Beam Penumbra Research Articles

DNA dosimeter measurements of beam profile using a novel simultaneous processing technique

A DNA dosimeter (DNAd) was previously developed that uses double-strand breaks (DSB) to measure dose. This dosimeter has been tested to measure dose in scenarios where transient-charged particle equilibrium (TCPE) has been established. The probability of double strand break (PDSBo), which is the ratio of broken double-stranded DNA (dsDNA) to the initial unbroken dsDNA in the dosimeter, was used to quantify DSBs and related to dose. The goal of this work is to produce a new technique to process and analyze the DNAd and quantify DNA-DSBs. This technique included simultaneously processing multiple DNAds and also establishing a new form to the probability of double strand break (PDSBn), which was then used to test the DNAd in a non-TCPE condition by taking beam penumbra measurements. The technique utilized a 384-well plate, and the measurements were made at the edge of a 10 × 10 cm field and compared to film measurements. During these penumbra measurements, while observing the positional differences in the higher gradient region at 4.1 and 4.55 cm from the center of the radiation field, the distance to agreement of PDSBo to film were 0.38 cm and 0.26 cm while the distance to agreement of PDSBn to film were 0.11 cm and 0.06 cm, respectively. Finally, the developed new separation technique reduced the time needed for the analysis of 25 samples from 200 min to 30 min.

Application of high field magnetic resonance microimaging in polymer gel dosimetry

PurposeThe purpose of this work was to examine the suitability of VIPARnd polymer gel–9.4 T magnetic resonance microimaging system for high spatial resolution dose distribution measurements.MethodsThe VIPARnd samples (3 cm in outside diameter and 12 cm in height) were exposed to ionizing radiation by using a linear accelerator (Varian TrueBeam, USA; 6 MV x‐ray beam). In the calibration stage, nine gel dosimeter vials were irradiated in a water phantom homogenously to the doses from 1.5 to 30 Gy in order to obtain R2‒dose relation. In the verification stage, two gel dosimeter vials were irradiated in the half beam penumbra area of 10 × 10 cm radiation field using the amount of monitor units appropriate to deliver 20 Gy at the field center. The gels were imaged on a vertical 9.4 T magnetic resonance (MR) microimaging scanner using single slice and multislice (9 slices) multiecho (90 × 7 ms) sequences at the spatial resolutions of 0.2–0.4 × 0.2–0.4 × 3 mm3 and 0.2–0.4 × 0.2–0.4 × 1 mm3 respectively. The gels were subjected to microimaging during the period of two weeks after irradiation. The reference data consisted of the dose profiles measured using the diode dosimetry, radiochromic film, ionization chamber, and the water phantom system.ResultsThe VIPARnd‒9.4 T MR microimaging system was characterized by the dose sensitivity of 0.067 ± 0.002 Gy−1 s−1 at day 3 after irradiation. The dose resolution at 10 Gy (at P = 95%) was equal to 0.42 Gy at day 3 after irradiation using a single slice sequence (0.2 × 0.2 × 3 mm3) and 2.0 Gy at day 4 after irradiation using a multislice sequence (0.2 × 0.2 × 1 mm3) for one signal acquisition (measurement time: 15 min). These values were improved by ~1.4‐fold when using four signal acquisitions in the single slice sequence, and by ~2.78‐fold for 12 signal acquisitions in the multislice sequence. Furthermore, decreasing the in‐plane resolution from 0.2 × 0.2 mm2 to 0.4 × 0.4 mm2 resulted in a dose resolution of 0.3 Gy and 1 Gy at 10 Gy (at P = 95%) for one signal acquisition in the single slice and multislice sequences respectively (measurement time: 7.5 min).As reveals from the gamma index analysis the dose distributions measured at days 3–4 postirradiation using both VIPARnd verification phantoms agree with the data obtained using a silicon diode, assuming 1 mm/5% criterion. A good interphantom reproducibility of the polymer gel dosimetry was proved by monitoring of two phantoms up to 10 days after irradiation. However, the agreement between the dose distributions measured using the diode and polymer gel started to get worse from day 5 after irradiation.ConclusionThe VIPARnd–9.4T MR microimaging system allows to obtain dose resolution of 0.42 Gy at 10 Gy (at P = 95%) for a spatial resolution of 0.2 × 0.2 × 3 mm3 (acquisition time: 15 min). Further studies are required to improve a temporal stability of the gel‐derived dose distribution.

Modelling of the focal spot intensity distribution and the off-focal spot radiation in kilovoltage x-ray tubes for imaging

X-ray tubes for medical applications typically generate x-rays by accelerating electrons, emitted from a cathode, with an interelectrode electric field, towards an anode target. X-rays are not emitted from one point, but from an irregularly shaped area on the anode, the focal spot. Focal spot intensity distributions and off-focal radiation negatively affect the imaging spatial resolution and broadens the beam penumbra. In this study, a Monte Carlo simulation model of an x-ray tube was developed to evaluate the spectral and spatial characteristics of off-focal radiation for multiple photon energies. Slit camera measurements were used to determine the horizontal and vertical intensity profiles of the small and the large focal spot of a diagnostic x-ray tube. First, electron beamlet weighting factors were obtained via an iterative optimization method to represent both focal spot sizes. These weighting factors were then used to extract off-focal spot radiation characteristics for the small and large focal spot sizes at 80, 100, and 120 kV. Finally, 120 kV simulations of a steel sphere (d = 4 mm) were performed to investigate image blurring with a point source, the small focal spot, and the large focal spot. The magnitude of off-focal radiation strongly depends on the anode size and the electric field coverage, and only minimally on the tube potential and the primary focal spot size. In conclusion, an x-ray tube Monte Carlo simulation model was developed to simulate focal spot intensity distributions and to evaluate off-focal radiation characteristics at several energies. This model can be further employed to investigate focal spot correction methods and to improve cone-beam CT image quality.

Beam commissioning of the first compact proton therapy system with spot scanning and dynamic field collimation.

To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.

Beam penumbra reduction of Gamma Knife machine model 4C using Monte Carlo simulation

Background and objectiveIn small radiation fields used in stereotactic radiosurgery penumbra is an important portion of the field size especially when critical organs at risk are located near the treatment sites. This study was aimed to reduce penumbra width (90%–50% isodose lines) of Gamma Knife (GK) machine by investigating of source to diaphragm distance (SDD) and designing compensating filter. MethodsCompensating filters at the end of the helmet collimators with the aim of reducing penumbra as well as reducing hot spots appeared near the edge of beam were modeled using Monte Carlo simulation code. Moreover, the SDD parameter was increased as one of the effective factors on penumbra width. ResultsResults showed that single beam penumbra width using optimal design of filters was decreased by 59.49%, 42.50%, 39.02% and 34.44% with attenuation of 30.53%, 13.67%, 11.43% and 9.82% for 4, 8, 14 and 18 mm field sizes, respectively. ConclusionsThe designed filters lead to considerable reductions in single beams penumbra width as well as a noticeable reduction in maximum dose emerged near the beam edge due to the curved lateral surface of filters.

Dosimetric analysis of breast cancer tumor bed boost: An interstitial brachytherapy vs. external beam radiation therapy comparison for deeply seated tumors

PurposeTo dosimetrically compare interstitial brachytherapy (MIBT) vs. EBRT (3DCRT and high-energy electron beams) for deep-seated tumor bed boosts (depth ≥4 cm) in early-stage breast cancer. Methods and MaterialsPlanning CTs of fifteen left-side breast cancer patients previously treated with MIBT boost chosen for this study. MIBT, 3DCRT (three-field technique), and enface high-energy electron (15–18 MeV) plans retrospectively generated on these images. To minimize intrapatient target contour inconsistency, due to a technical limitation for transferring identical contours from brachytherapy to EBRT planning system, spherical volumes delineated as hypothetical CTVs (CTV-H) (depth ≥4 cm with considering the geometry of the brachytherapy implant) instead of original lumpectomy cavities (which had irregular contours). In EBRT, PTV-H=CTV-H+5 mm. To account for beam penumbra, additional PTV-H to beam-edge margins added (3DCRT = 5 mm; electron = 10 mm). Included organs at risk (OARs) were ipsilateral breast, skin, ribs, lung, and heart. Prescribed dose-fractionations were 12 Gy/3fractions (MIBT) and 16 Gy/8fractions (EBRT) (BED = 24 Gy, breast cancer Alpha/Beta = 4 Gy). Biologically equivalent DVH parameters for all techniques compared. ResultsMean CTV-H depth was 6 cm. Normal breast V25%–V100%; skin V10%–V90%; rib V25%–V75%; lung V5%–V25%; heart V10%; mean lung dose; ribs/lung Dmax were lower in MIBT vs. 3CDRT. MIBT reduced breast V25%–V125%; skin V25%–V125%; rib V25%–V75% and V100%; lung V25%–V90%; heart V10%–V50%; skin/ribs/lung Dmax compared to electrons. In contrast, breast V125%–V250% and V175%–V250% were increased in MIBT vs. 3DCRT and electron plans, respectively. Electron plans had the minimum mean heart dose. ConclusionsFrom a dosimetric point of view, in deeply-seated lumpectomy beds, MIBT boost better protects OARs from exposure to medium and high doses of radiation compared to 3DCRT and high energy electron beams (except more ipsilateral breast hot spots).

Predicting Optimal IMRT Dose Objectives by Relating TCP and NTCP to Target Dose Heterogeneity

There is growing interest in relaxing homogeneity constraints in IMRT treatments for solid tumors that do not contain normal tissue. Knowledge-based dose predictions can improve plan quality, but depend on historical data typically incorporating homogeneity constraints. An analytical TCP and NTCP model could be used to estimate optimal target dose heterogeneity prior to plan optimization by assuming basic properties of the dose distribution, thereby improving prospective plan quality. We derived and validated an analytical model of TCP and NTCP based on the properties of co-planar VMAT including target dose heterogeneity, beam penumbra, and dose spillage outside of the target. We derived the TCP/NTCP model with input parameters including tumor size, tumor edge dose (prescription dose), tumor center dose, fractionation, and estimates of standard radio-biological parameters such as tumor clonogen density, radio-sensitivity mean and variance, and Lyman-Kutcher-Burman (LKB) NTCP model parameters. To validate the model, we created a virtual phantom incorporating a spherical tumor with 2 cm diameter and produced a series of VMAT plans in a treatment planning system with varying central target dose between 108% and 262% of the prescription while minimizing dose spillage outside of the target. Differential DVHs were exported and used to estimate TCP and NTCP using the Marsden and LKB models assuming a radio-resistant tumor prescribed 60 Gy in 30 fractions completely surrounded by a single cylindrical parallel organ. The TCP and NTCP were also calculated for these test cases using the analytical model, and compared to the values calculated using the treatment planning system data. The target heterogeneity and corresponding TCP and NTCP values estimated by the analytical model and planning system based approaches are summarized in Table 1. The errors in the TCP and NTCP estimates of the model were within 0.04 and 0.01, respectively, for all plans, despite not requiring a dose calculation. The analysis also demonstrated a steady increase in TCP with increased central target dose, and a transient decrease in NTCP. A central target dose of 111% minimized NTCP, and a central target dose of 124% maximized TCP while providing the same or lower NTCP as a homogeneous target dose. We have derived an analytical model of TCP and NTCP based on simple tumor and dose characteristics that can be applied immediately following contouring to estimate optimal target dose heterogeneity. This tool can be used to improve the dose objectives used in prospective clinical trials. We will present results following model validation in multiple clinical scenarios and demonstrate patient use cases.Abstract 3822; Table 1Central Target Dose (% of Rx)TCPNTCPPinnacleModelPinnacleModel1080.680.640.0500.0481110.720.680.0490.0471240.800.790.0500.0471610.920.930.0530.0522050.960.970.0590.0592340.980.980.0570.0632490.990.980.0580.0642621.000.980.0560.065 Open table in a new tab

An Integrated Framework Based on Full Monte Carlo Simulations for Double-Scattering Proton Therapy.

We developed an integrated framework that employs a full Monte Carlo (MC) model for treatment-plan simulations of a passive double-scattering proton system. We have previously validated a virtual machine source model for full MC proton-dose calculations by comparing the percentage of depth-dose curves, spread-out Bragg peaks, and lateral profiles against measured commissioning data. This study further expanded our previous work by developing an integrate framework that facilitates its clinical use. Specifically, we have (1) constructed patient-specific applicator and compensator numerically from the plan data and incorporated them into the beamline, (2) created the patient anatomy from the computed tomography image and established the transformation between patient and machine coordinate systems, and (3) developed a graphical user interface to ease the whole process from importing the treatment plan in the Digital Imaging and Communications in Medicine format to parallelization of the MC calculations. End-to-end tests were performed to validate the functionality, and 3 clinical cases were used to demonstrate clinical utility of the framework. The end-to-end tests demonstrated that the framework functioned correctly for all tested functionality. Comparisons between the treatment planning system calculations and MC results in 3 clinical cases revealed large dose difference up to 17%, especially in the beam penumbra and near the end of beam range. The discrepancy likely originates from a variety of sources, such as the dose algorithms, modeling of the beamline, and the dose metric. The agreement for other regions was acceptable. An integrated framework was developed for full MC simulations of double-scattering proton therapy. It can be a valuable tool for dose verification and plan evaluation.

Tungsten filled 3D printed field shaping devices for electron beam radiation therapy

PurposeElectron radiotherapy is a labor-intensive treatment option that is complicated by the need for field shaping blocks. These blocks are typically made from casting Cerrobend alloys containing lead and cadmium. This is a highly toxic process with limited precision. This work aims to provide streamlined and more precise electron radiotherapy by 3D using printing techniques.MethodsThe 3D printed electron cutout consists of plastic shells filled with 2 mm diameter tungsten ball bearings. Five clinical Cerrobend defined field were compared to the planned fields by measuring the light field edge when mounted in the electron applicator on a linear accelerator. The dose transmitted through the 3D printed and Cerrobend cutouts was measured using an IC profiler ion chamber array with 6 MeV and 16 MeV beams. Dose profiles from the treatment planning system were also compared to the measured dose profiles. Centering and full width half maximum (FWHM) metrics were taken directly from the profiler software.ResultsThe transmission of a 16MeV beam through a 12 mm thick layer of tungsten ball bearings agreed within 1% of a 15 mm thick Cerrobend block (measured with an ion chamber array). The radiation fields shaped by ball bearing filled 3D printed cutout were centered within 0.4 mm of the planned outline, whereas the Cerrobend cutout fields had shift errors of 1–3 mm, and shape errors of 0.5–2 mm. The average shift of Cerrobend cutouts was 2.3 mm compared to the planned fields (n = 5). Beam penumbra of the 3D printed cutouts was found to be equivalent to the 15 mm thick Cerrobend cutout. The beam profiles agreed within 1.2% across the whole 30 cm profile widths.ConclusionsThis study demonstrates that with a proper quality assurance procedure, 3D-printed cutouts can provide more accurate electron radiotherapy with reduced toxicity compared to traditional Cerrobend methods.

SparseCT: System concept and design of multislit collimators.

SparseCT, an undersampling scheme for compressed sensing (CS) computed tomography (CT), has been proposed to reduce radiation dose by acquiring undersampled projection data from clinical CT scanners (Koesters etal. in, SparseCT: Interrupted-Beam Acquisition and Sparse Reconstruction for Radiation Dose Reduction; 2017). SparseCT partially blocks the x-ray beam with a multislit collimator (MSC) to perform a multidimensional undersampling along the view and detector row dimensions. SparseCT undersamples the projection data within each view and moves the MSC along the z-direction during gantry rotation to change the undersampling pattern. It enables reconstruction of images from undersampled data using CS algorithms. The purpose of this work is to design the spacing and width of the MSC slits and the MSC motion patterns based on beam separation, undersampling efficiency, and image quality. The development and testing of a SparseCT prototype with the designed MSC will be described in a following paper. We chose a few initial MSC designs based on the guidance from two metrics: beam separation and undersampling efficiency. Both beam separation and undersampling efficiency were measured from numerically simulated photon distribution with MSC taken into consideration. Beam separation measures the separation between x-ray beams from consecutive slits, taking into account penumbra effects on both sides of each slit. Undersampling efficiency measures the dose-weighted similarity between penumbra undersampling and binary undersampling, in other words, the effective contribution of the incident dose to the signal to noise ratio of the projection data. We then compared the initially chosen MSC designs in terms of their reconstruction image quality. SparseCT projections were simulated from fully sampled patient projection data according to the MSC design and motion pattern, reconstructed iteratively using a sparsity-enforcing penalized weighted least squares cost function with ordered subsets/momentum algorithm, and compared visually and quantitatively. Simulated photon distributions indicate that the size of the penumbra is dominated by the size of the focal spot. Therefore, a wider MSC slit and a smaller focal spot lead to increased beam separation and undersampling efficiency. For fourfold undersampling with a 1.2mm focal spot, a minimum MSC slit width of three detector rows (projected to the detector surface) is needed for beam separation; for threefold undersampling, a minimum slit width of four detector rows is needed. Simulations of SparseCT projection and reconstruction indicate that the motion pattern of the MSC does not have a visible impact on image quality. An MSC slit width of three or four detector rows yields similar image quality. The MSC is the key component of the SparseCT method. Simulations of MSC designs incorporating x-ray beam penumbra effects showed that for threefold and fourfold dose reductions, an MSC slit width of four detector rows provided reasonable beam separation, undersampling efficiency, and image quality.

Characterization of a Novel 3 Megavolt Linear Accelerator for Dedicated Intracranial Stereotactic Radiosurgery.

The purpose of this article is to investigate and characterize from a physics perspective the Zap-X (ZAP Surgical Systems, Inc., San Carlos, CA), a new, dedicated self-contained and self-shielded radiosurgery system, focusing on beam energy and performance, leakage, radiation safety, dose delivery accuracy, regulations, quality assurance, and treatment planning. This investigation is required to establish the mechanical and overall performance specifications of the system and to establish baseline parameters for future clinical usage.The applied methods include measurements of energy, focal spot size, beam performance, dosimetry, beam data, treatment planning system, leakage radiation, acceptance testing, and commissioning.The results of the characterization reveal a 3 megavolt (MV) linear accelerator (linac) with a focal spot size of 2 mm, a dose rate of 1,500 MU/min at the isocenter with a dose linearity of 3%, a beam penumbra of less than 3 mm, and beam symmetry of less than 2%. Beam performance, as well as dosimetry characteristics, are suitable for intracranial radiosurgery.It can be concluded that the system was found to meet safety, accuracy, and performance requirements widely accepted in the radiation oncology and radiosurgery industry. Furthermore, the system was shown to meet the practical, clinical needs of the radiosurgery community.

Combined Megavoltage and Contrast-Enhanced Radiotherapy as an Intrafraction Motion Management Strategy in Lung SBRT.

Using Monte Carlo simulation and a realistic patient model, it is shown that the volume of healthy tissue irradiated at therapeutic doses can be drastically reduced using a combination of standard megavoltage and kilovoltage X-ray beams with a contrast agent previously loaded into the tumor, without the need to reduce standard treatment margins. Four-dimensional computed tomography images of 2 patients with a centrally located and a peripherally located tumor were obtained from a public database and subsequently used to plan robotic stereotactic body radiotherapy treatments. Two modalities are assumed: conventional high-energy stereotactic body radiotherapy and a treatment with contrast agent loaded in the tumor and a kilovoltage X-ray beam replacing the megavoltage beam (contrast-enhanced radiotherapy). For each patient model, 2 planning target volumes were designed: one following the recommendations from either Radiation Therapy Oncology Group (RTOG) 0813 or RTOG 0915 task group depending on the patient model and another with a 2-mm uniform margin determined solely on beam penumbra considerations. The optimized treatments with RTOG margins were imparted to the moving phantom to model the dose distribution that would be obtained as a result of intrafraction motion. Treatment plans are then compared to the plan with the 2-mm uniform margin considered to be the ideal plan. It is shown that even for treatments in which only one-fifth of the total dose is imparted via the contrast-enhanced radiotherapy modality and with the use of standard treatment margins, the resultant absorbed dose distributions are such that the volume of healthy tissue irradiated to high doses is close to what is obtained under ideal conditions

Study the effect of source to diaphragm distance (SDD) on beam penumbra width of Gamma Knife machine model 4C using Monte Carlo simulation

Introduction: Stereotactic radiosurgery (SRS) is one of the conformal radiotherapy methods and Gamma Knife is a non-invasive SRS technique to treat intracranial lesions with 201 Co- 60 sources without need to surgery and bleeding. Beam penumbra phenomenon leads to irradiation outside the field and normal tissues. Therefore, considering it is momentous to attain the acceptable treatment plan. Materials and Methods: In this study, Gamma Knife unit was modeled using EGSnrc/BEAMnrc Monte Carlo code. SDD (Source to Diaphragm Distance) parameter that is one of the affecting factors on physical penumbra width was studied. For this purpose, SDD was increased for 4, 8, 14 and 18 mm collimator sizes in five steps and each step as much as one centimeter. Single beam profiles were calculated using EGSnrc/DOSXYZnrc code at isocentre depth in a cubic Plexiglas phantom. MATLAB and EXCEL software were used to plot profiles and measure physical penumbra width . Results: Based on the results, physical penumbra width (90%-50%) does not show a regular trend with increasing SDD. This may be attributed to scatter radiation which is due to the low distance between the Gamma Knife collimator end and phantom surface. Best results were observed for 4 and 8 mm collimators in 1 cm increase in which penumbra width decreased from 0.785 and 0.790 mm to 0.749 and 0.736 mm for 4 and 8 mm collimator sizes, respectively. Conclusion: It is recommended to increase SDD of Gamma Knife 4C around 1 centimeter to reduce penumbra width for small tumor sizes located near the critical nerves.

Steeper dose gradients resulting from reduced source to target distance-a planning system independent study.

PurposeTo quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source‐to‐axis distance (SAD) for simple spherical target and different prescription isodoses (PI).MethodA TPS‐independent method was used to estimate three‐dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two‐dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi‐Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross‐sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI.ResultsFor practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%–12%; gradient indices (GI) were reduced by 3–39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%–33% lower for reduced SAD in the PI range 40%–70%. For PI = 80% and 90% the results depend on the circular collimator model.ConclusionStereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies.

Penumbra width determination of single beam and 201 beams of Gamma Knife machine model 4C using Monte Carlo simulation

AbstractBackgroundOne of the stereotactic radiosurgery techniques is Gamma Knife radiosurgery, in which intracranial lesions that are inaccessible or inappropriate for surgery are treated using 201 cobalt-60 sources in one treatment session. In this conformal technique, the penumbra width, which results in out-of-field dose in tumour-adjacent normal tissues should be determined accurately. The aim of this study is to calculate the penumbra widths of single and 201 beams for different collimator sizes of Gamma Knife machine model 4C using EGSnrc/BEAMnrc Monte Carlo simulation code and comparison the results with EBT3 film dosimetry data.Methods and materialsIn this study, simulation of Gamma Knife machine model 4C was performed based on the Monte Carlo codes of EGSnrc/BEAMnrc. To investigate the physical penumbra width (80−20%), the single beam and 201 beams profiles were obtained using EGSnrc/DOSXYZnrc code and EBT3 films located at isocentre point in a spherical Plexiglas head phantom.ResultsBased on the results, the single beam penumbra widths obtained from simulation data for 4, 8, 14 and 18 mm collimator sizes alongXaxis were 0·75, 0·77, 0·90 and 0·92 mm, respectively. The data for 201 beams obtained from simulation were 2·61, 4·80, 7·92 and 9·81 mm alongXaxis and 1·31, 1·60, 1·91 and 2·14 mm alongZaxis and from film dosimetry were 3·21, 4·90, 8·00 and 10·61 mm alongXaxis and 1·22, 1·69, 2·01 and 2·25 mm alongZaxis, respectively.ConclusionThe differences between measured and simulated penumbra widths are in an acceptable range. However, for more precise measurement in the penumbra region in which dose gradient is high, Monte Carlo simulation is recommended.

OA188] Passive ion beam modulation techniques for particle therapy raster scanning facilities

Purpose Raster scanning particle therapy offers potentially very conformal treatments. Limitation are long irradiation times and a sensitivity to intra-fractional motion. Passive energy modulators with an inhomogeneous mass distribution broaden the width of the mono-energetic Bragg peaks and reduce the accelerator energy shifts needed to homogeneously cover the target and thus the irradiation time. A modulator in clinical use is the 1D ripple filter (RiFi). Newer 2D RiFis improve comparatively the filter mass distribution, reducing the overall lateral beam width with a further reduced irradiation time. For certain cases a 3D RiFi can be designed, which with one energy covers the whole target within seconds. All designs can be produced using 3D printing. Furthermore, porous materials could function like passive energy modulators. Methods This work presents a general overview of these new modulators with focus on experimental data and treatment planning. We show carbon ion treatment plans with 1D and 2D RiFis for spheres in water, 8 NSCLC, 4 chordoma, and 3 prostate cases and proton plans for a 6 mm 2D RiFi compared to no RiFi for spheres and 9 NSCLC cases. A multi-ion facility was assumed and simulated. Baseline data for treatment planning were generated with the Monte Carlo code SHIELD-HIT12A and imported in the treatment planning systems TRiP98 and Eclipse. Results Measured Bragg curves confirm the functionality of the new modulators for protons and carbon ions. For carbon ions, all plans yielded comparable dosimetric results in terms of plan homogeneity and conformity but with up to half the irradiation time for thicker RiFis. For protons, the RiFi offers a similar coverage as without with up to only one-fourth of the necessary accelerator shifts but were found to be most beneficial in treatment cases with reduced nozzle-to-treatment-isocenter distance. Plan homogeneity and conformity were slightly improved for thinner RiFis with RiFi performances increasing with penetration depth. Porous plates could be used as passive energy modulators and simultaneously function as range shifters, which placed close to the patients leads to reduced beam penumbras for low penetration depths. Conclusions Passive energy modulators reduce the irradiation time in raster scanning particle therapy.

Abstract 4153: Commissioning and radiobiological verification of the small animal radiotherapy research platform (SARRP)

Abstract Preclinical radiation biology has become increasingly sophisticated due to the implementation of advanced small animal image guided radiation platforms into laboratory investigation. Small animal radiotherapy devices enable state-of-the-art image guided therapy (IGRT) research to be performed in small animal cancer models by combining high-resolution cone beam computed tomography (CBCT) imaging with an isocentric irradiation system. The technological evolution from simple, broad field irradiation configurations, to more sophisticated dose deliveries for preclinical radiobiology experiments has introduced new dosimetry challenges for preclinical small animal cancer research. Because of this, robust QA and dosimetry techniques are a key part of using novel treatment platforms using very small irradiation fields. In this study, we present a dosimetric study of the small animal radiotherapy research platform (SARRP, Xstrahl Inc.) focusing on the small field dosimetry. Physical dosimetry was assessed using ion chambers and radiochromic film, investigating the impact the beam focus size has on the dose rate output as well as beam characteristics (beam shape and penumbra variations). Two film reading modalities have been used to assess the dose output using the 0.5 mm diameter aperture. This study establishes that FilmQA Pro is a suitable tool for small field dosimetry, with a sufficiently small sampling area (0.1 mm) to ensure an accurate measurement. Furthermore, all small field dosimetry data generated in this study are equally comparable with our Monte Carlo simulations for both focal spot sizes. Overall, it was observed that the small focal spot, while having a lower dose-rate output was shown to produce a more stable and homogenous beam with minimum penumbra over time when compared to the larger focal spot. These findings suggest that when preclinical stereotactic irradiation fields are used, a practical compromise (time to deliver a desired dose and field homogeneity) needs to be considered when deciding the optimum treatment plan and beam configuration used. Citation Format: Jonathan L. Kane. Commissioning and radiobiological verification of the small animal radiotherapy research platform (SARRP) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4153.

Dosimetric verification and commissioning for a small animal image-guided irradiator

In recent years, small animal image-guided irradiators have been widely utilized in preclinical studies involving rodent models. However, the dosimetry commissioning of such equipment involving kilovoltage small-field dosimetry has not received as much interest as the megavoltage small-field dosimetry used clinically. To date, a paucity of measured kilovoltage beam data, especially for field sizes less than 3 mm, can be found in the literature. For improvement of rodent treatments in the future, this work aims to provide comprehensive and accurate beam data for the small animal radiation research platform (SARRP, Xstrahl) using EBT3 Gafchromic films and Monte Carlo calculation, with submillimeter resolution and accuracy.This work includes three primary tasks: (1) establish an optimized film measurement protocol for small field dosimetry of kilovoltage photon beam. (2) Acquire dosimetric data including (a) depth dose curves from the surface to 6 cm depth (b) beam profiles, (c) penumbra, (d) cone factors and (e) 2D dose distribution. These tasks were undertaken for a 220 kVp photon beam with five different small field widths and 33 cm source to surface distance (0.5 mm and 1 mm circular fields, 3 × 3 mm2, 5 × 5 mm2, 10 × 10 mm2 square fields). Beam data was measured with EBT3 films. (3) Provide comparative dosimetry for film measurements, Monte Carlo calculations, and the dose calculations performed with the SARRP treatment planning system, Muriplan.For the majority of parameters, film measurement agreed with Monte Carlo simulation within 1%. There were, however, discrepancies between measured beam data and Muriplan treatment planning data. Specifically, for PDD, Muriplan underestimates the dose for field sizes of 0.5 mm and 1 mm. For beam profiles comparisons, the calculation from Muriplan predicts a smaller lateral distance between the 50% isodose lines compared to film measurement. There is a difference of 0.18, 0.72, 0.6 mm between Muriplan and film for field sizes of 3, 5, 10 mm, respectively.This work demonstrates that accurate and precise kilovoltage small-field dosimetry can be conducted using EBT3 Gafchromic film with an optimized protocol. In addition, discrepancies between measured beam data and Muriplan were identified.

Design and development of an add-on automated multi-leaf collimator for telecobalt therapy machine and study of its characteristics

Purpose: The purpose of the paper is to present the design, development and dosimetric characteristics of an automated Multi-leaf collimator (MLC) integrated along with an inexpensive Treatment Planning System (ROPS), developed for existing telecobalt units as an add-on without making any modifications to the treatment machine. Method: The prototype MLC design consists of 28 tungsten alloy leaves (14 leaf pairs) having a mass density of 18 g cm -3 . Each of the leaves projects 10 mm width at the isocenter, which is at 80 cm from the source. The automation has been achieved with a dedicated linear actuator for each leaf. Radiochromic films and IC PROFILER™ (Sun Nuclear Corporation, Melbourne, FL) were used for the measurement of beam profiles and the profiles were analyzed to arrive at radiation field width, beam flatness, symmetry and beam penumbra. Results: The prototype MLC can define radiation fields of up to 14 × 14 cm² within the prescribed tolerance values of 2 mm. The flatness and symmetry were found to be within the prescribed tolerance value of 3%. The penumbra for a 10 × 10 cm² field size is 9.5 mm which is less than the generally acceptable value of 12 mm for a telecobalt machine. The maximum leakage through the leaf ends in closed condition was observed to be 7.3% which is less than the values reported for other MLCs designed for medical linear accelerators. Conclusion: It is concluded that dosimetric parameters and the leakage radiation of the prototype MLC are well below their recommended tolerance values. The MLC can be used for carrying out conformal radiotherapy with existing telecobalt machines.

EP-2071: Determination of the flattening filter free beam penumbra of the MR-linac without a reference beam

EP-2071: Determination of the flattening filter free beam penumbra of the MR-linac without a reference beam