DICOM RT Dose and W2CAD files was developed using MatLab integrated development environment. TPS calculated dose distributions, in DICOM RT Dose format, and dose values measured in different Varian Clinac beams, in W2CAD format, were compared. Experimental beam data used were those acquired for beam commissioning, collected on a water phantom with a 2D automatic beam scanning system. Two methods were chosen to evaluate dose distributions fitting: gamma analysis and point tests described in Appendix E of IAEA TECDOC‐1583. Depth dose curves and beam profiles were evaluated for both open and wedged beams. Tolerance parameters chosen for gamma analysis are 3% and 3 mm dose and distance, respectively.Absolute dose was measured independently at points proposed in Appendix E of TECDOC‐1583 to validate software results.Results:TPS calculated depth dose distributions agree with measured beam data under fixed precision values at all depths analyzed. Measured beam dose profiles match TPS calculated doses with high accuracy in both open and wedged beams. Depth and profile dose distributions fitting analysis show gamma values &lt; 1. Relative errors at points proposed in Appendix E of TECDOC‐1583 meet therein recommended tolerances.Independent absolute dose measurements at points proposed in Appendix E of TECDOC‐1583 confirm software results.Conclusion:Automatic validation of megavoltage beams modeled for their use in the clinic was accomplished. The software tool developed proved efficient, giving users a convenient and reliable environment to decide whether to accept or not a beam model for clinical use. Validation time before beam‐on for clinical use was reduced to a few hours.

OC-0278: On the commissioning of Monte Carlo beam models and dose profile corrections on stereotactic photon beams.

OC-0278: On the commissioning of Monte Carlo beam models and dose profile corrections on stereotactic photon beams.