Field Size Factor Research Articles

FRED: a fast Monte Carlo code on GPU for quality control in Particle Therapy

Charged Particle Therapy is a non-invasive technique for radio-resistant tumor treatment performed with protons or light ions, aiming to deliver a high precision treatment. Compared to conventional radiotherapy, ions allow for a higher dose deposition in the tumor region while sparing the surrounding healthy tissue. To really exploit the potential benefits of this technique, the highest possible accuracy in the calculation of dose and its spatial distribution is required in treatment planning. Commonly used Treatment Planning Software solutions adopt a simplified beam-body interaction model. An alternative is the use of Monte Carlo simulations which explicitly take into account the interaction of charged particles with actual human tissues hence providing highly accurate results. However, Monte Carlo simulations are used in a restricted number of cases due to substantial computational resources required. The code FRED has been developed to allow a fast optimization of the treatment plans in Charged Particle Therapy while profiting from the dose release accuracy of a Monte Carlo tool. Currently, the most refined module is the transport of proton beams in water. A comparison with measurements shows that the lateral dose tails are reproduced within 2% in the field size factor test up to 20 cm. Models for the interaction of ion with the matter are currently under development in the FRED code. The status of new developments and the performance of FRED will be presented.

Nuclear halo measurements for accurate prediction of field size factor in a Varian ProBeam proton PBS system.

PurposeFor pencil‐beam scanning proton therapy systems, in‐air non‐Gaussian halo can significantly impact output at small field sizes and low energies. Since the low‐intensity tail of spot profile (halo) is not necessarily modeled in treatment planning systems (TPSs), this can potentially lead to significant differences in patient dose distribution. In this work, we report such impact for a Varian ProBeam system.MethodsWe use a pair magnification technique to measure two‐dimensional (2D) spot profiles of protons from 70 to 242 MeV at the treatment isocenter and 30 cm upstream of the isocenter. Measurements are made with both Gafchromic film and a scintillator detector coupled to a CCD camera (IBA Lynx). Spot profiles are measured down to 0.01% of their maximum intensity. Field size factors (FSFs) are compared among calculation using measured 2D profiles, calculation using a clinical treatment planning algorithm (Raystation 8A clinical Monte Carlo), and a CC04 small‐volume ion chamber. FSFs were measured for square fields of proton energies ranging from 70 to 242 MeV.ResultsAll film and Lynx measurements agree within 1 mm for full width at half maximum beam intensity. The measured radial spot profiles disagree with simple Gaussian approximations, which are used for modeling in the TPS. FSF measurements show the magnitude of disagreements between beam output in reality and in the TPS without modeling halo. We found that the clinical TPS overestimated output by as much as 6% for small field sizes of 2 cm at the lowest energy of 70 MeV while the film and Lynx measurements agreed within 4% and 1%, respectively, for this FSF.ConclusionsIf the in‐air halo for low‐energy proton beams is not fully modeled by the TPS, this could potentially lead to under‐dosing small, shallow treatment volumes in PBS treatment plans.

Application of a Relative Visual Performance Model in a Virtual Reality Immersive System.

As part of an evaluation process of user experience realism in a Virtual Reality (VR) system, we focus in this paper on one of the core characteristics of vision: the relationship between contrast and luminance. The experiment aims at validating in VR reaction time predictions given by Rea and Ouellette's model. The subjects have to distinguish, as fast as they can, a target object from an uniform background. Our results did not match the predictions of the model. Our subjects showed higher performance in performing the task than expected. At low level of contrast, our subjects could easily perceive a target they should not have been able to see at all. This is explained by the size of the visual field surrounding the target: at low level of visibility, the larger the surrounding, the easier perception the is. We conclude that the Rea and Ouellette's model could be applied in VR if a specific visual field size factor was added.

A novel pencil beam model for carbon-ion dose calculation derived from Monte Carlo simulations

An accurate kernel model is of vital importance for pencil-beam dose algorithm in charged particle therapy using precise spot-scanning beam delivery, in which an accurate depiction of the low dose envelope is especially crucial. Based on the Monte Carlo method, we investigated the dose contribution of secondary particles to the total dose and proposed a novel beam model to depict the lateral dose distribution of carbon-ion pencil beam in water. We demonstrated that the low dose envelope in single-spot profiles in water could be adequately modelled with the addition of a logistic distribution to a double Gaussian one, which was verified in both single carbon-ion pencil beam and superposed fields of different sizes with multiple pencil beams. Its superiority was mainly manifested at medium depths especially for high-energy beams with small fields compared with single, double and triple Gaussian models, where the secondary particles influenced the total dose considerably. The double Gaussian-logistic model could reduce the deviations from 4.1%, 1.7% to 0.3% in the plateau and peak regions, and from 19.2%, 4.9% to 1.2% in the tail region compared for the field size factor (FSF) calculations of 344 MeV/u carbon-ion pencil beam with the single and double Gaussian models. Compared with the triple Gaussian one, our newly-proposed model was on a par with it, even better than it in the plateau and peak regions. Thus our work will be helpful for improving the dose calculation accuracy for carbon-ion therapy.

Radial asymmetries in population receptive field size and cortical magnification factor in early visual cortex

Human visual cortex does not represent the whole visual field with the same detail. Changes in receptive field size, population receptive field (pRF) size and cortical magnification factor (CMF) with eccentricity are well established, and associated with changes in visual acuity with eccentricity. Visual acuity also changes across polar angle. However, it remains unclear how RF size, pRF size and CMF change across polar angle. Here, we examine differences in pRF size and CMF across polar angle in V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper and lower) visual field quadrants. Differences increase with eccentricity, approximately in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower than upper quadrant, and again differences increase with eccentricity. However, pRF size differences between lower and upper quadrants change direction with eccentricity. Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and V3, though this difference is very small, and we find no differences in V1 and no differences in CMF. Moreover, differences in pRF size and CMF vary gradually with polar angle and are not limited to the meridians or visual field map discontinuities. PRF size and CMF differences do not consistently follow patterns of cortical curvature, despite the link between cortical curvature and polar angle in V1. Thus, the early human visual cortex has a radially asymmetric representation of the visual field. These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.

Fred: a GPU-accelerated fast-Monte Carlo code for rapid treatment plan recalculation in ion beam therapy

Ion beam therapy is a rapidly growing technique for tumor radiation therapy. Ions allow for a high dose deposition in the tumor region, while sparing the surrounding healthy tissue. For this reason, the highest possible accuracy in the calculation of dose and its spatial distribution is required in treatment planning. On one hand, commonly used treatment planning software solutions adopt a simplified beam–body interaction model by remapping pre-calculated dose distributions into a 3D water-equivalent representation of the patient morphology. On the other hand, Monte Carlo (MC) simulations, which explicitly take into account all the details in the interaction of particles with human tissues, are considered to be the most reliable tool to address the complexity of mixed field irradiation in a heterogeneous environment. However, full MC calculations are not routinely used in clinical practice because they typically demand substantial computational resources. Therefore MC simulations are usually only used to check treatment plans for a restricted number of difficult cases. The advent of general-purpose programming GPU cards prompted the development of trimmed-down MC-based dose engines which can significantly reduce the time needed to recalculate a treatment plan with respect to standard MC codes in CPU hardware. In this work, we report on the development of fred, a new MC simulation platform for treatment planning in ion beam therapy. The code can transport particles through a 3D voxel grid using a class II MC algorithm. Both primary and secondary particles are tracked and their energy deposition is scored along the trajectory. Effective models for particle–medium interaction have been implemented, balancing accuracy in dose deposition with computational cost. Currently, the most refined module is the transport of proton beams in water: single pencil beam dose–depth distributions obtained with fred agree with those produced by standard MC codes within 1–2% of the Bragg peak in the therapeutic energy range. A comparison with measurements taken at the CNAO treatment center shows that the lateral dose tails are reproduced within 2% in the field size factor test up to 20 cm. The tracing kernel can run on GPU hardware, achieving 10 million primary on a single card. This performance allows one to recalculate a proton treatment plan at 1% of the total particles in just a few minutes.

Radial asymmetries in population receptive field size and cortical magnification factor in early visual cortex

Radial asymmetries in population receptive field size and cortical magnification factor in early visual cortex

An efficient method to determine double Gaussian fluence parameters in the eclipse™ proton pencil beam model.

To find an efficient method to configure the proton fluence for a commercial proton pencil beam scanning (PBS) treatment planning system (TPS). An in-water dose kernel was developed to mimic the dose kernel of the pencil beam convolution superposition algorithm, which is part of the commercial proton beam therapy planning software, eclipse™ (Varian Medical Systems, Palo Alto, CA). The field size factor (FSF) was calculated based on the spot profile reconstructed by the in-house dose kernel. The workflow of using FSFs to find the desirable proton fluence is presented. The in-house derived spot profile and FSF were validated by a direct comparison with those calculated by the eclipse TPS. The validation included 420 comparisons of the FSFs from 14 proton energies, various field sizes from 2 to 20 cm and various depths from 20% to 80% of proton range. The relative in-water lateral profiles between the in-house calculation and the eclipse TPS agree very well even at the level of 10-4. The FSFs between the in-house calculation and the eclipse TPS also agree well. The maximum deviation is within 0.5%, and the standard deviation is less than 0.1%. The authors' method significantly reduced the time to find the desirable proton fluences of the clinical energies. The method is extensively validated and can be applied to any proton centers using PBS and the eclipse TPS.

SU‐F‐T‐138: Commissioning and Evaluating Dose Computation Models for a Dedicated Proton Line Scanning Beam Nozzle in Eclipse Treatment Planning System

Purpose:In this study, we present an effective method to derive low dose envelope of the proton in‐air spot fluence at beam positions other than the isocenter to reduce amount of measurements required for planning commission. Also, we demonstrate commissioning and validation results of this method to the Eclipse treatment planning system (version 13.0.29) for a Sumitomo dedicated proton line scanning beam nozzle.Methods:The in‐air spot profiles at five beam‐axis positions (±200, ±100 and 0 mm) were obtained in trigger mode using a MP3 Water tank (PTW‐Freiburg) and a pinpoint ionization chamber (model 31014, PTW‐Freiburg). Low dose envelope (below 1% of the center dose) of the spot profile at isocenter was obtained by repeated point measurements to minimize dosimetry uncertainty. The double Gaussian (DG) model was used to fit and obtain optimal σ1, σ2 and their corresponding weightings through our in‐house MATLAB (Mathworks) program. σ1, σ2 were assumed to expand linearly along the beam axis from a virtual source position calculated by back projecting fitted sigmas from the single Gaussian (SG) model. Absolute doses in water were validated using an Advanced Markus chamber at the depth of 2cm with Pristine Peak (BP) R90d ranging from 5–32 cm for 10×10 cm2 scanned fields. The field size factors were verified with square fields from 2 to 20 cm at 2cm and before BP depth.Results:The absolute dose outputs were found to be within ±3%. For field size factor, the agreement between calculated and measurement were within ±2% at 2cm and ±3% before BP, except for the field size below 2×2 cm2.Conclusion:The double Gaussian model was found to be sufficient for characterizing the Sumitomo dedicated proton line scanning nozzle. With our effective double Gaussian fitting method, we are able to save significant proton beam time with acceptable output accuracy.

SU‐E‐T‐134: Assessing the Capabilities of An MU Model for Fields as Small as 2cm in a Passively Scattered Proton Beam

Purpose:To assess and expand the capabilities of the current MU model for a passively scattered proton beam. The expanded MU model can potentially be used to predict the dose/MU for fields smaller than 2cm in diameter and reduce time needed for physical calibrations.Methods:The current MU model accurately predicted the dose/MU for more than 800 fields when compared to physical patient calibrations. Three different ion chambers were used in a Plastic Water phantom for physical measurements: T1, PIN, and A‐16. The original MU model predicted output for fields that were affected by the bolus gap factor (BGF) and nozzle extension factor (NEF). As the system was tested for smaller treatment fields, the mod wheel dependent field size factor (MWDFSF) had to be included to describe the changes observed in treatment fields smaller than 3cm. The expanded model used Clarkson integration to determine the appropriate value for each factor (field size factor (FSF), BGF, NEF, and MWDFSF), to accurately predict the dose/MU for fields smaller than 2.5cm in effective diameter.Results:The expanded MU model demonstrated agreement better than 2% for more than 800 physical calibrations that were tested. The minimum tested fields were 1.7cm effective diameter for 149MeV and 2.4cm effective diameter for 186MeV. The inclusion of Clarkson integration into the MU model enabled accurate prediction of the dose/MU for very small and irregularly shaped treatment fields.Conclusion:The MU model accurately predicted the dose/MU for a wide range of treatment fields used in the clinic. The original MU model has been refined using factors that were problematic to accurately predict the dose/MU: the BGF, NEF, and MWDFSF. The MU model has minimized the time for determining dose/MU and reduced the time needed for physical calibrations, improving the efficiency of the patient treatment process.

SU‐E‐T‐743: The Simple Monitor Unit Calculation for Irregular Field in Passive Proton Beam

Purpose:We try to apply the monitor unit (MU) calculation procedure for passively proton beam therapy. For estimation of field size factor (FSF) in the process, we used root A method for conversion from irregular field to equivalent field so far. In this time, we changed the conversion from root A method to Clarkson method. The purpose of this study is to ensure the calculation accuracy by comparative evaluation between root A and Clarkson methods.Methods:As fundamental study, we measured absolute dose with several circular field. Then we performed experimental fitting of conversion curve. With this method, the conversion from the equivalent circular field to equivalent square field is available by the assumed equation. As clinical study, we compared the measured MU with the calculated MU (root A and Clarkson method) in 28 prostate patients (76 fields) for the purpose of evaluating the feasibility of our procedure. To simplify the comparison of the calculation procedure, we built the dedicated MU calculation software.Results:As the Result of fundamental study, the assumed equation for conversion from circular field to square field was estimated. As the Result of clinical study, the differences between the measured MU and the calculated MU were within 2% in all cases. All calculated MU were calculated by our software. The differences of MU calculation between measurement MU and calculation MU (root A and Clarkson) were less than 2% for all cases. The average difference and standard deviation of MU calculation between the root A and Clarkson was 0.05±0.05%. However, the calculation accuracy of Clarkson method should be verified in the complicated cases.Conclusion:The MU calculation by use of Clarkson method was supposed and verified in this study. For prostate cases, the Clarkson method is feasible for MU calculation procedure.

SU-E-T-586: Field Size Dependence of Output Factor for Uniform Scanning Proton Beams: A Comparison of TPS Calculation, Measurement and Monte Carlo Simulation

Purpose: Output dependence on field size for uniform scanning beams, and the accuracy of treatment planning system (TPS) calculation are not well studied. The purpose of this work is to investigate the dependence of output on field size for uniform scanning beams and compare it among TPS calculation, measurements and Monte Carlo simulations. Methods: Field size dependence was studied using various field sizes between 2.5 cm diameter to 10 cm diameter. The field size factor was studied for a number of proton range and modulation combinations based on output at the center of spread out Bragg peak normalized to a 10 cm diameter field. Three methods were used and compared in this study: 1) TPS calculation, 2) ionization chamber measurement, and 3) Monte Carlos simulation. The XiO TPS (Electa, St. Louis) was used to calculate the output factor using a pencil beam algorithm; a pinpoint ionization chamber was used for measurements; and the Fluka code was used for Monte Carlo simulations. Results: The field size factor varied with proton beam parameters, such as range, modulation, and calibration depth, and could decrease over 10% from a 10 cm to 3 cm diameter field for a large range proton beam. The XiO TPS predicted the field size factor relatively well at large field size, but could differ from measurements by 5% or more for small field and large range beams. Monte Carlo simulations predicted the field size factor within 1.5% of measurements. Conclusion: Output factor can vary largely with field size, and needs to be accounted for accurate proton beam delivery. This is especially important for small field beams such as in stereotactic proton therapy, where the field size dependence is large and TPS calculation is inaccurate. Measurements or Monte Carlo simulations are recommended for output determination for such cases.

SU-E-T-75: Commissioning Optically Stimulated Luminescence Dosimeters for Fast Neutron Therapy

Purpose: Fast neutrons therapy used at the University of Washington is clinically proven to be more effective than photon therapy in treating salivary gland and other cancers. A nanodot optically stimulated luminescence (OSL) system was chosen to be commissioned for patient in vivo dosimetry for neutron therapy. The OSL-based radiation detectors are not susceptible to radiation damage caused by neutrons compared to diodes or MOSFET systems. Methods: An In-Light microStar OSL system was commissioned for in vivo use by radiating Landauer nanodots with neutrons generated from 50.0 MeV protons accelerated onto a beryllium target. The OSLs were calibrated the depth of maximum dose in solid water localized to 150 cm SAD isocenter in a 10.3 cm square field. Linearity was tested over a typical clinical dose fractionation range i.e. 0 to 150 neutron-cGy. Correction factors for transient signal fading, trap depletion, gantry angle, field size, and wedge factor dependencies were also evaluated. The OSLs were photo-bleached between radiations using a tungsten-halogen lamp. Results: Landauer sensitivity factors published for each nanodot are valid for measuring photon and electron doses but do not apply for neutron irradiation. Individually calculated nanodot calibration factors exhibited a 2–5% improvement over calibration factors computed by themore » microStar InLight software. Transient fading effects had a significant impact on neutron dose reading accuracy compared to photon and electron in vivo dosimetry. Greater accuracy can be achieved by calibrating and reading each dosimeter within 1–2 hours after irradiation. No additional OSL correction factors were needed for field size, gantry angle, or wedge factors in solid water phantom measurements. Conclusion: OSL detectors are a useful for neutron beam in vivo dosimetry verification. Dosimetric accuracy comparable to conventional diode systems can be achieved. Accounting for transient fading effects during the neutron beam calibration is a critical component for achieving comparable accuracy.« less

Experimental characterization of two-dimensional spot profiles for two proton pencil beam scanning nozzles

Dose calculation for pencil beam scanning proton therapy requires accurate measurement of the broad tails of the proton spot profiles for every nozzle in clinical use. By applying a pair/magnification method and merging film data, 200 mm × 240 mm dose kernels extending to 10−4 of the central spot dose are generated for six selected energies of the IBA dedicated and universal nozzles (DN and UN). One-dimensional, circular profiles up to 100 mm in radius are generated from the asymmetric profiles to facilitate spot profile comparison. For the highest energy, 225 MeV, the output of both the DN and the UN for field sizes from 40 to 200 mm increases in parallel, slowest at the surface (∼1%) and fastest at a depth of 150 mm (∼9%). In contrast, at the lowest energy, 100 MeV, the output of the DN across the same range of field sizes increases 3–4% versus 6–7% for the UN throughout all the depths. The charge deficits in the measured depth-dose of Bragg peaks are similar between the UN and the DN. At 100 MeV, the field size factor difference at the surface between two orientations of a rectangular 40 mm × 200 mm field is 1.4% at isocentre for the DN versus 2% for the UN. Though the one-dimensional distributions are similar for the primary and tail components at different positions, the primary components of the DN spots are more elliptical 270 mm upstream than at isocentre.

SU‐E‐T‐12: Clinical Implementation of the Dedicated Program for MU Calculation in Proton Beam Therapy

Purpose: The purpose of this study is to develop the dedicated in‐house program for calculation of the monitor unit (MU) of proton therapy beam and to verify the feasibility of its clinical implementation in proton beam therapy with a combination of range modulation wheel (RMW).Materials and Methods: The MU calculation for the proton beam was suggested by Dr.Sahoo in 2008 and given by the formalism using the multiplication of several factors with regard to the structure of the proton beam system. Following his theory, we implemented eight factors into the formalism for MU calculation as follows: Relative output factor (ROF), Spread‐out Bragg peak factor (SOBPF), Range shifter factor (RSF), SOBP off‐center factor (SOBPOCF), Off‐center ratio (OCR), Field size factor (FSF), Inverse square factor (ISF), and Compensator and patient scatter factor (CPSF). Our program was built by Microsoft Visual Basic language. The MU could be calculated by division of the prescribed dose with (d/MU). After building the program, the calculated doses by the formalism were compared with the measured dose. Results: The (d/MU) was calculated by the multiplication using eight factors by use of our program. Once the energy and the settings were input to the program, these factors were estimated from the measurement data. Then MU calculation was done immediately. It was simply and satisfactory fast for the user to complete calculating (d/MU). As the results of the comparison between the measured and calculated doses, the discrepancies in all referential settings were within 2%. Conclusions: The in‐house program provides the fast MU calculation for proton beams. The accuracy of the dose calculation is good in the referential settings. However, further improvement is required for the complicated condition.

An Integrated 2-dimensional Dosimeter and Electronic Portal Imaging Device for In Vivo Dosimetry: A Feasibility Study

Purpose/Objective(s): As radiation therapy becomes more conformal and the delivery technology becomes more complex, in-vivo dosimetry is becoming more necessary but at the same time more technically challenging. This work investigates the feasibility of a novel integrated in-vivo imaging and dosimetry device to meet the current and future requirements of radiation therapy treatment verification whereby imaging and dosimetry could be achieved concurrently. Materials/Methods: The dose response of a commercial 2-D ion-chamber array, in two configurations was investigated; (1) the experimental configuration with the ion-chamber array positioned directly under an electronic portal imaging device (EPID), (2) the standard dosimetry configuration with the ion-chamber array and 5cm solid water build-up. All measurements were conducted with the ion chamber array at isocentric distance and no additional objects in the beam. The measured dose was compared with the calculated dose from a treatment planning system (TPS). For the experimental configuration (1) the ion chamber array was positioned on the treatment couch with the EPID sitting directly on top of it. An additional 3.2 cm solid water was added to fill the air gap between the array and EPID. All the experiments were conducted at zero gantry angle. The central axis dose per Monitor unit and dose profiles for a 10x10 cm open field was measured in both configurations. The water equivalent depth of the experimental configuration (1) was determined by comparison with the TPS calculated dose. This depth was subsequently used for TPS dose calculations for further open beams, IMRT test patterns, and clinical IMRT beams. The field size factor and off-axis dose response measured with configuration (1) for open fields was compared with TPS data. The gamma analysis was performed with 2% and 3 mm criteria. Agreement between ion-chamber array and TPS was compared for both configurations. Results: The water equivalent depth of the ion chamber array in configuration (1) was 8.2 cm. The maximum dose difference between measured dose profiles with configuration (1) and TPS was 3.5% for 20x20 cm field size. The field size output factor with configuration (1) agreed within 2% with TPS. For a given set of gamma criteria the percentage of gamma pass agreed to with 0.5% for all the measurements. Conclusion: An ion chamber array integrated with an EPID maintains accurate water equivalent dose response. This configuration has the potential to provide a new technique of accurate 2D in vivo dosimetry. Further work is underway to assess the impact of the ion chamber array on imaging, and the performance of the device with an object in the beam to more closely simulate the clinical setting. Author Disclosure: S. Deshpande: None. L. Holloway: None. P. Metcalfe: None. P. Vial: None.

SU-E-T-238: Annual QA of Proton Gantry with Robotic Table.

To perform the annual QA of proton gantry with a robotic table. A new proton gantry with robotic table has been commissioned and is being used in clinic for patient treatment. The gantry is equipped with a robotic table with 6-degrees of freedom and dual cardinal angle KV imagers for patient registration. The system allows direct movement from one beam location to another without additional image registration, which effectively reduces portal setup time and increases treatment efficiency. The annual QA has four main components: Beam parameter checks included proton depth dose, output, linearity, modulation factor, field size factor, effective source distance, compensator gap factor, and monitor unit comparison between model calibration and physical measurements for every energy. Mechanical checks included gantry and robotic table isocenter, gantry and robotic couch isocentricity, and mechanical movement of fully loaded couch and corresponding digital readout. Imaging system checks included proton, X-ray beam, laser and image receptor alignment, image quality of KV imagers, and image registration accuracy. The last were the system safety checks. Methods used to perform these checks, especially those pertaining to robotic positioner will be discussed. The new proton gantry and robotic table had the isocentric accuracy of about 1 mm. The accuracy of mechanical movements of the robotic table was within 1mm/0.5 degree in the clinical motion range. The accuracy of proton outputs determined by IAEA TRS 398 protocol was within +/-2% and the consistency of beam range for all clinical energies at cardinal gantry angles was within 1mm. The results of the gantry annual QA demonstrate that the machine satisfies the highest standards of quality assurance for proton radiation treatment. The annual QA verifies the proton output, robotic table movement accuracy, image registration and safety of the machine and thus increases our confidence level in the uncertainties of daily proton treatments.

A Monte Carlo pencil beam scanning model for proton treatment plan simulation using GATE/GEANT4

This work proposes a generic method for modeling scanned ion beam delivery systems, without simulation of the treatment nozzle and based exclusively on beam data library (BDL) measurements required for treatment planning systems (TPS). To this aim, new tools dedicated to treatment plan simulation were implemented in the Gate Monte Carlo platform. The method was applied to a dedicated nozzle from IBA for proton pencil beam scanning delivery. Optical and energy parameters of the system were modeled using a set of proton depth–dose profiles and spot sizes measured at 27 therapeutic energies. For further validation of the beam model, specific 2D and 3D plans were produced and then measured with appropriate dosimetric tools. Dose contributions from secondary particles produced by nuclear interactions were also investigated using field size factor experiments. Pristine Bragg peaks were reproduced with 0.7 mm range and 0.2 mm spot size accuracy. A 32 cm range spread-out Bragg peak with 10 cm modulation was reproduced with 0.8 mm range accuracy and a maximum point-to-point dose difference of less than 2%. A 2D test pattern consisting of a combination of homogeneous and high-gradient dose regions passed a 2%/2 mm gamma index comparison for 97% of the points. In conclusion, the generic modeling method proposed for scanned ion beam delivery systems was applicable to an IBA proton therapy system. The key advantage of the method is that it only requires BDL measurements of the system. The validation tests performed so far demonstrated that the beam model achieves clinical performance, paving the way for further studies toward TPS benchmarking. The method involves new sources that are available in the new Gate release V6.1 and could be further applied to other particle therapy systems delivering protons or other types of ions like carbon.

SU‐E‐T‐364: Measurements of Field Size Factor for Proton Therapy

Purpose: To measure the field size factor and establish reference values for clinical use Methods and Materials: The field size factors (FSF), which is the ratio of output factor of a certain field size to the output of a reference field size, were measured for five clinical proton beams. The proton ranges of the selected beams are from 5.35 cm to 13.0 cm and modulations are from 3.3 cm and 8.9 cm. for each beam, the output was measured using a 0.01 cc compact chamber for field sizes of 11.6 cm diameter (open field), 5.6×5.6 cm2, 4.5×4.5 cm2, 3.0×3.0 cm2, 2.0×2.0 cm2 and 1.1 cm diameter. A 1D‐translational water phantom was used. The point of the measurement is on the central axis in the center of the Spread Out Bragg Peak (SOBP), aligned with the isocenter. Results: For field size larger than 3.0 cm diameter, the FSF is close to 100%. The FSF started to drop when field size became smaller than 3.0 cm diameter for the five beams. It decreased to about 90% for field size of 2.0 cm diameter and further decreased to about 45% for field size of 1.1 cm diameter. Conclusions: The FSFs were measured for five beams with either square or circular apertures. It was observed the FSF changes significantly with field size. The values of FSF measured with square or circular apertures may not be directly used for irregular shaped patient fields, they however provide reference values for these fields.

SU-GG-T-478: A Comparison of Proton Beam Monitor Units Predicted by the Treatment Planning System and Obtained from Physical Calibration

Purpose: To compare the proton monitor units predicted by the treatment planning system and obtained from physical calibration.Method and Materials: Monitor units for proton beam calibration are product of the machine calibration factor (CF), mod factor (MF), Spread out Bragg Peak depth dose (SOBPPdd), field size factor (FSF), inverse square factor (ISF), off center ratio (OCR), and bolus gap factor (BGF). The CF is defined as 1cGy/MU at the center of modulation for the mod wheel 60. The SOBPPdd, MF, FSF, BGF and OCR were measured during the commissioning of proton machines for each beam energy. The proton beam monitor units in treatment planning system are calculated based on the OCR, MF, SOBPdd, ISF from virtual SAD and proton scatter parameters. The differences in the monitor unit calculation between the treatment planning system and from the physical calibration arise from the BGF, FSF and patient scatter. The BGF and FSF are handled correctly in the physical calibration. The patient scattering and heterogeneity are handled by the proton pencil beam dose kernel in the treatment planning system. The proton monitor units from the treatment planning system and physical calibration were compared for prostate, lung and brain patients. Results: The differences in monitor units between the treatment planning system and physical calibration were within 1% for breast cases, and 3%for most of the other cases in the study. The differences increased at higher energy and for thicker compensators. There were also larger differences for some of the cases where the field sizes were smaller than 3 cm. The treatment planning algorithm does not always accommodate special treatment situations adequately. Conclusion:Treatment planning algorithms cannot always accommodate special treatment situations. For such cases, in addition to routine MU calculation, it is important to carry out a physical calibration of treatment fields.