Beam Penumbra Research Articles

Measurements of lateral penumbra for uniform scanning proton beams under various beam delivery conditions and comparison to the XiO treatment planning system

The main purposes of this study were to (1) investigate the dependency of lateral penumbra (80%-20% distance) of uniform scanning proton beams on various factors such as air gap, proton range, modulation width, compensator thickness, and depth, and (2) compare the lateral penumbra calculated by a treatment planning system (TPS) with measurements. First, lateral penumbra was measured using solid-water phantom and radiographic films for (a) air gap, ranged from 0 to 35 cm, (b) proton range, ranged from 8 to 30 cm, (c) modulation, ranged from 2 to 10 cm, (d) compensator thickness, ranged from 0 to 20 cm, and (e) depth, ranged from 7 to 15 cm. Second, dose calculations were computed in a virtual water phantom using the XiO TPS with pencil beam algorithm for identical beam conditions and geometrical configurations that were used for the measurements. The calculated lateral penumbra was then compared with the measured one for both the horizontal and vertical scanning magnets of our uniform scanning proton beam delivery system. The results in the current study showed that the lateral penumbra of horizontal scanning magnet was larger (up to 1.4 mm for measurement and up to 1.0 mm for TPS) compared to that of vertical scanning magnet. Both the TPS and measurements showed an almost linear increase in lateral penumbra with increasing air gap as it produced the greatest effect on lateral penumbra. Lateral penumbra was dependent on the depth and proton range. Specifically, the width of lateral penumbra was found to be always lower at shallower depth than at deeper depth within the spread out Bragg peak (SOBP) region. The lateral penumbra results were less sensitive to the variation in the thickness of compensator, whereas lateral penumbra was independent of modulation. Overall, the comparison between the results of TPS with that of measurements indicates a good agreement for lateral penumbra, with TPS predicting higher values compared to measurements. Lateral penumbra of uniform scanning proton beams depends on air gap, proton range, compensator thickness, and depth, whereas lateral penumbra is not dependent on modulation. The XiO TPS typically overpredicted lateral penumbra compared to measurements, within 1 mm for most cases, but the difference could be up to 2.5 mm at a deep depth and large air gap.

Proton Arc Reduces Range Uncertainty Effects and Improves Conformality Compared With Photon Volumetric Modulated Arc Therapy in Stereotactic Body Radiation Therapy for Non-Small Cell Lung Cancer

To describe, in a setting of non-small cell lung cancer (NSCLC), the theoretical dosimetric advantages of proton arc stereotactic body radiation therapy (SBRT) in which the beam penumbra of a rotating beam is used to reduce the impact of range uncertainties. Thirteen patients with early-stage NSCLC treated with proton SBRT underwent repeat planning with photon volumetric modulated arc therapy (Photon-VMAT) and an in-house-developed arc planning approach for both proton passive scattering (Passive-Arc) and intensity modulated proton therapy (IMPT-Arc). An arc was mimicked with a series of beams placed at 10° increments. Tumor and organ at risk doses were compared in the context of high- and low-dose regions, represented by volumes receiving >50% and <50% of the prescription dose, respectively. In the high-dose region, conformality index values are 2.56, 1.91, 1.31, and 1.74, and homogeneity index values are 1.29, 1.22, 1.52, and 1.18, respectively, for 3 proton passive scattered beams, Passive-Arc, IMPT-Arc, and Photon-VMAT. Therefore, proton arc leads to a 30% reduction in the 95% isodose line volume to 3-beam proton plan, sparing surrounding organs, such as lung and chest wall. For chest wall, V30 is reduced from 21 cm(3) (3 proton beams) to 11.5 cm(3), 12.9 cm(3), and 8.63 cm(3) (P=.005) for Passive-Arc, IMPT-Arc, and Photon-VMAT, respectively. In the low-dose region, the mean lung dose and V20 of the ipsilateral lung are 5.01 Gy(relative biological effectiveness [RBE]), 4.38 Gy(RBE), 4.91 Gy(RBE), and 5.99 Gy(RBE) and 9.5%, 7.5%, 9.0%, and 10.0%, respectively, for 3-beam, Passive-Arc, IMPT-Arc, and Photon-VMAT, respectively. Stereotactic body radiation therapy with proton arc and Photon-VMAT generate significantly more conformal high-dose volumes than standard proton SBRT, without loss of coverage of the tumor and with significant sparing of nearby organs, such as chest wall. In addition, both proton arc approaches spare the healthy lung from low-dose radiation relative to photon VMAT. Our data suggest that IMPT-Arc should be developed for clinical use.

Implementation and commissioning of an integrated micro-CT/RT system with computerized independent jaw collimation

To design, construct, and commission a set of computer-controlled motorized jaws for a micro-CT∕RT system to perform conformal image-guided small animal radiotherapy. The authors designed and evaluated a system of custom-built motorized orthogonal jaws, which allows the delivery of off-axis rectangular fields on a GE eXplore CT 120 preclinical imaging system. The jaws in the x direction are independently driven, while the y-direction jaws are symmetric. All motors have backup encoders, verifying jaw positions. Mechanical performance of the jaws was characterized. Square beam profiles ranging from 2×2 to 60×60 mm2 were measured using EBT2 film in the center of a 70×70×22 mm3 solid water block. Similarly, absolute depth dose was measured in a solid water and EBT2 film stack 50×50×50 mm3. A calibrated Farmer ion chamber in a 70×70×20 mm3 solid water block was used to measure the output of three field sizes: 50×50, 40×40, and 30×30 mm2. Elliptical target plans were delivered to films to assess overall system performance. Respiratory-gated treatment was implemented on the system and initially proved using a simple sinusoidal motion phantom. All films were scanned on a flatbed scanner (Epson 1000XL) and converted to dose using a fitted calibration curve. A Monte Carlo beam model of the micro-CT with the jaws has been created using BEAMnrc for comparison with the measurements. An example image-guided partial lung irradiation in a rat is demonstrated. The averaged random error of positioning each jaw is less than 0.1 mm. Relative output factors measured with the ion chamber agree with Monte Carlo simulations within 2%. Beam profiles and absolute depth dose curves measured from the films agree with simulations within measurement uncertainty. Respiratory-gated treatments applied to a phantom moving with a peak-to-peak amplitude of 5 mm showed improved beam penumbra (80%-20%) from 3.9 to 0.8 mm. A set of computer-controlled motorized jaws for a micro-CT∕RT system were constructed with position reliably better than a tenth of a millimeter. The hardware system is ready for image-guided conformal radiotherapy for small animals with capability of respiratory-gated delivery.

TU‐E‐108‐02: Dynamic Electron Arc Radiotherapy (DEAR): A Feasibility Study

Purpose: Electron beam therapy is under‐utilized in radiation treatment of cancer and has remained practically unchanged for decades. We propose a new technique, Dynamic Electron Arc Radiotherapy (DEAR) with synchronized couch motion. DEAR utilizes modulation in gantry rotation, dose rate, and intensity to create desirable dose distributions. This study investigates the feasibility of DEAR in terms of deliverability and dosimetry. Methods: DEAR plans were designed and delivered across various energies, doses, and dose rates on a cylindrical phantom. Deliverability was evaluated by verifying the accuracy of the MU, gantry, and couch axes: The agreement between planned and actual axes values recorded in the trajectory log during delivery was quantified by the root mean square error (RMSE). Subsequently, couch, gantry, and collimator velocities were derived and evaluated. Maximum velocities were verified by delivering 10 MU at 1000 MU/min while an axis covered a large distance/rotation, thus forcing a dose rate drop resulting in maximum velocity. Dosimetry was evaluated with radiographic film for static and DEAR plans by comparing in‐plane and cross‐plane beam penumbra (20–80%) to those calculated with the eMC algorithm in Eclipse v10. Dose homogeneity within target region was evaluated. Results: Trajectory log analysis of planned and actual axes values for DEAR plans had small RMSE (&lt;0.1 MU, &lt;0.1° gantry, &lt;0.1 cm couch positions). Velocities for couch lateral, vertical, and gantry ranged from 0.1–1.1cm/s, 0.1–1.7cm/s, and 0.3– 6° /s, respectively. Maximum velocities agreed with manufacturer's specifications and were 3.8, 2, and 7.4cm/s for couch lateral, vertical, and longitudinal, respectively; and 2.9, 5.9, and 14.2°/s for couch rotational, gantry, and collimator, respectively. Dosimetric evaluation showed good agreement for the penumbrae (within 1 mm). Further, dose homogeneity was within ±2% of the mean dose. Conclusion: Preliminary findings show that DEAR has the potential to deliver electron beam therapy with high accuracy and dose homogeneity.

TU‐E‐108‐11: Design and Simulation of a Short, 10 MV, S‐Band Linear Accelerator

Purpose: To design a short (27.5 cm) S‐band linear accelerator capable of producing a 10 MV x‐ray beam to enable higher photon energies for linear accelerator‐MRI hybrid systems, where the waveguide length and linac‐MR orientation is a concern. Methods: Finite element software COMSOL Multiphysics was used to design an accelerator cavity matching one published in an experiment on breakdown, to ensure that the threshold electric field published are applicable to the final waveguide geometry. This cavity was used as the basis for designing an accelerator waveguide, and the full RF field solution within the waveguide was calculated. This RF solution and an electron‐gun phase space generated using Opera3D/SCALA are input into electron tracking software PARMELA to compute the electron phase space striking the x‐ray target. The resulting electron beam is used for Monte Carlo simulations to calculate PDD curves and the beam penumbra, and compared against those produced by a Varian 10 MV. Results: The shunt impedance, Q factor, and ratio of peak‐to‐mean electric fields of the cavity were matched to those in the published study to within 0.1% error. After tuning the full waveguide, the peak surface fields are 207 MV/m, well below the breakdown threshold of 239 MV/m. The peak of the electron spectrum is centered at 10.4 MeV, with a width of 0.2 MeV, compared with 10.5 MeV and 0.3 MeV, respectively, for a Varian 10 MV linac. Monte Carlo results show dmax is 2.15 cm for a 10×10 cm2 field and the penumbra is 5.0 mm wide at 10 cm depth, compared with dmax at 2.3 cm and 5.6 mm width for a Varian 10 MV linac. Conclusion: This work demonstrates the design and modeling of a waveguide capable of producing 10 MV energy x‐rays yet short enough to be compatible with a linac‐MR system.

SU-E-T-333: A New Approach in Radiation Therapy: Proton Grid Therapy

Purpose: To evaluate the interest of the development of grid therapy with protons. The main aim of this proof of concept was twofold: first, the evaluation of a possible gain in tissue sparing thanks to the spatial fractionation of the dose provided by the grid and second, to show that the more precise ballistics of protons offer a further gain with respect to MV GRID therapy. Methods: Monte Carlo simulations (GATE v.6) have been used as a method to calculate the ratio of the peak-to-valley doses (PVDR) at different depths in a water phantom for several grid configurations (0.7 mm of beam width and 1.4, 2.1, 2.8 and 3.5 mm center-to-center distances). The beam energy considered was 105 MeV. The beam penumbras were also assessed as an important parameter for tissue sparing. Results: Very high values of PVDR, significantly superior than the ones achieved in MV Grid therapy, were obtained. In addition, the penumbras are much more smaller than in the case of MV Grid therapy and of Gamma Knife radiosurgery. Conclusion: The high PVDR obtained, the small penumbras and the absence of deposited dose after the Bragg Peak suggest a potential gain in tissue sparing in this new radiotherapy approach. This could be used to deposit higher and potentially curative doses in clinical cases where tissue tolerances are a limit for conventional methods. Biological experiments are warranted to assess the effects of proton grid therapy.

SU-E-T-531: Spatial Fractionation of the Dose in Heavy Ion Therapy

Purpose: To evaluate the possible gain in tissue sparing in heavy ion therapy when a spatial fractionation of the dose is used. Methods: Monte Carlo simulations (GATE v.6) have been used as a method to calculate the dose distributions in a cylindrical water phantom (16 cm height and 16 cm of diameter). The phantom was irradiated with carbon and oxygen minibeams of 0.7 mm of beam width and 1.4, 2.1, 2.8 and 3.5 mm center‐to‐center distances. The ratio of the peak‐to‐valley doses (PVDR), that is considered to be a very relevant dosimetric parameter, was evaluated at different depths. The beam penumbras were also assessed as an important parameter for tissue sparing. Results: Very high values of PVDR, significantly superior than the ones achieved in other spatial fractionated radiotherapy techniques, like in synchrotron minibeam radiation therapy were obtained. In addition, the penumbras are much more smaller than in the case of MV Grid therapy and of Gamma Knife radiosurgery. Of the two types of ions studied, oxygen is the one that provides the more advantageous dose distributions. Conclusion: A gain in tissue sparing might be expected in this new radiotherapy approach thanks to the high PVDR obtained, the small penumbras and the absence of deposited dose after the Bragg Peak. This could be used to treat radioresistant tumors or non‐cancer diseases, like epilepsy.

SU-E-T-143: Dosimetric Characteristics of TrueBeam STx System with High Definition Multi-Leaf Collimator: Commissioning of Stereotactic Radiosurgery

IntroductionDosimetric characteristics of one TrueBeam STx treatment unit with a 120‐leaf high definition multileaf collimator (HD120MLC) are systematically measured for commissioning of stereotactic radiosurgery (SRS). Due to limited data available, it is important to make the dosimetric data available for reference.MethodsSRS commissioning data were acquired for two photon beam calculation algorithms of the treatment planning system iPlan (BrainLAB iPlan4.5): Clarkson method and Pencil Beam dose calculation (PBC) method. Major beam data required include percent depth dose (PDD), beam profile, and relative scatter factors for various field sizes. Generally, the PBC is used for HD120MLC based treatment planning, while Clarkson method is for cone‐based planning. ResultsPDD, beam profile and relative scatter data for the circular cones with diameters of 4, 6.0, 7.5, 10, 12.5, 15 mm were measured for 6X flattening‐filter‐free (FFF) and 10X FFF. The relative scatter factors range from 1.12 to 1.45 for 6X FFF and 0.81 to 1.23 for 10XFFF. Beam penumbra of 6XFFF and 10XFFF of each the circular cone were less than 2.5 mm for the 20% to 80% criterion. PDD and relative scatter data for HD120MLC were measured for various MLC fields with corresponding jaw settings for various energy modes: 6X, 6X FFF, 10X, 10X FFF and 15X, with an MLC filed down to 5mmx5mm. The transverse and diagonal beam profiles for HD120MLC were measured at different depths to generate source function correction models in iPlan. HD120MLC leakage for open jaw and close jaw are ∼1% and 0.0% for all the energy modes. The measured HD120MLC dynamic leaf shifts are 0.15, 0.11, 0.19, 0.18, 0.21 mm for 6X, 6XFFF, 10X, 10XFFF and 15X, less than 1/2 of those of the Novalis Tx system previous published.ConclusionThe commissioning data provided us valuable insights and might be useful for future reference.

Determination of ion recombination for absolute dosimetry in microbeam radiation therapy

Introduction Microbeam Radiation Therapy (MRT) uses a 3rd generation synchrotron X-ray source. MRT is based on the dose volume effect by applying an array of microbeams 25–100 lm wide with a center-to-center spacing of 100–400 lm. The synchrotron X-ray beam is minimally divergent over distances on a scale of 1–2 m, and is very intense, with in-beam dose rates as high as 13 kGy/s. These physical properties lead to sharp beam penumbras in tissue using an energy spectrum between 50 to 350 keV (as exists at the ID17 Biomedical beamline at the ESRF). Such dose rates make the dosimetry and in particular the determination the ion recombination correction factor to be applied to the cylindrical ionization chamber readings a challenging task. Material and methods We used three different approaches for the determination of the ion recombination correction factor: 1. the ‘‘two-voltage” method, 2. a cross-calibration with Alanine dosimeters, and 3. a storage ring current (mA) ramping method. This last method involves quantifying the decrease in ionization chamber response while increasing the storage ring current, which is attributable to the lack of ion recombination correction. The MRT beam has a 352 MHz fine structure, which can be considered, from the point of view of the charge collection process in the ionisation chamber, as a continuous beam. Consequently, among the different ‘‘two-voltage” methods used the approach of a continuous beam was applied. Results The ‘‘two-voltage” method resulted in a mean ion recombination correction Alanine dosimeter, an average ion recombination of 5.5% was found. The current ramping method gave a correction of 4.6% at a ring current of 200 mA. Conclusion The current ramping method is the most suitable approach to ion recombination correction, if we wish to use an ionization chamber measurement as the standard dosimetric reference. Indeed, with the current ramping method, the ion recombination correction can be determined reliably at a given current in the machine, assuming negligible ion recombination at very low current and a linear relationship between the ring current and the dose rate.

SU‐E‐T‐628: Impact of Large SSD On the Plan Quality of Energy‐Intensity Modulated Electron Therapy

Purpose: In photon MLC (pMLC) delivered energy‐intensity modulated electron therapy, a short SSD was preferred for reducing beam penumbra caused by electron in‐air scatter. However, due to some restrictions (e.g. table height limit on some accelerators), a large SSD may be an only choice. This work presents our preliminary results of the impact of a large SSD on an ultimate MERT plan. Methods: A house‐grown Monte Carlo (MC)‐based optimization/dose calculation code was used to generate plans for the Artiste linac with a breast phantom, at 60 cm and 90 cm SSD, respectively. The plans employed four different electron energies (6, 9, 12, 15 MeV) and was normalized to ensure 85% of the CTV to receive 100% of the prescription dose. Their DVHs are then evaluated for target (breast), left lung and heart. Results: When a 90 cm SSD was used instead of a 60 cm SSD, the maximum dose of the CTV increased by 5% while the minimum dose decreased by 3.6%. The left lung showed a 10% increase in the maximum dose while D10% (dose to 10% of volume) and D30% of the lung increased by a factor of 1.39 and 2.23, respectively. The maximum dose to the heart increased by 20% and its D10% and D30% were about 1.5 times larger. Conclusion: With a larger SSD (e.g., 90 cm as compared to 60 cm), increased air‐scattered electrons may lead to substantial deterioration of the overall plan quality ‐ increased target dose heterogeneity and higher doses to critical structures. However, absolute doses received by these critical structures may be still low and further investigations are warranted to find out as to whether MERT with a large SSD is still a better treatment as compared with conventional treatments for some disease sites.

Optically stimulated luminescence for diagnostic and therapeutic low to medium energy X-ray beams experimental dosimetry

Introduction Optically stimulated luminescence (OSL) using aluminium oxide crystals (Al2O3:C) is a radiation detection technique, which has been validated as a reliable and promising alternative to ion chambers measurements in radiation therapy with significantly smaller active volumes and good tissue equivalence at diagnostic and therapeutic X-rays energies. However the precision achieved at lower energies is directly linked to the ability to characterise, model and correct the energy dependence of these detectors. The aim of this study was to characterize the OSL detectors in terms of energy dependence using monochromatic diagnostic and therapeutic beams in the low-medium energy range (25–90 keV); and to evaluate the potential of this technique for exeprimental dosimetry in radiologic and therapeutic applications at these energies. Materials and methods The OSL/OF (Optical Fibers) system is composed of a multichannel reader and OSL OFS (Optical Fibre Sensors) incorporating dosimetric-grade alumina crystals. Al2O3:C (0.09 mm3). Irradiations have been performed at the European synchrotron radiation facility medical beamline using square irradiation fields at 8 energy points from 25 to 90 keV. Absolute dose measurements have been performed at 80 keV at 2 cm depth in a water equivalent phantom. Once irradiated the detector has been read over 300 s to collect all the light available from the trapped electrons, which is directly linked to the absorbed dose. Results The OSL detector exhibit an energy dependance ranking from 10% to 45% from 25 to 90 keV. The use of the OSL detector for absolute dosimetry in a water equivalent phantom at 80 keV has shown a good reproducibility with a standard deviation lower than 5% for a 4 Gy single dose, using 5 × 5 cm2 field. Conclusion Those measurements demonstrated that the OSL energy dependance decreases with the beam energy. Moreover there is a good reproducibility in the OSL signal if being used in absolute dosimetry. The next step of this study is to use OSLD for beam penumbra caracterisation, where conventional detectors exhibit a severe energy dependance at these low energies. In vivo dosimetry applications in synchrotron stereotactic radiotherapy are also currently being investigated.

SU-E-T-715: Evaluation of Off-Target Isocenter Placement On TArget Coverage and Critical Structure Dose

Purpose: To evaluate the effect of placing the isocenter away from the PTV (so that collision is avoided during VMAT treatment) on tumor coverage or critical structure doses in SBRT. Method: CT images of lung SBRT patients were acquired for pre-treatment alignment using CT-on-rails and imported back in to the planning system. The daily images were registered with the original simulation CT using the GTV as the alignment structure. We re planned the patient (on original CT) using 2 additional scenarios 1) isocenter at the midline of the patient at the AP and Sup-Inf level of the GTV and 2) The isocenters at the center of the vertebral body at the Sup-Inf level of the GTV. In both cases, the plans were normalized so that PTV coverage is similar to the original clinical plan. The treatment plans were recomputed on the daily CTs using original isocenter and the two off target isocenters and doses to target and serial critical structure cord were tabulated. The MU units remained the same between the plans on original CT and daily CT. Result: The target doses were observed to be not affected by the off target placement of isocenter. On average, the dose to 99% GTV and 95% PTV differed from planned value by 0.25 and 0.35 Gy respectively. The 0.1 and 1cc cord doses differed by .23 and .85 Gy respectively. Conclusions: Off center placement of isocenter does not affect target coverage and serial critical structure dose such as cord. However, care should be exercised not to beam penumbra adjacent to critical structure.

SU‐E‐T‐251: 3D Collision Viewer for Proton Therapy Treatment Planning

Purpose: In proton treatment, in order to reduce beam penumbra, it is desirable to place the treatment snout as close as possible to the patient skin. However, collision between treatment couch/patient and snout is not predicted by current treatment planning system. We developed and validated an interactive 3D Collision Viewer for patient clearance check during treatment planning. Methods: The 3D Collision Viewer calculates the shortest distance between two major groups of objects, couch group and nozzle group. The couch group includes a couch and a patient and the nozzle group includes a nozzle, a snout and extensions for snout. The software displays the object coordinates and the shortest distance in the 3D Viewer. The tool was developed with a video game development framework to realize 3D interactive performance. Surface coordinates of equipment including the patient were defined relative to the iso‐center and created by commonly used CAD software. The equipment coordinates and the shortest distance are interactively updated on any change of equipment setting by the user. The video game framework takes care of equipment surface coordinate motion according to each equipment setting. The shortest distance was obtained by comparing distances between all combination of nozzle meshes and couch meshes. To confirm the numerical accuracy of the shortest distance calculation, the distance between nozzle and 2 cm diameter sphere was compared with the expected distance. Results: Numerical accuracy of the distance calculation is better than about 1/40 of mesh size of the sphere. The accuracy mainly depends on the surface data preparation. Average calculation time for a sample configuration, which had 156 vertices and 94 meshes for the nozzle and 2707 vertices and 3758 meshes for couch, was less than 0.6 seconds. Conclusion: 3D Collision Viewer is capable of detecting potential collision during treatment planning.

SU-E-T-117: Optically Stimulated Luminescence for Diagnostic and Therapeutic Low to Medium Energy X-Ray Beams Experimental Dosimetry

Purpose: Optically stimulated luminescence (OSL) using aluminium oxide crystals (Al2O3:C) is a dosimetry technique (1), already validated in radiation therapy (2). The aim of this study was to characterize the OSL detectors in terms of energy dependence using monochromatic beams in the low-medium energy range (25-85 keV); and to evaluate their potential for experimental dosimetry in therapeutic applications at these energies. Methods: The OSL system is composed of a multichannel reader and alumina crystals (Al2O3:C, 0.09 mm3) that are remotely read by laser light. The OSL signal is directly linked to the absorbed dose. Irradiations in were performed at 8 energy points from 25 to 90 keV using a synchrotron source. Absolute OSL dose measurements were also performed at 80 keV at 2 cm depth in water equivalent phantoms (3) (1-8 Gy range). The detectors have been read over 300 s to collect all the light available from the trapped electrons. Results: The OSL detector show a lower energy dependence (< 10%) in the 25 to 50 keV range than at higher energies (∼ 45% from 50 to 90 keV). OSL is a reproducible absolute dosimetry technique at 80 keV (sigma<5 %). The field size has no other influence on the OSL signal than the one due to phantom scatter output factors (3). Conclusion: We demonstrated that the OSL energy dependance decreases with the beam energy. The conventional models (4) of OSL response in function of beam energy and delivered dose were confronted to our results. The next step of this study is to use OSL for beam penumbra caracterisation, where conventional detectors exhibit a severe energy dependance due to low energy spectra. In vivo dosimetry applications in low-medium enregy radiotherapy are foreseen due to the good reproducibility in OSL signal for absolute dose measurements.

Improvement of the penumbra for small radiosurgical fields using flattening filter free low megavoltage beams

BackgroundIn stereotactic radiosurgery, sharp beam edges have clear advantages to spare normal tissues. In general, the dose gradient is a limiting factor in minimizing dose to nearby critical structures for clinical cases. Therefore the penumbral width should be diminished. MethodsA Varian Clinac 2100 linear accelerator equipped with in-house designed radiosurgical collimator was modeled using the EGSnrc/BEAMnrc Monte Carlo code and compared with the measurements. The 0.015 cm3 PinPoint chamber was used to measure the 6 MV photon beam characteristics and to validate Monte Carlo calculations. Additional to the standard (STD) linac, a flattening filter free (FFF) linac was simulated. Percent depth doses, beam profiles and output factors were calculated for small field sizes with diameter of 5, 10, 20 and 30mm with DOSXYZnrc. The mean energy and photon fluence at the water surface were calculated with BEAMDP for both FFF linac and STD linacs. ResultsThe penumbra width (80%-20%) was decreased by 0.5, 0.3, 0.2 and 0.2mm for field sizes of 5, 10, 20 and 30mm respectively when removing the FF. The fluence of photons at the surface increased up to 3.6 times and the mean energy decreased by a factor of 0.69 when removing the FF. The penumbra width (80%-20%) decreased by 17% when a 2 MeV monoenergetic electron pencil beam incident on the target is used instead of 6.2 MeV. ConclusionsIt was found that the penumbra of small field sizes is decreased by removing the FF. Likewise using low megavoltage photons reduced the beam penumbra maintaining adequate penetration and skin sparing.

Proton‐minibeam radiation therapy: A proof of concept

This Monte Carlo simulation work aims at studying a new radiotherapy approach called proton-minibeam radiation therapy (pMBRT). The main objective of this proof of concept was the evaluation of the possible gain in tissue sparing, thanks to the spatial fractionation of the dose, which could be used to deposit higher and potentially curative doses in clinical cases where tissue tolerances are a limit for conventional methods. Monte Carlo simulations (GATE v.6) have been used as a method to calculate the ratio of the peak-to-valley doses (PVDR) for arrays of proton minibeams of 0.7 mm width and several center-to-center distances, at different depths in a water phantom. The beam penumbras were also evaluated as an important parameter for tissue sparing, for example, in the treatment of non-cancer diseases like epilepsy. Two proton energies were considered in this study: a clinically relevant energy (105 MeV) and a very high energy (1 GeV), to benefit from a reduced lateral scattering. For the latter case, an interlaced geometry was also evaluated. Higher or similar PVDR than the ones obtained in x-rays minibeam radiation therapy were achieved in several pMBRT configurations. In addition, for the two energies studied, the beam penumbras are smaller than in the case of Gamma Knife radiosurgery. The high PVDR obtained for some configurations and the small penumbras in comparison with existing radiosurgery techniques, suggest a potential gain in healthy tissue sparing in this new technique. Biological studies are warranted to assess the effects of pMBRT on both normal and tumoral tissues.

Transfer line scattering model of therapeutic hadron beams and applications to nozzle and gantry optimization

The field of hadron therapy is growing rapidly with several facilities currently being planned, under construction or in commissioning worldwide. In the ``active scanning'' irradiation technique, the target is irradiated using a narrow pencil beam that is scanned transversally over the target while the penetration depth is altered with the beam energy. Together, the target dose can thereby be conformed in all three dimensions to the shape of the tumor. For applications where a sharp lateral beam penumbra is required in order to spare critical organs from unwanted dose, beam size blowup due to scattering in on-line beam diagnostic monitors, air gaps and passive elements like the ripple filter must be minimized. This paper presents a model for transverse scattering of therapeutic hadron beams along arbitrary multislab geometries. The conventional scattering formulation, which is only applicable to a drift space, is extended to not only take beam optics into account, but also non-Gaussian transverse beam profiles which are typically obtained from the slow resonant extraction from a synchrotron. This work has been carried out during the design phase of the beam delivery system for MedAustron, an Austrian hadron therapy facility with first patient treatment planned for the end of 2015. Irradiation will be performed using active scanning with proton and carbon ion beams. As a direct application of the scattering model, design choices for the MedAustron proton gantry and treatment nozzles are evaluated with respect to the transverse beam profile at the focal point; in air and at the Bragg peak.

Projection imaging of photon beams by the Čerenkov effect

A novel technique for beam profiling of megavoltage photon beams was investigated for the first time by capturing images of the induced Čerenkov emission in water, as a potential surrogate for the imparted dose in irradiated media. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire 2D projection images of Čerenkov emission from a 4 × 4 cm(2) 6 MV linear accelerator (LINAC) x-ray photon beam operating at a dose rate of 400 MU∕min incident on a water tank with transparent walls. The ICCD acquisition was gated to the LINAC sync pulse to reduce background light artifacts, and the measurement quality was investigated by evaluating the signal to noise ratio and measurement repeatability as a function of delivered dose. Monte Carlo simulations were used to derive a calibration factor for differences between the optical images and deposited dose arising from the anisotropic angular dependence of Čerenkov emission. Finally, Čerenkov-based beam profiles were compared to a percent depth dose (PDD) and lateral dose profile at a depth of d(max) from a reference dose distribution generated from the clinical Varian ECLIPSE treatment planning system (TPS). The signal to noise ratio was found to be 20 at a delivered dose of 66.6 cGy, and proportional to the square root of the delivered dose as expected from Poisson photon counting statistics. A 2.1% mean standard deviation and 5.6% maximum variation in successive measurements were observed, and the Monte Carlo derived calibration factor resulted in Čerenkov emission images which were directly correlated to deposited dose, with some spatial issues. The dose difference between the TPS and PDD predicted by Čerenkov measurements was within 20% in the buildup region with a distance to agreement (DTA) of 1.5-2 mm and ±3% at depths beyond d(max). In the lateral profile, the dose difference at the beam penumbra was within ±13% with a DTA of 0-2 mm, ±5% in the central beam region, and 2%-3% in the beam umbra. The results from this initial study demonstrate the first documented use of Čerenkov emission imaging to profile x-ray photon LINAC beams in water. The proposed modality has several potential advantages over alternative methods, and upon future refinement may prove to be a robust and novel dosimetry method.

Anatomy-based volumetric modulated arc therapy for a prostate cancer patient with a hip prosthesis

Anatomy-based volumetric modulated arc therapy for a prostate cancer patient with a hip prosthesis

Commissioning and dosimetric characteristics of TrueBeam system: Composite data of three TrueBeam machines

A TrueBeam linear accelerator (TB-LINAC) is designed to deliver traditionally flattened and flattening-filter-free (FFF) beams. Although it has been widely adopted in many clinics for patient treatment, limited information is available related to commissioning of this type of machine. In this work, commissioning data of three units were measured, and multiunit comparison was presented to provide valuable insights and reliable evaluations on the characteristics of the new treatment system. The TB-LINAC is equipped with newly designed waveguide, carousel assembly, monitoring control, and integrated imaging systems. Each machine in this study has 4, 6, 8, 10, 15 MV flattened photon beams, and 6 MV and 10 MV FFF photon beams as well as 6, 9, 12, 16, 20, and 22 MeV electron beams. Dosimetric characteristics of the three new TB-LINAC treatment units are systematically measured for commissioning. High-resolution diode detectors and ion chambers were used to measure dosimetric data for a range of field sizes from 10 × 10 to 400 × 400 mm(2). The composite dosimetric data of the three units are presented in this work. The commissioning of intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), image-guided radiation therapy, and gating systems are also illustrated. Critical considerations of P(ion) of FFF photon beams and small field dosimetric measurements were investigated. The authors found all PDDs and profiles matched well among the three machines. Beam data were quantitatively compared and combined through average to yield composite beam data. The discrepancies among the machines were quantified using standard deviation (SD). The mean SD of the PDDs among the three units is 0.12%, and the mean SD of the profiles is 0.40% for 10 MV FFF open fields. The variations of P(ion) of the chamber CC13 is 1.2 ± 0.1% under 6 MV FFF and 2.0 ± 0.5% under 10 MV FFF from dmax to the 18 cm-off-axis point at 35 cm depth under 40 × 40 cm(2). The mean penumbra of crossplane flattened photon beams at collimator angle of 0° is measured from 5.88 ± 0.09 to 5.99 ± 0.13 mm from 4 to 15 MV at 10 cm depth of 100 × 100 mm(2). The mean penumbra of crossplane beams at collimator angle of 0° is measured as 3.70 ± 0.21 and 4.83 ± 0.04 mm for 6 MV FFF and 10 MV FFF, respectively, at 10 cm depth with a field size of 5 × 5 cm(2). The end-to-end test procedures of both IMRT and VMAT were performed for various energy modes. The mean ion chamber measurements of three units showed less than 2% between measurement and calculation; the mean MultiCube ICA measurements demonstrated over 90% pixels passing gamma analysis (3%, 3 mm, 5% threshold). The imaging dosimetric data of KV planar imaging and CBCT demonstrated improved consistency with vendor specifications and dose reduction for certain imaging protocols. The gated output verification showed a discrepancy of 0.05% or less between gating radiation delivery and nongating radiation delivery. The commissioning data indicated good consistency among the three TB-LINAC units. The commissioning data provided us valuable insights and reliable evaluations on the characteristics of the new treatment system. The systematically measured data might be useful for future reference.