Central Axis Percentage Depth Dose Research Articles

Skin Dose Modeling and Measurement in a High Field In-Line MRI-Linac System

The Australian MRI-Linac prototype radiotherapy system has been shown to generate significant entry skin or surface dose increases. This arises from electron contamination focusing toward the isocenter caused by the 1 T MRI field being in-line with the x-ray beam. The aim of this study is to present accurate Monte Carlo modeling of these skin dose changes and to compare them with previous experimental measurements. Accurate skin dose modeling will improve confidence in the pathway forward to treatment planning for clinical trials. A COMSOL Multiphysics model of the Australian MRI-Linac system was used to generate a 3D magnetic field map to be used in corresponding Geant4 Monte Carlo simulations. The Geant4 simulations included the x-ray source (6 MV Linac), multileaf collimators (MLCs), and a 30 cm × 30 cm × 30 cm water phantom located with its front surface at the beam isocenter. Simulations were performed with a source to surface distance (SSD) of 1,819 mm for nominal field sizes 2 cm × 2 cm, 6 cm × 6 cm, and 10 cm × 10 cm. Central axis percentage depth dose (PDD) and surface (or skin) doses at 70 μm depth were calculated by using high-resolution scoring voxels of 10 μm thickness. The results were compared with corresponding experimental data collected using MOSkin™ on the Australian MRI-Linac prototype system. The accurate modeling provides great detail into how the electron contamination is heavily confined and focused toward the beam central axis due to the presence of in-line magnetic field. This concentration significantly increases the skin dose up to 320% for the field size of 10 cm × 10 cm. For 2 cm × 2 cm and 6 cm × 6 cm, the surface skin dose is 128% and 217%, respectively, as compared to the skin dose in the absence of the magnetic field. The simulation results are in generally good agreement, ±10%, with previously collected experimental data for the same nominal field sizes. For the first time, detailed Geant4 Monte Carlo simulations of the electron contamination in the Australian MRI-Linac system have been performed and confirmed to be sufficiently accurate. These simulations will provide a solid framework for estimating the skin dose changes in more clinically relevant treatment plan scenarios that are envisaged in the near future.

Treatment Planning System for Electron FLASH Radiation Therapy: Open-Source for Clinical Implementation

To present a Monte Carlo (MC) beam model and its implementation in a clinical treatment planning system (TPS, Varian Eclipse) for a modified ultrahigh dose-rate electron FLASH radiation therapy (eFLASH-RT) linear accelerator (LINAC) using clinical accessories and geometry. The gantry head without scattering foils or targets, representative of the LINAC modifications, was modeled in the Geant4-based GAMOS MC toolkit. The energy spectrum (σE) and beam source emittance cone angle (θcone) were varied to match the calculated open-field central-axis percent depth dose (PDD) and lateral profiles with Gafchromic film measurements. The beam model and its Eclipse configuration were validated with measured profiles of the open field and nominal fields for clinical applicators. An MC forward dose calculation was conducted for a mouse whole-brain treatment, and an eFLASH-RT plan was compared with a conventional (Conv-) RT electron plan in Eclipse for a human patient with metastatic renal cell carcinoma. The eFLASH beam model agreed best with measurements at σE=0.5 MeV and θcone=3.9° ± 0.2°. The model and its Eclipse configuration were validated to clinically acceptable accuracy (the absolute average error was within 1.5% for in-water lateral, 3% for in-air lateral, and 2% for PDDs). The forward calculation showed adequate dose delivery to the entire mouse brain while sparing the organ at risk (lung). The human patient case demonstrated the planning capability with routine accessories to achieve an acceptable plan (90% of the tumor volume receiving 95% and 90% of the prescribed dose for eFLASH and Conv-RT, respectively). To our knowledge, this is the first functional beam model commissioned in a clinical TPS for eFLASH-RT enabling planning and evaluation with minimal deviation from the Conv-RT workflow. It facilitates the clinical translation because eFLASH-RT and Conv-RT plan quality were comparable for a human patient involving complex geometries and tissue heterogeneity. The methods can be expanded to model other eFLASH irradiators with different beam characteristics.

Treatment Planning System for Clinical Translation of Electron FLASH Radiotherapy

Electron ultrahigh dose rate FLASH radiotherapy (UHDR/eFLASH-RT) is currently being extensively tested and modeled, and was studied in several rodent and large animal experiments and one clinical cancer patient. These studies suggest prospective dosimetry assessments are necessary to ensure safe delivery and widespread clinical translation of eFLASH-RT would clearly benefit from prior dose prediction and a treatment planning process that accounts for anatomical heterogeneity and complex geometries. In this study, a GAMOS Monte Carlo beam model was developed and implemented in a clinical treatment planning system for a modified UHDR electron LINAC to demonstrate eFLASH-RT planning of patients.The gantry head was modelled without scattering foils or targets, representative of the LINAC modifications. Centered at the nominal energy of 10 MeV, the energy spectrum (σE) was varied to match the central axis percent depth dose (PDD) profiles of an open field measured with radiographic film. The beam's emittance cone angle (θcone) was determined by matching calculated and film measured lateral profiles, at several depths. The beam model and its configuration in the treatment planning system were validated with film measured lateral profiles and PDD's of the open field and nominal fields for clinical applicators. A GAMOS forward dose calculation was conducted for a mouse whole brain treatment and an eFLASH-RT plan was compared to a conventional electron plan for a human patient with a rib metastasis.Our eFLASH beam model agrees best with measurements at σE = 0.5MeV and θcone = 3.9o ± 0.2o. The model and its treatment planning system configuration were validated to clinically acceptable accuracy (the absolute average error was within 1.5% for in water lateral, 3% for in air lateral, and 2% for PDD's). The forward dose calculation showed dose was delivered to the entire mouse brain with adequate conformality (90% of the tumor volume receiving 90% of the prescribed dose). The comparison plans for the patient undergoing treatment for a rib metastasis demonstrated the capability to achieve an acceptable eFLASH-RT plan with routine accessories (90% of the tumor receiving 95% and 90% of the prescribed dose for eFLASH and conventional, respectively) in relatively complex geometry (right posterior oblique treatment with patient specific applicator cutout).To the best of our knowledge, this study presents the first functional beam model commissioned in a clinical TPS for eFLASH-RT, enabling treatment planning and evaluation with minimal deviation from conventional radiotherapy workflow. By producing plans with the quality comparable to conventional plans, specifically for human patients involving complex geometries and tissue heterogeneity, the clinical translation of eFLASH-RT can be expedited. The open-source methods (described in https://github.com/mr3536/eFLASHBeamModeltoTPS) can model other eFLASH irradiators with different beam characteristics.

NanoDot™ OSLDs in verifying radiotherapy dose calculations in the presence of metal implants: A Monte Carlo assisted research

The goal of this work is to evaluate the dosimetric accuracy of nanoDot optically stimulated luminescence dosimeters (OSLDs) for small field sizes both at phantom-implant interfaces and deeper depths of a phantom. The central axis percentage depth-dose (PDD) curves were obtained by 6 MV flattening-filter-free (FFF) photon beams in conjunction with the homogeneous and nonhomogeneous RW3 slab phantoms with aluminum (Al), titanium alloy grade 5 (Ti5), and stainless steel grade 316 L (SS) implant material sheets inserted into, in small fields: 2 × 2 cm2 and 4 × 4 cm2. Three different depths were defined within the heterogeneous phantoms created to compare the measured and calculated doses: The soft tissue/metal implant upper interface (level I), metal implant/soft tissue lower interface (level II), and deeper depth from the implant (level III). The assessment of OSLDs was performed at defined depths comparing by Monte Carlo simulation (EGSnrc-based DOSXYZnrc/BEAMnrc) and Acuros XB (AXB) algorithm with GafChromic™ EBT3 film and PTW 31014 pinpoint ion chamber (IC) measurements. The PDDs using OSLDs were compared to those predicted by MC, and AXB, and those measured by EBT3 and IC and found to agree with uncertainty (<3%) in a homogeneous medium. The mean relative differences between OSLD and MC simulation at the level I interface of the Al, Ti5, and SS implants were analyzed as 2.5%, 5.6%, and 9.8% for 2 × 2 cm2 and 3.0%, 5.2%, and 8.8% for 4 × 4 cm2, respectively. When using implant material with lower electron density such as Al, the OSLD measurement values were found to agree with each measurement and calculation technique. At level II, the relative doses measured using OSLDs were similar to those of MC calculations for both field sizes. The maximum difference was found at 3.2% for SS implant. Except for the vicinity of the metal implant/RW3 phantom slab interfaces, the OSLD relative dose values were similar to those of PDD measurements in homogeneous media at depths away from the implant material. The OSLDs present a high capacity for radiotherapy dosimetry in homogeneous medium.But we can not adequately consider the dose to the interface between tissue and high Zeff materials in the case of the Ti5 and SS implant materials for small fields using OSLDs.

Characteristics of very high-energy electron beams for the irradiation of deep-seated targets.

Driven by advances in accelerator technology and the potential of exploiting the FLASH effect for the treatment of deep-seated targets (>5cm), there is an active interest in the construction of devices to deliver very high-energy electron (VHEE) beams for radiation therapy. The application of novel VHEE devices, however, requires an assessment of the tradeoffs between the different beam parameter choices including beam energies, beam divergences, and maximal field sizes. This study systematically examines the dosimetric beam properties of VHEE beams, determining their clinical usefulness while marking their limits of applications for different beam configurations. We performed Monte Carlo simulations of the dose distributions of electron beams for different energies (25-250MeV), source-to-surface distances (SSD) (50cm, 100cm, parallel), and field sizes (2cm2 ×2cm2 to 15cm2 ×15cm2 ) in water using a research version of the RayStation treatment planning system (RaySearch Labs 9A IONPG). The beam was simulated using a monoenergetic point source and perfect collimation. Central axis percentage depth dose (PDD) and transverse dose profiles at multiple depths were evaluated and compared to those of MV photon beams. Profile characteristics including therapeutic range (TR) at 90%, proximal fall-off (PFO) at 90%, lateral penumbra (LP) at 90%-10%, and field width (FW) at 90% were obtained. Very high-energy electrons beams with SSD 100cm and parallel beams (infinite SSD) exhibit a linear to near-linear increase of TR as a function of energy in the simulated energy range and reach values well beyond the typical depths of lesions encountered in clinics (<20cm). Their TR show a marked field size dependence only for field sizes <10cm2 ×10cm2 . For VHEE beams with SSD 50cm, TR are largely reduced (4-8cm). For beam energies >150MeV with large SSD (>100cm), for many configurations, there is no substantial difference in PDD when adding an opposed beam. This may potentially reduce the number of VHEE beams needed for treatment by a factor of two compared to a treatment using lower energies and lower SSD. In order to cover deep-seated targets homogeneously, VHEE devices with a parallel beam must provide a maximum field size up to several centimeters larger than the tumor size. For the investigated diverging beams, there is not such a significant field width reduction with depth for larger fields as it is compensated by divergence. Penumbrae of VHEE beams are smaller than those of clinical MV photon beams for lower depths (<5cm) but increase quickly for larger depths. There is only a relatively small dependence of penumbra on the SSD of the beam. The findings presented in this study assess the performance of VHEE beams and offer a first estimate of treatment indications and tradeoffs for a given design of a VHEE device. SSD >100cm results in clinically more favorable PDD. Beam energies of 100MeV and above are needed to cover common tumors (5-15cm in-depth) conformally. Higher energies provide an additional benefit specifically for small and deep-seated lesions due to their reduced lateral penumbrae.

In-line MRI-LINAC depth dose measurements using an in-house plastic scintillation dosimeter

Plastic scintillation dosimeters (PSDs) have many properties that make them desirable for relative dosimetry with MRI-LINACs. An in-house PSD, Farmer ionisation chamber and Gafchromic EBT3 film were used to measure central axis percentage depth dose distributions (PDDs) at the Australian MRI-LINAC Mean errors were calculated between each detector’s responses, where the in-house PSD was on average within 0.7% of the Farmer chamber and 1.4% of film, while the Farmer chamber and film were on average within 1.1% of each other. However, the PSD systematically over-estimated the dose as depth increased, approaching a maximum overestimation of the order of 3.5% for the smallest field size measured. This trend was statistically insignificant for all other field sizes measured; further investigation is required to determine the source of this effect. The calculated values of mean absolute error are comparable to the those of trusted dosimeters reported in the literature. These mean absolute errors, and the ubiquity of desirable dosimetric qualities inherent to PSDs suggest that PSDs in general are accurate for relative dosimetry with the MRI-LINAC. Further investigation is required into the source of the reported systematic trends dependent on field-size and depth of measurement.

Improved Monte Carlo clinical electron beam modelling

PurposeAn EGSnrc based electron model was developed and validated for an Elekta Synergy® 160-leaf Agility™ linear accelerator. It was able to reproduce measured central axis (CAX) percentage depth dose (PDD) curves and off-axis profiles (OAPs) within 2%/2 mm, and relative output factors (ROFs) within 3%. MethodsBEAMnrc component modules were used to model the accelerator accurately according to vendor supplied specifications. The electron beam focal spot size and input energy spectrum were determined through their effects on electron CAX PDDs and OAPs as benchmarked against water tank data. Phase space files were used as source input in DOSXYZnrc water phantom simulations. Dose distributions were calculated for six electron nominal energies, 11 field sizes and two source-to-surface distances. ResultsThe full width at half maximum of the focal spot (assuming a Gaussian intensity distribution) was determined to be 1.50 mm. An asymmetrical input electron energy spectrum with a low-energy tail produced good agreement with measured data and solved the match in the build-up (BU) region for all electron energies used in this study. ConclusionsThe improved input electron spectra for the electron model could predict dose distributions within 2%/2 mm of measured data. The model’s success is embedded in the asymmetrical energy spectrum which provided a valuable free parameter which, by fine adjustment, improved the match in the BU region of dose distributions. Furthermore, focal spot parameters could be determined by means of iterative simulations.

An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry.

Purpose:An experimental method using the linear portion of the relative film dose–response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.Materials and Methods:The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.Results:The maximum percentage deviation of PDDFS with PDDIC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDDFS with PDDIC for electron beams was within 3% for energy range studied of 8–15 MeV. The gamma passing rates of PDDFS with PDDIC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDDFS with PDDIC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROFFS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDDFS and PDDMC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROFFS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.Conclusions:The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

Remote Cherenkov imaging-based quality assurance of a magnetic resonance image-guided radiotherapy system.

Tools to perform regular quality assurance of magnetic resonance image-guided radiotherapy (MRIgRT) systems should ideally be independent of interference from the magnetic fields. Remotely acquired optical Cherenkov imaging-based dosimetry measurements in water were investigated for this purpose, comparing measures of dose accuracy, temporal dynamics, and overall integrated IMRT delivery. A 40×30.5×37.5cm3 water tank doped with 1g/L of quinine sulfate was imaged using an intensified charge-coupled device (ICCD) to capture the Cherenkov emission while being irradiated by a commercial MRIgRT system (ViewRay™). The ICCD was placed down-bore at the end of the couch, 4m from treatment isocenter and behind the 5-Gauss line of the 0.35-T MRI. After establishing optimal camera acquisition settings, square beams of increasing size (4.2×4.2cm2 , 10.5×10.5cm2 , and 14.7×14.7cm2 ) were imaged at 0.93 frames per second, from an individual cobalt-60 treatment head, to develop projection measures related to percent depth dose (PDD) curves and cross beam profiles (CPB). These Cherenkov-derived measurements were compared to ionization chamber (IC) and radiographic film dosimetry data, as well as simulation data from the treatment planning system (TPS). An intensity-modulated radiotherapy (IMRT) commissioning plan from AAPM TG-119 (C4:C-Shape) was also imaged at 2.1 frames per second, and the single linear sum image from 509s of plan delivery was compared to the dose volume prediction generated by the TPS using gamma index analysis. Analysis of standardized test target images (1024×1024pixels) yielded a pixel resolution of 0.37mm/pixel. The beam width measured from the Cherenkov image-generated projection CBPs was within 1mm accuracy when compared to film measurements for all beams. The 502 point measurements (i.e., pixels) of the Cherenkov image-based projection percent depth dose curves (pPDDs) were compared to pPDDs simulated by the treatment planning system (TPS), with an overall average error of 0.60%, 0.56%, and 0.65% for the 4.2, 10.5, and 14.7cm square beams, respectively. The relationships between pPDDs and central axis PDDs derived from the TPS were used to apply a weighting factor to the Cherenkov pPDD, so that the Cherenkov data could be directly compared to IC PDDs (average error of -0.07%, 0.10%, and -0.01% for the same sized beams, respectively). Finally, the composite image of the TG-119 C4 treatment plan achieved a 95.1% passing rate using 4%/4mm gamma index agreement criteria between Cherenkov intensity and TPS dose volume data. This is the first examination of Cherenkov-generated pPDDs and pCBPs in an MR-IGRT system. Cherenkov imaging measurements were fast to acquire, and minimal error was observed overall. Cherenkov imaging also provided novel real-time data for IMRT QA. The strengths of this imaging are the rapid data capture ability providing real-time, high spatial resolution data, combined with the remote, noncontact nature of imaging. The biggest limitation of this method is the two-dimensional (2D) projection-based imaging of three-dimensional (3D) dose distributions through the transparent water tank.

Comparison of measured electron energy spectra for six matched, radiotherapy accelerators

This study compares energy spectra of the multiple electron beams of individual radiotherapy machines, as well as the sets of spectra across multiple matched machines. Also, energy spectrum metrics are compared with central‐axis percent depth‐dose (PDD) metrics.MethodsA lightweight, permanent magnet spectrometer was used to measure energy spectra for seven electron beams (7–20 MeV) on six matched Elekta Infinity accelerators with the MLCi2 treatment head. PDD measurements in the distal falloff region provided R 50 and R 80–20 metrics in Plastic Water®, which correlated with energy spectrum metrics, peak mean energy (PME) and full‐width at half maximum (FWHM).ResultsVisual inspection of energy spectra and their metrics showed whether beams on single machines were properly tuned, i.e., FWHM is expected to increase and peak height decrease monotonically with increased PME. Also, PME spacings are expected to be approximately equal for 7–13 MeV beams (0.5‐cm R90 spacing) and for 13–16 MeV beams (1.0‐cm R90 spacing). Most machines failed these expectations, presumably due to tolerances for initial beam matching (0.05 cm in R 90; 0.10 cm in R 80–20) and ongoing quality assurance (0.2 cm in R 50). Also, comparison of energy spectra or metrics for a single beam energy (six machines) showed outlying spectra. These variations in energy spectra provided ample data spread for correlating PME and FWHM with PDD metrics. Least‐squares fits showed that R 50 and R 80–20 varied linearly and supralinearly with PME, respectively; however, both suggested a secondary dependence on FWHM. Hence, PME and FWHM could serve as surrogates for R 50 and R 80–20 for beam tuning by the accelerator engineer, possibly being more sensitive (e.g., 0.1 cm in R 80–20 corresponded to 2.0 MeV in FWHM).ConclusionsResults of this study suggest a lightweight, permanent magnet spectrometer could be a useful beam‐tuning instrument for the accelerator engineer to (a) match electron beams prior to beam commissioning, (b) tune electron beams for the duration of their clinical use, and (c) provide estimates of PDD metrics following machine maintenance. However, a real‐time version of the spectrometer is needed to be practical.

Abstract ID: 145 Monte Carlo for CyberKnife radiosurgery with the InCise Multileaf Collimator

This work investigates the accuracy and efficiency of Monte Carlo dose calculation with the InciseTM 2 Multileaf Collimator (MLC) for the CyberKnife® M6TM System in Accuray PrecisionTM Treatment Planning System 1.1. Phase space above the MLC is derived from a virtual source model calculated from beam commissioning measurements (profiles, TPRs, and output factors). Transport through the MLC is performed by ray tracing, accounting for leaf tip geometry, leaf-bank tilt, beam hardening, and shifts applied by the MLC controller to compensate for variable offset between the radiographic and geometric beam edge at each leaf tip. Patient model transport uses material and density specific photon mean-free-paths in combination with track-repeating of precomputed electron tracks from mono-energetic photon beams in water, modified using local density. Variance reduction techniques include forced interaction, particle splitting, and range rejection. This version improves on previous implementations by linearly propagating electrons below a cutoff energy in some circumstances instead of depositing their remaining energy in one voxel. The algorithm is implemented in C++, multithreaded over 12 CPU cores, and can be used pre- or post- beam weight optimization. Calculations were compared against measurement for single beams and composed plans in homogeneous and inhomogeneous phantoms. Single beam measurements included central axis percent depth dose (PDD) using diode and microdiamond detectors, and planar dose distributions with EBT3 film. For composed plans, point measurements were performed using air-filled micro-ionization chambers and TLD. Calculation speed was characterized by evaluating lung SBRT plans consisting of between 13 and 126 treatment beams, and PTV’s between 14cc and 72cc. The PDD curves showed good agreement in solid water and water-lung-water phantoms between calculation and measurement. Films had ⩾90% pixels meeting a 2%/2 mm gamma criterion for all 12 aperture shapes in similar phantoms. Composite plans had point dose differences ⩽3% in heterogeneous lung phantoms. Calculation speeds at native 512 × 512 CT resolution (approximately 1 mm3 voxels) and 2% requested uncertainty were 9 min on average, reducing to 3 min when medium resolution (approximately 4 mm3 voxels) was used. Methods to reduce calculation times are under investigation. Preliminary results suggest possible 40%–75% reductions without underlying algorithm or hardware changes.

A hybrid phantom Monte Carlo-based method for historical reconstruction of organ doses in patients treated with cobalt-60 for Hodgkin's lymphoma

Historical radiotherapy treatment plans lack 3D images sets required for estimating mean organ doses to patients. Alternatively, Monte Carlo-based models of radiotherapy devices coupled with whole-body computational phantoms can permit estimates of historical in-field and out-of-field organ doses as needed for studies associating radiation exposure and late tissue toxicities. In recreating historical patient treatments with 60Co based systems, the major components to be modeled include the source capsule, surrounding shielding layers, collimators (both fixed and adjustable), and trimmers as needed to vary field size. In this study, a computational model and experimental validation of the Theratron T-1000 are presented. Model validation is based upon in-field commissioning data collected at the University of Florida, published out-of-field data from the British Journal of Radiology (BJR) Supplement 25, and out-of-field measurements performed at the University of Wisconsin’s Accredited Dosimetry Calibration Laboratory (UWADCL). The computational model of the Theratron T-1000 agrees with central axis percentage depth dose data to within 2% for 6 × 6 to 30 × 30 cm2 fields. Out-of-field doses were found to vary between 0.6% to 2.4% of central axis dose at 10 cm from field edge and 0.42% to 0.97% of central axis dose at 20 cm from the field edge, all at 5 cm depth. Absolute and relative differences between computed and measured out-of-field doses varied between ±2.5% and ±100%, respectively, at distances up to 60 cm from the central axis. The source-term model was subsequently combined with patient-morphometry matched computational hybrid phantoms as a method for estimating in-field and out-of-field organ doses for patients treated for Hodgkin’s Lymphoma. By changing field size and position, and adding patient-specific field shaping blocks, more complex historical treatment set-ups can be to recreated, particularly those for which 2D or 3D image sets are unavailable.

A measurement-based generalized source model for Monte Carlo dose simulations of CT scans

The goal of this study is to develop a generalized source model for accurate Monte Carlo dose simulations of CT scans based solely on the measurement data without a priori knowledge of scanner specifications. The proposed generalized source model consists of an extended circular source located at x-ray target level with its energy spectrum, source distribution and fluence distribution derived from a set of measurement data conveniently available in the clinic. Specifically, the central axis percent depth dose (PDD) curves measured in water and the cone output factors measured in air were used to derive the energy spectrum and the source distribution respectively with a Levenberg–Marquardt algorithm. The in-air film measurement of fan-beam dose profiles at fixed gantry was back-projected to generate the fluence distribution of the source model. A benchmarked Monte Carlo user code was used to simulate the dose distributions in water with the developed source model as beam input. The feasibility and accuracy of the proposed source model was tested on a GE LightSpeed and a Philips Brilliance Big Bore multi-detector CT (MDCT) scanners available in our clinic. In general, the Monte Carlo simulations of the PDDs in water and dose profiles along lateral and longitudinal directions agreed with the measurements within 4%/1 mm for both CT scanners. The absolute dose comparison using two CTDI phantoms (16 cm and 32 cm in diameters) indicated a better than 5% agreement between the Monte Carlo-simulated and the ion chamber-measured doses at a variety of locations for the two scanners. Overall, this study demonstrated that a generalized source model can be constructed based only on a set of measurement data and used for accurate Monte Carlo dose simulations of patients’ CT scans, which would facilitate patient-specific CT organ dose estimation and cancer risk management in the diagnostic and therapeutic radiology.

Analytic derivation of central axis percent depth dose calculations in transition zones with loss of electronic equilibrium

Purpose: The study of megavoltage photon dose distribution behind and near small areas of low and high density material is best understood with Monte Carlo (MC) dose calculation or direct measurements which may not be always be possible. This is especially true for air-tissue area where the replacement of soft tissue scattering material by air results in the loss of electronic equilibrium and changes in the lateral spread of the beam as well. Monte Carlo calculations are the standards to correctly evaluate in homogeneities in transition zones. If one could develop a model with sufficient accuracy to obtain similar results, this would be very helpful clinically. Methods: To this end, we have developed an exponential model and derive an explicit expression that accounts for the under dosage. The model is an extension of a much earlier work done with electrons and photons. Our analytic model is based on the experience of the underlying physics assuming exponential attenuation of photons in matter. Results: It differs from a similar work by solving the problem correctly and introducing parameters that can be traced to direct measurements without the need of extensive statistical data analysis. It combines the generation of free electrons through ionization and their attenuation to a simple differential equation for the central axis depth dose. It involves two parameters, which can be obtained from 1) direct beam measurements, 2) primary photon attenuation coefficients from physics tables and 3) iteration techniques. Conclusion: The simplicity of the model allows us to extend our derivation to situations such as transitions zones of different densities in areas such as head and neck and lung. A clinical example is illustrated to demonstrate the problems encountered in treating cancer of the larynx.

Percentage depth dose calculation accuracy of model based algorithms in high energy photon small fields through heterogeneous media and comparison with plastic scintillator dosimetry.

Small fields smaller than 4×4 cm2 are used in stereotactic and conformal treatments where heterogeneity is normally present. Since dose calculation accuracy in both small fields and heterogeneity often involves more discrepancy, algorithms used by treatment planning systems (TPS) should be evaluated for achieving better treatment results. This report aims at evaluating accuracy of four model‐based algorithms, X‐ray Voxel Monte Carlo (XVMC) from Monaco, Superposition (SP) from CMS‐Xio, AcurosXB (AXB) and analytical anisotropic algorithm (AAA) from Eclipse are tested against the measurement. Measurements are done using Exradin W1 plastic scintillator in Solid Water phantom with heterogeneities like air, lung, bone, and aluminum, irradiated with 6 and 15 MV photons of square field size ranging from 1 to 4 cm2. Each heterogeneity is introduced individually at two different depths from depth‐of‐dose maximum (Dmax), one setup being nearer and another farther from the Dmax. The central axis percentage depth‐dose (CADD) curve for each setup is measured separately and compared with the TPS algorithm calculated for the same setup. The percentage normalized root mean squared deviation (%NRMSD) is calculated, which represents the whole CADD curve's deviation against the measured. It is found that for air and lung heterogeneity, for both 6 and 15 MV, all algorithms show maximum deviation for field size 1×1 cm2 and gradually reduce when field size increases, except for AAA. For aluminum and bone, all algorithms' deviations are less for 15 MV irrespective of setup. In all heterogeneity setups, 1×1 cm2 field showed maximum deviation, except in 6 MV bone setup. All algorithms in the study, irrespective of energy and field size, when any heterogeneity is nearer to Dmax, the dose deviation is higher compared to the same heterogeneity far from the Dmax. Also, all algorithms show maximum deviation in lower‐density materials compared to high‐density materials.PACS numbers: 87.53.Bn, 87.53.kn, 87.56.bd, 87.55.Kd, 87.56.jf

SU‐E‐T‐416: Experimental Evaluation of a Commercial GPU‐Based Monte Carlo Dose Calculation Algorithm

Purpose:A new commercial GPU‐based Monte Carlo dose calculation algorithm (GPUMCD) developed by the vendor Elekta™ to be used in the Monaco Treatment Planning System (TPS) is capable of modeling dose for both a standard linear accelerator and for an Elekta MRI‐Linear accelerator (modeling magnetic field effects). We are evaluating this algorithm in two parts: commissioning the algorithm for an Elekta Agility linear accelerator (the focus of this work) and evaluating the algorithm's ability to model magnetic field effects for an MRI‐linear accelerator.Methods:A beam model was developed in the Monaco TPS (v.5.09.06) using the commissioned beam data for a 6MV Agility linac. A heterogeneous phantom representing tumor‐in‐lung, lung, bone‐in‐tissue, and prosthetic was designed/built. Dose calculations in Monaco were done using the current clinical algorithm (XVMC) and the new GPUMCD algorithm (1 mm3 voxel size, 0.5% statistical uncertainty) and in the Pinnacle TPS using the collapsed cone convolution (CCC) algorithm. These were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2, 5×5 cm2, and 10×10 cm2 field sizes.Results:The calculated central axis percentage depth doses (PDDs) in homogeneous solid water were within 2% compared to measurements for XVMC and GPUMCD. For tumor‐in‐lung and lung phantoms, doses calculated by all of the algorithms were within the experimental uncertainty of the measurements (±2% in the homogeneous phantom and ±3% for the tumor‐in‐lung or lung phantoms), except for 2×2 cm2 field size where only the CCC algorithm differs from film by 5% in the lung region. The analysis for bone‐in‐tissue and the prosthetic phantoms are ongoing.Conclusion:The new GPUMCD algorithm calculated dose comparable to both the XVMC algorithm and to measurements in both a homogeneous solid water medium and the heterogeneous phantom representing lung or tumor‐in‐lung for 2×2 cm2‐10×10 cm2 field sizes.Funding support was obtained from Elekta.

Verification and Dosimetric Impact of Acuros XB Algorithm for Stereotactic Body Radiation Therapy (SBRT) and RapidArc Planning for Non-Small-Cell Lung Cancer (NSCLC) Patients

Purpose: The experimental verification of the Acuros XB (AXB) algorithm was conducted in a heterogeneous rectangular slab phantom, and compared to the Anisotropic Analytical Algorithm (AAA). The dosimetric impact of the AXB for stereotactic body radiation therapy (SBRT) and RapidArc planning for 16 non-small-cell lung cancer (NSCLC) patients was assessed due to the dose recalculation from the AAA to the AXB. Methods: The calculated central axis percentage depth doses (PDD) in a heterogeneous slab phantom for an open field size of 3 ×3 cm2 were compared against the PDD measured by an ionization chamber. For 16 NSCLC patients, the dose-volume parameters from the treatment plans calculated by the AXB and the AAA were compared using identical jaw settings, leaf positions, and monitor units (MUs). Results: The results from the heterogeneous slab phantom study showed that the AXB was more accurate than the AAA; however, the dose underestimation by the AXB (up to ?3.9%) and AAA (up to ?13.5%) was observed. For a planning target volume (PTV) in the NSCLC patients, in comparison to the AAA, the AXB predicted lower mean and minimum doses by average 0.3% and 4.3% respectively, but a higher maximum dose by average 2.3%. The averaged maximum doses to the heart and spinal cord predicted by the AXB were lower by 1.3% and 2.6% respectively; whereas the doses to the lungs predicted by the AXB were higher by up to 0.5% compared to the AAA. The percentage of ipsilateral lung volume receiving at least 20 and 5 Gy (V20 and V5 respectively) were higher in the AXB plans than in the AAA plans by average 1.1% and 2.8% respectively. The AXB plans produced higher target heterogeneity by average 4.5% and lower plan conformity by average 5.8% compared to the AAA plans. Using the AXB, the PTV coverage (95% of the PTV covered by the 100% of the prescribed dose) was reduced by average 8.2% than using the AAA. The AXB plans required about 2.3% increment in the number of MUs in order to achieve the same PTV coverage as in the AAA plans. Conclusion: The AXB is more accurate to use for the dose calculations in SBRT lung plans created with a RapidArc technique; however, one should also note the reduced PTV coverage due to the dose recalculation from the AAA to the AXB.

Verification and dosimetric impact of Acuros XB algorithm on intensity modulated stereotactic radiotherapy for locally persistent nasopharyngeal carcinoma

The main aim of the current study was to assess the dosimetric impact on intensity modulated stereotactic radiotherapy (IMSRT) for locally persistent nasopharyngeal carcinoma (NPC) due to the recalculation from the Anisotropic Analytical Algorithm (AAA) to the recently released Acuros XB (AXB) algorithm. The dosimetric accuracy of using AXB in predicting air∕tissue interface doses from an open single small field in a simple geometric phantom and intensity modulated small fields in an anthropomorphic phantom was also investigated. The central axis percentage depth doses (PDD) of a rectangular phantom containing an air cavity were calculated by both AAA and AXB from 6 MV beam with small field sizes (2 × 2 to 5 × 5 cm(2)). These data were compared to PDD measured by thin thermoluminescent dosimeters (TLDs) and Monte Carlo simulations. The doses predicted by AAA and AXB near air∕tissue interfaces from five different IMSRT plans were compared to the TLD measured doses in an anthropomorphic phantom. The PTV coverage, conformity and doses to organs at risk (OARs) calculated by AAA and AXB were compared for 12 patients, using identical beam setup, leaves movement and monitor units. Testing using the simple rectangular phantom demonstrated that AAA and AXB overestimated the PDD at the air∕tissue interfaces by up to 41% and 6%, respectively, from a 2 × 2 cm(2) field. The secondary build-up curves predicted by AXB caught up well with the measured data at around 2 mm beyond the air cavity. Testing using the anthropomorphic phantom showed that AAA overestimated the doses by up to 10%, while the measured doses matched those of the AXB to within 3%. Using AAA, the planning target coverage represented by 100% of the reference dose was estimated to be 4% higher than using AXB. The averaged minimum dose to the PTV predicted by AAA was about 4% higher and OARs doses 3% to 6% higher compared to AXB. AXB should be used whenever possible as the standard reference for IMSRT boost of NPC cases. The more accurate AXB indicating lower target coverage and lower minimum target dose compared to AAA should be noted.

SU-E-T-498: A Preliminary Monte Carlo Simulation Study of the Varian TrueBeam Linear Accelerator.

To benchmark the quality of Monte Carlo simulation results with the commissioning data for a Varian TrueBeam accelerator. IAEA phase space files of a 6MV TrueBeam accelerator provided by Varian were implemented using GEANT4 Monte Carlo code. The present application consisted of upper and lower jaws and a cubic water phantom of 0.125 cubic meters in volume. Both radial and transverse dose profiles (in 5 different depths) and a central axis percentage depth dose (PDD) curve were recorded in the phantom. Field sizes of as small as 4×4 cm2 and as large as 30×30 cm2 were simulated with 2.0E9 incident particles each. The results were then compared with our commissioning data performed in a Wellhoffer Blue Phantom with a 0.13cc ion-chamber and a 0.8×0.8mm diode. The GEANT4 simulated PDD curve compared favorably within ∼2% against the measured ion-chamber PDD for all field sizes and against the measured diode PDD for all fields less then 20×20cm2 . The simulated in-plane and cross-plane profiles compared well within 2 mm at the 50% level against the measured profiles for all field sizes. These results demonstrate the feasibility of utilizing Monte-Carlo simulated beam data in the commissioning of a linear accelerator. The increasing speed and capability of the desktop computer will Result in the adoption of Monte- Carlo techniques for dosimetric calculations.

Ge-doped optical fibres as thermoluminescence dosimeters for kilovoltage X-ray therapy irradiations

Abstract We investigate key dosimetric parameters for the thermoluminescence (TL) of Ge-doped silica optical fibres irradiated by X-rays generated at 90 and 300 kVp. The parameters include dose response, reproducibility and fading. Relative dose measurements were performed, obtaining central axis percentage depth dose (PDD) values, use being made of doped fibres irradiated in water and solid water phantoms. TL yields were compared with published data and ionisation chamber measurements. Linearity to dose was demonstrated over the investigated range (0.1–6 Gy), with reproducibility to within±2%. TL fading was found to be minimal, at