Beam Penumbra Research Articles

Water equivalent PRESAGE® for synchrotron radiation therapy dosimetry.

Synchrotron Radiation Therapy techniques are currently being trialed and commissioned at synchrotrons around the world. The patient treatment planning systems (TPS) developed for these treatments use simulated data of the synchrotron x-ray beam to produce the dosimetry in the treatment plan. The purpose of this study was to investigate a water equivalent PRESAGE® dosimeter capable of 3D dosimetry over an energy range suitable for synchrotron x-ray beams. Water equivalent PRESAGE® dosimeters were fabricated with a radiological effective atomic number similar to water over an energy range of 10 keV to 10 MeV. The dosimeters were irradiated at various energies, scanned using optical CT (OCT) scanning and compared to ion chamber measurements. Percentage depth dose and beam profiles of the synchrotron beam were compared to Monte Carlo (MC) model simulations. The PDD profiles of the water equivalent PRESAGE® agreed with ion chamber measurements and MC calculations within 2% for all keV energies investigated. The PRESAGE® also showed good agreement to the MC model for depths below 5 mm of the synchrotron beam where ion chamber data do not exist. The spatial resolution of the OCT was not sufficient to accurately measure the penumbra of the synchrotron beams compared to MC calculations or EBT3 film; however, the water equivalent PRESAGE® was able to verify dose profile characteristics of the MC model. The radiological response of a water equivalent PRESAGE® dosimeter has been validated for synchrotron x-ray beam energies along with the ability to independently verify dose distributions of a MC model.

Dosimetric Validation of Commissioning Data Validation of Xio Treatment Planning System on a Philips Linear Accelerator

Treatment planning is one of the main steps in radiotherapy. It includes dose, isodose and monitor units (MUs) calculations. The dose calculation is based on algorithms implemented in the treatment planning system (TPS). For a suitable clinical use, these algorithms must calculate the dose as accurately as possible. The aim of this work is the assessment of treatment planning system installed in Aswan Oncology Institute to perform dosimetric validation of various parameters. Measurements have been performed using existing Elekta linear accelerator, (scanditronix-wellhofer dosimetry) system, and water phantom. A variety of 3D conformal fields were created in Xio TPS on a combined Polymethyl methacrylate (PMMA) phantom and validated against measurements with a calibrated ion chamber. Some of the parameters such as (Tissue phantom ratio (TPR), surface dose, buildup, flatness, symmetry, penumbra, contamination dose) varied including source to surface distance, field size, gantry angle, and depth for all photon and electron energies. The difference between measurements and calculation of flatness and symmetry values at different electron energies were between -0.4% to 1.7% and 6 MV didn’t exceed ± 0.8%. The mean difference in value of penumbra of electron beams was -4.97% and 6 MV was ± 5%. TPR and surface dose at 6 MV increased with the field size (FS) increasing. All the resulted difference of measurements and calculation were in agreement with IAEA-TRS430 and Venselaar et al. which didn’t exceed ± 2% at flatness, symmetry and ±15% at penumbra. This investigation on dosimetric validation ensures accuracy of Xio TPS thereby improving patient safety.

Technical Note: Penumbral width trimming in solid lung dose profiles for 0.9 and 1.5 T MRI‐Linac prototypes

Longitudinal magnetic fields narrow beam penumbra and tighten lateral spread of secondary electrons in air cavities, including lung tissue. Gafchromic® EBT3 film was used to investigate differences between penumbra in solid water and solid lung, without a magnetic field (0 T) and with two field strengths (0.9 and 1.5 T). The first prototype of the Australian MRI-linac consisted of a 1.5 T Siemens Sonata MRI and Varian industrial linatron (nominal 4 MV). The second prototype replaced the Sonata with a 1.0 T Agilent split-bore magnet. Measurements were completed at 0.9 T to maintain the same source-to-surface distance between set-ups. Gammex-rmi® solid water with 50 mm of CIRS solid lung inserted as a lung cavity was positioned inside each magnet. This was compared to the same set-up with solid water only, where film measurements were completed at solid water equivalent depths corresponding to entrance interface/mid/exit interface positions of solid lung from the first set-up. Multileaf collimator (MLC)-defined field sizes were set to 3 × 3 cm2 and 10 × 10 cm2 . The 80%-20% penumbral width was determined. Under 1.5 T conditions, penumbra narrowing occurred up to 4.4 ± 0.1 mm compared to 0 T. As expected, the effect was less for 0.9 T, which resulted in a maximum narrowing of 2.5 ± 0.1 mm. Exit profile penumbra were more affected than entrance penumbra by up to 2.6 ± 0.2 mm. The 1.5 T field brought the solid water and lung penumbral widths more into alignment by a maximum difference of 0.4 ± 0.1 mm. The trimming of penumbral widths due to magnetic fields in solid water and lung was demonstrated and compared to 0 T. The 0.9 and 1.5 T field trimmed the penumbra by up to 2.5 ± 0.1 mm and 4.4 ± 0.1 mm respectively.

A comparative study between flattening filter-free beams and flattening filter beams in radiotherapy treatment

Abstract Flattening filter-free (FFF) beams generated by medical linear particle accelerators (linacs) have recently been used in radiotherapy clinical practice. FFF beams have fundamental physical parameter differences with respect to standard flattening filter (FF) beams, such that the generally used dosimetric parameters and definitions are not always viable. This study investigates dosimetric parameters for use in the quality assurance of FFF beams generated by medical linacs in radiotherapy. The main characteristics of the photon beams are analyzed using specific data generated by a Varian TrueBeam linac having both FFF and FF beams of 6 and 10 MV (megavolt) energy, respectively. Definitions for dose profile parameters are suggested, starting from the renormalization of the FFF with respect to the corresponding FF beam. From this point, the flatness concept is translated into one of “un-flatness”, and other definitions are proposed, maintaining a strict parallelism between FFF and FF parameter concepts. The quality controls used in establishing a quality assurance program when introducing FFF beams into the clinical environment are given, maintaining similarity to those used for standard FF beams, and recommendations for the introduction of FFF beams into clinical radiotherapy application for breast cancer patients are provided as an example for comparison between FFF and FF for dose distribution and coverage for a target volume. Although there are many advantages of using a FFF beam, especially for advanced radiotherapy techniques, there are a few limitations (e.g., using a relatively higher energy photon beam for stereotactic radiotherapy (SRT), limited speed of current multileaf collimators (MLCs), and off-axis distance-dependent modulation in intensity-modulated radiation therapy (IMRT)) as well as challenges (e.g., criteria for beam quality evaluation and penumbra, establishment of dosimetry methods, and consequences of photon target burn-up) that need to be addressed for establishing the FFF beam as a viable alternative to the FF beam.

Small field dosimetry for the small animal radiotherapy research platform (SARRP)

BackgroundPreclinical radiation biology has become increasingly sophisticated due to the implementation of advanced small animal image guided radiation platforms into laboratory investigation. These small animal radiotherapy devices enable state-of-the-art image guided therapy (IGRT) research to be performed by combining high-resolution cone beam computed tomography (CBCT) imaging with an isocentric irradiation system. Such platforms are capable of replicating modern clinical systems similar to those that integrate a linear accelerator with on-board CBCT image guidance.MethodsIn this study, we present a dosimetric evaluation of the small animal radiotherapy research platform (SARRP, Xstrahl Inc.) focusing on small field dosimetry. Physical dosimetry was assessed using ion chamber for calibration and radiochromic film, investigating the impact of beam focus size on the dose rate output as well as beam characteristics (beam shape and penumbra). Two film analysis tools) have been used to assess the dose output using the 0.5 mm diameter aperture.ResultsGood agreement (between 1.7–3%) was found between the measured physical doses and the data provided by Xstrahl for all apertures used. Furthermore, all small field dosimetry data are in good agreement for both film reading methods and with our Monte Carlo simulations for both focal spot sizes. Furthermore, the small focal spot has been shown to produce a more homogenous beam with more stable penumbra over time.ConclusionsFilmQA Pro is a suitable tool for small field dosimetry, with a sufficiently small sampling area (0.1 mm) to ensure an accurate measurement. The electron beam focus should be chosen with care as this can potentially impact on beam stability and reproducibility.

Design of a 3D Printed Immobilization Device for Radiation Therapy of Experimental Tumors in Mice

Transplantable murine tumor models are critical for radiobiological studies of proton and photon radiation, and immobilization devices are essential for accurate tumor (often <1 cm) targeting in these models. We utilized three-dimensional (3D) printing technology to design and produce an immobilization device for this purpose. All procedures were approved by the institutional animal care committee. CT imaging of a C57BL/6 mouse was performed, and the Blender software package was used to generate a surface contour. A clamshell device was designed with a negative impression of the animal surface contour, and a lateral aperture for flank tumors. A brass block was affixed to reduce beam penumbra. The device was printed from polylactic acid polymer using an Ultimaker 2 3D printer and tested on a mouse harboring a subcutaneous flank tumor (EL4 Thymoma). Tumor motion was tracked in the x, y, and z planes using video monitoring over 15 minutes. Animals (n=3) were safely immobilized with no signs of trauma or distress. Minimal tumor movement was observed in the x, y, and z planes. Proton and photon treatment plans were generated using Eclipse software, demonstrating uniform targeting of the tumor with <10% of the prescribed dose delivered to the abdominal or thoracic cavity. We present the rapid and cost-effective design and production of an immobilization device using 3D printing for the treatment of experimental murine tumors with photon or proton beam radiotherapy.

A Technique to Irradiate Scar-like Line Target With Dynamic Electron Arc Radiation Therapy

For superficial target such as surgical scar, standard radiotherapy technique is the static electron beam. Its use can become cumbersome when target area is long and/or the skin has large curvature, because the combination of multiple beams often produces hot and cold spots in the junction regions and surrounding healthy tissues can be over-irradiated. In addition, manual setup of each field is required which is often impractical clinically. In this study, we investigate a new technique to irradiate any line target with the dynamic electron arc radiotherapy (DEAR) where both the couch position and Linac gantry angle are synchronized with the radiation beam. The electron applicator is kept in DEAR to preserve beam penumbra. To avoid collision between Linac and patient, couch is moved simultaneously with gantry rotation to maintain a constant source to skin distance (SSD) and an en face beam direction at all times. The DEAR plan, in the form of XML, follows the MU-position trajectory model using control point (CP) similar to IMRT and VMAT, is currently deliverable on Truebeam Linac in research mode. The line target is represented by a series of connected points whose 3D coordinates are first extracted from treatment planning system. They are used to determine the gantry angle, couch position, collimator angle, and MU at each CP. The XML plan also includes some fixed parameters: energy, applicator, cutout, and dose rate. The plan and CT are fed to the Virtual Linac Monte Carlo simulation platform for the final dose calculation and prescription normalization. Finally the plan is verified dosimetrically on phantom with radiochromic film. Various target orientations and lengths were studied on flat, cylindrical, and anthropomorphic phantoms. Either 6 or 9 MeV electron beams were chosen depending on the depth of the target. The smallest applicator (6 × 6 cm2) is used for increased clearance. Cutout shape is in square (3 × 3 cm2), rectangular (3 × 1 cm2), or circular (dia = 3 cm). Dose rate is based on the prescription dose, speed of couch motion and gantry rotation, and delivery time. Plans are generated in both forward and reverse motion directions. Any number of setup beams during the motion trajectory can be created and mode up during the dry run. Gantry angle is chosen so that beam is always perpendicular to the target. For circular cutout, the collimator angle is fixed; for others, it is parallel to the scanning direction. Finally, the MU at each CP is proportional to the distance between the current and the neighboring target points. For a 20 cm long target with 200 cGy prescription dose, the delivery can be completed within 2 minutes at dose rate of 1000 MU/min. The film measurements agree well with the Monte Carlo dose calculations. We have developed a technique to irradiate any line target using DEAR. It improves dose distributions substantially over the standard electron beam. Further investigations are in progress to expand into two dimensional targets.

The accuracy of treatment planning system dose modelling in the presence of brass mesh bolus.

This work assesses the dosimetric accuracy of three commercial treatment planning system (TPS) photon dose calculation algorithms in the presence of brass mesh used as a bolus. Bolus material is used in radiotherapy to provide dose build-up where superficial tissues require irradiation. They are generally water equivalent but high density materials can also be used. Dose calculations were performed on Monaco and Masterplan TPS (Elekta AB, Sweden) using phantoms defined by the three DICOM CT image sets of water equivalent blocks (no bolus, 1 layer and 2 layers of brass mesh) exported from the CT scanner. The effect of the mesh on monitor units, build-up dose, phantom exit dose and beam penumbra were compared to measured data. Dose calculations for 6 and 15MV photon beams on plain water equivalent phantoms were seen to agree well with measurement validating the basic planning system algorithms and models. Dose in the build-up region, phantom exit dose and beam penumbra were poorly modelled in the presence of the brass mesh. The beam attenuation created by the bolus material was overestimated by all three calculation algorithms, at both photon energies, e.g. 1.6% for one layer and up to 3.1% for two layers at 6MV. The poor modelling of the physical situation in the build-up region is in part a consequence of the high HU artefact caused by the mesh in the CT image. CT imaging is not recommended with the brass mesh bolus in situ due to the poor accuracy of the subsequent TPS modelling.

Evaluation of different physical parameters based on standard photon beam versus flattening filterfreein treatment cancer patients

cccc In radiotheraby, the purpose of using standard photon beam (Flattening Filter; FF) is to convert the forward peaked MV bremsstrahlung photon intensity into uniform intensity pattern for obtaining clinically acceptable beam profile. Recently a number of studies were carried out on existing medical linac by removing the flattening filter to produce the unflat photon beam and demonstrated their feasibility in the implementation of advanced radiotherapy techniques. Flattening Filter Free (FFF) beam have a fundamental physical parameter differences with respect to the standard filter flattened beams, making the generally used dosimetric parameters and definitions not always viable. In this concern, the current paper is a trial to shed further light on studying some dosimetric parameters for use in quality assurance of FFF beams generated by medical linacs in radiotherapy. The main characteristics of the photon beams have been analyzed using specific data generated by a Varian TrueBeamlinac having bothFFF and FF beams of 6 and 10 MV energy. Definitions for dose profile parameters are suggested starting from the renormalization of the FFF with respect to the corresponding FF beam. From this point the flatness concept has been translated into one of “unflatness” and other definitions have been proposed, maintaining a strict parallelism between FFF and FF parameter concepts. In summary, although there are a number of advantages of using a FFF beam especially for advanced radiotherapy techniques there are a few challenges (e.g., criteria for beam quality evaluation and penumbra, establishment of dosimetry methods, and consequences of photon target burn-up) which need to be addressed for establishing this beam as an alternate to the FF beam.

Optimization of minibeam generation by mechanical collimation in proton minibeam radiation therapy

Introduction The dose tolerances of normal tissues continue being the main barrier in radiotherapy. To lower it, we recently proposed a novel concept: proton minibeam radiation therapy (pMBRT). It allies the inherent advantages of protons with the normal tissue preservation observed when irradiated with submillimetric spatially fractionated beams. The tumor receives a homogeneous dose distribution, while normal tissues benefit from the spatial fractionation of the dose. Purpose We have recently implemented this technique at the Orsay proton therapy clinical center. This work aims at optimizing the minibeam generation by means of a mechanical collimation. Materials and methods Monte Carlo simulations (GATEv7.1) were used to evaluate different irradiation configurations, e.g. collimator dimensions and materials (Brass, Tungsten, Iron and Nickel). Clinically relevant energies were used. Neutron contamination, Peak-to-valley dose ratios (PVDR) and beam penumbras were used as figures-of-merit. Results The neutron dose due to the minibeam collimator was found to be lower than 0.002% of the primary dose. A tungsten collimator provides the higher PVDR, but the generated neutron yield is 3 times higher than in other materials. Brass seems to provide the best compromise between PVDR values and neutron contamination. Conclusion A mechanical collimation only increases the biologic neutron dose by 0.04% of the peak surface dose therefore it is suitable for pMBRT. An optimized collimation for pMBRT, providing the best compromise between a high PVDR and low neutron generation, has been obtained.

Photon and electron beam penumbra determination using different measuring systems

Introduction The penumbra region is an intrinsic characteristic of any beam, weather photon or electron. Therefore, its correct determination constitutes a vital part in the dosimetric chain of a modern radiation therapy department. Purpose The purpose of this study is to compare and analyze results concerning the penumbra size determination obtained using six different measuring systems. Materials and methods Penumbra measurements were performed for two photon beams (6, 15 MV) for 4×4, 10×10 and 25×25 cm2 square field sizes and for five electron beams (6, 9, 12, 16, 20 MeV) using a 10×10 cm2 applicator at several depths. Six different measuring systems were used: (a) a Semiflex, (b) a Markus, (c) a Roos ionisation chamber, (d) a one dimensional Linnear Array (LA48, PTW), (e) a p-type diode (PTW 60008) and (f) an e-type diode (PTW 60017). All measurements were taken with standard water phantom geometry (MP3, PTW) using water as medium. Results Our results indicate that penumbra size determination is strongly dependant on the measuring system. More specifically, the diodes showed the narrowest penumbras followed by the LA48, while the largest penumbras were presented by the ionisation chambers. In terms of effectiveness, the LA48 method showed significant lower beam-on time. Conclusion Regarding penumbra size determination, the LA48 method is acceptable in terms of accuracy, and it is associated with considerable time-saving compared to other measuring systems. When the greatest possible accuracy is required, the use of silicon diode dosimeters is recommended.

SU-F-T-375: Optimization of a New Co-60 Machine for Intensity Modulated Radiation Therapy: A Monte Carlo Characterization Study

Purpose: To provide a wide range of dose output for intensity modulation purposes while minimizing the beam penumbra for a new rotating cobalt therapy system. The highest dose rate needs to be maximized as well. Methods: The GEPTS Monte Carlo system is used to calculate the dose distribution from each tested Co-60 head for a wide range of field sizes (1×1 to 40×40 cm2). This includes the transport of photons (and secondary electrons) from the source through the collimation system (primary collimator, Y and × jaws, and MLCs) and finally in the water phantom. Photon transport includes Compton scattering (with electron binding effect), Rayleigh scattering, Photoelectric effect (with detailed simulation of fluorescence x-rays). Calculations are done for different system designs to reduce geometric penumbra and provide dose output modulation. Results: Taking into account different clinical requirements, the choice of a movable head (SAD = 70 to 80 cm) is made. The 120-leaf MLC (6-cm thick) entrance is at 32 cm from the bottom of the source (to reduce penumbra while allowing larger patient clearance). Three system designs (refereed here as S1–3) were simulated with different effective source sizes (2mm, 10mm and 17mm diameter). The effective point source is at mid-height of the 25-mm-long source. Using a 12000-Ci source, the designed Co-60 head can deliver a wide range of dose outputs (0.5 − 4 Gy/mn). A dose output of 2.2 Gy/mn can be delivered for a 10cm × 10cm field size with 1-cm penumbra using a 10mm effective source. Conclusion: A new 60Co-based VMAT machine is designed to meet different clinical requirements in term of dose output and beam penumbra. Outcomes from this study can be used for the design of 60Co machines for which a renewed interest is seen.

SU‐F‐T‐529: Dosimetric Investigation of a Rotating Gamma Ray System for ImagedGuided Modulated Arc Radiotherapy

Purpose:Because of their effectiveness and efficiency, rotational arc radiotherapy (MAT) techniques have been developed on both specialty machines such as Tomotherapy and conventional linear accelerators. This work investigates a new rotating Gamma therapy system for image guided MAT and SBRT of intra/extracranial tumors.Methods:The CyberMAT system (Cyber Medical Corp., China) consists of a ring gantry with a gamma source (effective source size 1cm and 1.7cm respectively), a 120‐leaf MLC, a kV CBCT and an EPID. The treatment couch provides 6‐degrees‐of‐freedom motion compensation and the kV CBCT system has a spatial resolution of 0.4mm for target localization. The maximum dose rate is &gt;4.0 Gy/min and the maximum field size is 40cm × 40cm. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 125 previously treated SBRT patients was performed to evaluate the dosimetric characteristics of CyberMAT in comparison with existing VMAT systems.Results:Monte Carlo results confirmed the CyberMAT design parameters including output factors and 3D dose distributions. Its beam penumbra is 5mm to 10mm for field sizes 1cm to 10cm, respectively and its isocenter accuracy is &lt;0.5mm. Compared to the 6 MV photons of Tomotherapy and conventional linacs, Cobalt beams produce lower‐energy secondary electrons that exhibit better dose properties in low‐density lung tissues. Cobalt beams are ideal for peripheral lung tumors with half‐arc arrangements to spare the opposite lung and other critical structures. Superior dose distributions have been obtained for brain, head and neck, breast, spine and lung tumors with half/full arc arrangements.Conclusion:Because of the unique dosimetric properties of Cobalt sources and its accurate stereotaxy/dose delivery CyberMAT is ideally suited for image guided MAT and SBRT. Full‐arc arrangements are superior for brain and H&amp;N treatments while half‐arc arrangements produce best dose distributions for thoracic tumors.

SU‐F‐T‐481: Physics Evaluation of a Newly Released InCise™ Multileaf Collimator for CyberKnife M6™ System

Purpose:This work reports the results of the physics evaluation of a newly released InCise™2 Multileaf Collimator (MLC) installed in our institution.Methods:Beam property data was measured with unshielded diode and EBT2 films. The measurements included MLC leaf transmission, beam profiles, output factors and tissue‐phantom ratios. MLC performance was evaluated for one month after commissioning. Weekly Garden Fence tests were performed for leaf / bank positioning in standard (A/P) and clinically relevant non‐standard positions, before and after MLC driving exercises of 10+ minutes. Daily Picket Fence test and AQA test, End‐to‐End tests and dosimetric quality assurance were performed to evaluate the overall system performance.Results:All measurements including beam energy, flatness and symmetry, were within manufacture specifications. Leaf transmission was 0.4% &lt;0.5% specification. The values of output factors ranged from 0.825 (7.6 mm × 7.5 mm) to 1.026 (115.0 mm × 100.1 mm). Average beam penumbra at 10 cm depth ranged from 2.7mm/2.7mm(7.6 mm × 7.5 mm) to 6.0 mm/6.2mm(84.6 mm × 84.7 mm). Slight penumbra difference (&lt;10% from average penumbra for fields &gt;20 mm) was observed in the direction perpendicular to leaf motion due to the tilting of the leaf housing. Mean leaf position offsets was −0.08±0.07mm and −0.13 ± 0.08 for X1 and X2 leaf banks in 13 Garden Fence tests. No significant difference on average leaf positioning offsets was observed between different leaf orientations and before/after MLC driving exercises. Six End‐to‐End tests showed 0.43±0.23mm overall targeting accuracy. Picket‐Fence and AQA showed stable performance of MLC during the test period. Dosimetric point dose measurements for test cases agreed with calculation within 3%. All film measurements on relative dose had Gamma (2%, 2mm) passing rate of &gt;95%.Conclusion:The Incise™2 MLC for CyberKnife M6™ was proven to be accurate and reliable, and it is currently in clinical use.Stanford was one of the physics evaluation sites for the newly released InCise 2 MLC for Accuray Inc.

SU‐F‐T‐548: An Extraction Method for Correction of Detector Volume Effect in Small Field Profile Measurement

Purpose:To investigate the appropriateness of Gaussian detector response function in SRS profile measurement.Methods:In‐air profile data for a 6MV beam were collected by using ionization chambers and radiochromic film. Two ionization chambers (FC65‐G(IBA) and PinPoint(PTWTW31009)) were used to scan the beam profile. Film was used to measure the beam penumbra. A half‐blocked beam was acquired with stepping size of 0.2mm and gradient of the measured profile represents the detector response function. The field widths of 2cm and 3cm were measured at SAD. Two detector response functions were evaluated, i.e. a traditional single Gaussian and a three‐Gaussian symmetric function. Two corresponding models were used to fit measured data. One was error functions with one shaping parameter. The other model was a symmetrical superposition of three sets of error functions with fixed separation and two different shaping parameters. In both fitting, resultant Gaussian kernels and corresponding σs could be calculated analytically. SNR of the fitted curves were obtained and compared.Results:Our measured data indicated that the ionization chambers had a three‐peak response function. From the first modeling, SNR for FC65‐G were 36.0dB and 38.3dB for 2cm and 3cm field respectively. The corresponding SNR for the PinPoint were 43.6dB and 42.5dB. From the second modeling, SNR for FC65‐G were 42.5dB and 47.4dB for 2cm and 3cm field respectively. The corresponding SNR for the PinPoint were 44.0dB and 44.5 dB. Three‐gaussian model had an observable improvement for FC65‐G chamber. Film measured penumbra was 3–5 times smaller than that of ionization‐chambers.Conclusion:Superposition of three Gaussian kernels resembles the detector response due to central electrode and chamber wall and results in a better fitting to the measurement data than a single Gaussian kernel. Our study indicated that the Three‐Gaussian model was more suitable for the detector with sigma comparable to penumbra width.

SU‐F‐T‐438: A Practical Technique to Avoid the Radioactive Otitis Media in Whole Brain Radiotherapy

Purpose:Radioactive otitis media is a common complication in whole brain radiotherapy. In this study, we present a practical technique to avoid the injury for whole brain radiotherapy patients.Methods:For this study, 10 whole brain radiotherapy patients with planning CT were selected. For the target, 40Gy/20fractions was prescribed. In the treatment planning, two opposing lateral fields were used, from the left and right side of the patient. The MLC (0.5cm width) was used to shape the fields around the brain with a 10 mm margin to allow for the beam penumbra. For 5 patients, six leafs were positioned to block the auris media. For other patients, there were no leafs blocked to auris media according to the routine technique. The target coverage and the mean dose of the auris media were evaluated to assess the new technique in avoiding the radioactive otitis media.Results:The target coverage of new technique decreased by 1.7% (97.6%±1.1% vs. 99.3%±0.5%) compared routine technique. The mean dose of left auris media decreased by 79% (7.92 Gy±0.81Gy vs. 37.82 Gy±2.33Gy). The mean dose of right auris media decreased by 77% (8.8 Gy±0.93Gy vs. 38.4 Gy±2.65Gy). There was no difference for other OARs.Conclusion:An excellent sparing of the auris media is achievable while maintaining high coverage to the target volume. The simple technique to deliver whole brain may avoid the radioactive otitis media prospectively.

The influence of physical wedges on penumbra and in-field dose uniformity in ocular proton beams

A physical wedge may be partially introduced into a proton beam when treating ocular tumours in order to improve dose conformity to the distal border of the tumour and spare the optic nerve. Two unwanted effects of this are observed: a predictable broadening of the beam penumbra on the wedged side of the field and, less predictably, an increase in dose within the field along a relatively narrow volume beneath the edge (toe) of the wedge, as a result of small-angle proton scatter. Monte Carlo simulations using MCNPX and direct measurements with radiochromic (GAFCHROMIC® EBT2) film were performed to quantify these effects for aluminium wedges in a 60 MeV proton beam as a function of wedge angle and position of the wedge relative to the patient. For extreme wedge angles (60° in eye tissue) and large wedge-to-patient distances (70 mm in this context), the 90–10% beam penumbra increased from 1.9 mm to 9.1 mm. In-field dose increases from small-angle proton scatter were found to contribute up to 21% additional dose, persisting along almost the full depth of the spread-out-Bragg peak. Profile broadening and in-field dose enhancement are both minimised by placing the wedge as close as possible to the patient. Use of lower atomic number wedge materials such as PMMA reduce the magnitude of both effects as a result of a reduced mean scattering angle per unit energy loss; however, their larger physical size and greater variation in density are undesirable.

Dosimetric characteristics of brass mesh as bolus under megavoltage photon irradiation.

This article presents a set of dosimetric measurements describing the properties of brass mesh (Whiting and Davis, Attleboro Falls, MA) under megavoltage photon irradiation conditions, with particular relevance to its use in breast radiotherapy. The effectiveness of brass mesh as a bolus material was investigated using 6-, 15- and 6-MV flattening filter-free photon beams. The effect on dose build-up at the entrance surface, build-down at the beam-exit surface, dose with surface entrance obliquity, beam profiles, penumbra and percentage depth doses were investigated. One layer of the brass mesh produces a build-up effect equivalent to 1.1 mm of water at 6 MV and 1.9 mm at 15 MV. The brass generates a backscattered component of dose, if the photon beam exits through it. Percentage depth-dose curves are largely unaffected by the mesh and are shown to be equivalent to plain-field data. Beam penumbra and profiles are unchanged by the brass except within the first millimetre (mm) of phantom, where a periodic pattern of dose enhancement is seen. The data presented demonstrate that one layer of brass mesh provides a similar dose build-up effect equivalent to only a few millimetres of water. However, backscatter from the high atomic number (Z) mesh, at the beam exit, contributes appreciably to the overall dose surface enhancement. This dosimetric consequence cannot be neglected and indeed should be considered and accounted for, when determining the bolus effect of the brass mesh in the case of tangential breast irradiation. Advance in knowledge: This article provides dosimetric data necessary for the introduction of brass mesh bolus into the clinical setting for external-beam breast radiotherapy.

Spatial fractionation of the dose using neon and heavier ions: A Monte Carlo study.

This work explores a new radiation therapy approach which might trigger a renewed use of neon and heavier ions to treat cancers. These ions were shown to be extremely efficient in radioresistant tumor killing. Unfortunately, the efficient region also extends into the normal tissue in front of the tumor. The strategy the authors propose is to profit from the well-established sparing effect of thin spatially fractionated beams, so that the impact on normal tissues might be minimized while a high tumor control is achieved. The main goal of this work is to provide a proof of concept of this new approach. With that aim, a dosimetric study was carried out as a first step to evaluate the interest of further explorations of this avenue. The gate/geant4 v.6.1 Monte Carlo simulation platform was employed to simulate arrays of rectangular minibeams (700 μm × 2 cm) of four ions (Ne, Si, Ar, and Fe). The irradiations were performed with a 2 cm-long spread-out Bragg peak centered at 7 cm-depth. Dose distributions in a water phantom were scored considering two minibeams center-to-center distances: 1400 and 3500 μm. Peak and valley doses, peak-to-valley dose ratios (PVDRs), beam penumbras, and relative contribution of nuclear fragments and electromagnetic processes were assessed as figures of merit. In addition, the type and proportion of the secondary nuclear fragments were evaluated in both peak and valley regions. Extremely high PVDR values (>100) and low valley doses were obtained. The higher the atomic number (Z) of the primary ion is, the lower the valleys and the narrower the penumbras. Although the yield of secondary nuclear products increases with Z, the actual dose being deposited by the secondary nuclear fragments in the valleys starts to be the dominant contribution at deeper points, helping in the sparing of proximal normal tissues. Additionally, a wider center-to-center distance leads to a minimized contribution of heavier secondary fragments in valleys. The computed dose distributions suggest that a spatial fractionation of the dose combined to the use of submillimetric field sizes might allow profiting from the high efficiency of neon and heavier ions for the treatment of radioresistant tumors, while preserving normal tissues. The authors' results support the further exploration of this avenue. Next steps include the realization of biological experiment to confirm the shifting of normal tissue complication probability curves.

Single-energy intensity modulated proton therapy

In this note, an intensity modulated proton therapy (IMPT) technique, based on the use of high single-energy (SE-IMPT) pencil beams, is described.The method uses only the highest system energy (226 MeV) and only lateral penumbra to produce dose gradient, as in photon therapy. In the study, after a preliminary analysis of the width of proton pencil beam penumbras at different depths, SE-IMPT was compared with conventional IMPT in a phantom containing titanium inserts and in a patient, affected by a spinal chordoma with fixation rods.It was shown that SE-IMPT has the potential to produce a sharp dose gradient and that it is not affected by the uncertainties produced by metal implants crossed by the proton beams. Moreover, in the chordoma patient, target coverage and organ at risk sparing of the SE-IMPT plan resulted comparable to that of the less reliable conventional IMPT technique. Robustness analysis confirmed that SE-IMPT was not affected by range errors, which can drastically affect the IMPT plan.When accepting a low-dose spread as in modern photon techniques, SE-IMPT could be an option for the treatment of lesions (e.g. cervical bone tumours) where steep dose gradient could improve curability, and where range uncertainty, due for example to the presence of metal implants, hampers conventional IMPT.