Beam Flatness Research Articles

Beam profile characteristics of a Varian linear accelerator across different photon energy levels

Radiotherapy employs variety of energy radiation to destroy cancer cells. A linear accelerator(also called LINAC) is the machine used to delivered external beam radiation therapy.In order to confirm the treatment planning systems that deliver the best radiation to the tumors while sparing the surrounding normal tissues, extensive measurements of dosimetric parameters are part of the commissioning procedure of a LINAC for clinical usage. This work aimed to compare and assess photon beam profiles with respect to penumbra width, symmetry, and beam flatness. Beam profile measurements were performed for this study atTMSS Cancer Center, Bogura, Bangladesh, using a linear accelerator (Varian VitalBeam SN: 5199) with 6MV,10MV and 15MV photon energies for a set of field sizes (4 × 4, 10 × 10 and 20 × 20 cm2) and various reference depths keeping the same environmental conditions. A 3D water phantom, CC13 ionization chamber (SN:18635)as a reference chamber, CC04 ionization field chamber (SN:18616) as a and IBA myQA Accept software version 1.6 were used to measure the profiles of photon energies 6 MV, 10 MV 15 MV respectively. Utilizing the Eclipse (Version: 16.1) external treatment planning system, the profile calculation was carried out. According to the manufacturer's and IEC specifications, the photon beam profile data from the current investigation are compatible, and all of the tolerances fall within the ranges of clinically acceptable tolerance.

Effects of magnetic field topography on the ion thruster discharge chamber performance

It is the magnetic field topography that determines the ion thruster discharge performance. A three-dimension numerical model of the 20 cm ion thruster discharge chamber was established, and a particle-in-cell method was used to model the discharge process to investigate the effect of the magnetic field topography on the ionization rate and plasma distribution. The results showed that compared with the cylindrical-cusp magnetic field topography, the ring-cusp field made the energetic electrons primarily confined for their increased path length prior to loss to the anode wall and caused the ionization rate to be increased, but the plasma distributed uniformly upstream of the screen grid and the beam flatness was compared to be decreased to 14.8%. When the magnet size was increased and the distance between magnets was changed, the magnetic field contour and the magnetic flux density were modified, and then an improved beam flatness was obtained, from 0.556 to 0.658, by 15.5%.

Using KMeans Clustering to Evaluate and Alert for Deviations of Linac Photon Beam Parameters.

To analyse the daily measured Dosimetric Quality Assurance (QA) parameters of linear accelerator (linac) using Unsupervised Machine Learning (ML) Algorithm thereby evaluating the current machine status and to highlight the probable cause of the 'out-of-range' measured parameter. Five Parameters measured using PTW QuickCheckwebline device in a linac is subjected to KMeans clustering technique. The measured parameters comprise of Central Axis Dose (CAX), Beam Flatness, SymmetryLR, SymmetryGT and Beam Quality (BQF). Data from Varian with 55- and 107-day's measurements and from Elekta with 75 days measurements from 2 beam matched linacs were used in this clustering technique. This evaluation is used to review the current linac status and obtain 1) Upper and lower limits of each parameter (CAX, Flatness, Symmetry, Beam Quality), 2) Frequency of the days when the linac parameters are closer to the target value and when they deviate from the target value. 3) The date when these parameters deviate from the estimated limits. 4) The probable reason for the deviation and 5) Finally if the machine requires maintenance. This methodology ensures that the machine is always closest to the target value, thus providing quality radiation treatment for the cancer patients. Moreover, the performance of the linac is studied meticulously and the need for maintenance is alerted before the linac beam shows marked deviation from the base value. KMeans clustering is a very simple and easy to use ML tool. With quick computation time and with lesser data it can arrive at the actual limits of the linac parameters and help to determine if the linac needs maintenance well in advance.

Electron Beams Commissioning and Initial Measurements on an Elekta Synergy Platform Linear Accelerator

A linear accelerator (Linac) must be commissioned for use in treating cancer patients. This research work aims to analyze part of the electron beam data produced by the Elekta Synergy Platform Linac at the radiotherapy department at Tripoli University Hospital, Tripoli, Libya. In this paper, percentage depth dose (PDD), beam profile and applicators factor were studied at five different electron beam energies (4, 6, 8, 10 and 12 MeV). The relative measurement part of this study was carried out using a PTW MP3-M 3D water scanning system, and the absolute measurement was taken using a plane parallel ionization chamber and a unidose E electrometer. The analysis for these measurements has yielded the following results: a) The Penetrative qualities of all the electron beam energies were within the manufacturer's tolerance limits of ±1%; b) the maximum values of beam flatness of 2.94 and beam symmetry of 1.93 are within the accepted limits of International Electrotechnical Commission criteria; c) A penumbra maximum value of 1.30 was measured at the 4-MeV electron beam energy and using a 14×14 cm2 applicator size. All the obtained parameters were within the permissible limits. Therefore, the electron beams can be safely used for clinical purposes.

Monitoring linear accelerator beam with daily quality assurance phantom

This paper aims to analyze the output constancy of a medical linear accelerator using PTW, and to study the suitability as a daily quality assurance device. It is assumed that the device is sensitive enough to detect minor variations in central axis beam, flatness, symmetry, and beam quality factor. PTW, QUICKCHECKwebline is the most efficient wireless device used in daily quality assurance. The measurements of output doses of photons ( 6MV and 15 MV) and electrons (6,9,12, and 15MeV) from a medical linear accelerator before the daily treatment have been graphically analyzed. This study assures the output stability of Varian Clinac iX 2100 CD linear accelerator at Bhaktapur Cancer Hospital, Bhaktapur, Nepal. The beam flatness, symmetry, beam quality factor, and central axis of photon and electron beam have been analyzed for 10x10cm2 and 20x20cm2 field size in separate ways. Among the measurements, observed parameters lie under the tolerance limit as recommended by American Association of Physicists in Medicine viz. ±3%. The outcomes of the measurements are in the acceptable range for the treatment procedure of patients.

Dosimetric Analysis of Rhizophora-based Phantom Material in Radiation Therapy Applications Using Monte Carlo GATE Simulation.

This study aims to determine the percentage depth dose (PDD) of a phantom material made from soy-lignin bonded Rhizophora spp. particleboard coated with a gloss finish by using Monte Carlo Geant4 Application for Tomographic Emission (GATE) simulation. The particleboard was fabricated using a hot pressing technique at target density of 1.0 g·cm-3 and the elemental fraction was recorded for the simulation. The PDD was simulated in the GATE simulation using the linear accelerator Elekta Synergy model for the water phantom and Rhizophora phantom, and the results were compared with the experimental PDD performed by several studies. Beam flatness and beam symmetry were also measured in this study. The simulated PDD for Rhizophora and water was in agreement with the experimental PDD of water with overall discrepancies of 0% to 8.7% at depth ranging from 1.0 to 15.0 cm. In the GATE simulation, all the points passed the clinical 3%/3 mm criterion in comparison with water, with the final percentage of 2.34% for Rhizophora phantom and 2.49% for the water phantom simulated in GATE. Both the symmetries are all within the range of an acceptable value of 2.0% according to the recommendation, with the beam symmetry of the water phantom and Rhizophora phantom at 0.58% and 0.28%, respectively. The findings of this study provide the necessary foundation to confidently use the phantom for radiotherapy purposes, especially in treatment planning.

Assessing Clinical Feasibility of a LINAC-Based Electron FLASH Radiotherapy System Using an Anthropomorphic Phantom under Realistic Clinical Conditions

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (UHDR) radiotherapy (RT) at ≥40 Gy/s has been shown preclinically to reduce toxicity with similar tumor control compared to conventional RT, a phenomenon known as the FLASH effect. We previously described a configuration of a clinical-use LINAC that can output ∼17 MeV electron UHDR RT at 45-80 Gy/s with clinically feasible beam profiles, field sizes (3-20 cm), and SSDs (70-90 cm). Safe and effective clinical trials are needed to advance translation of FLASH treatment in humans, such as one proposed at our institution for cutaneous lymphomas. This study characterizes the beam output of our electron UHDR RT set-up under realistic clinical conditions for common anatomic sites using an anthropomorphic phantom, to assess its feasibility for clinical translation. <h3>Materials/Methods</h3> Our electron UHDR RT set-up utilizes a treatment delivery system with a configured program board of a decommissioned electron energy to adjust the beam output and control, without hardware modification. The electron cone is replaced with a flat applicator with custom cut-outs that lies in the accessory mount. We tested this set-up under realistic clinical conditions on a RANDO phantom for treatment of the chest/breast, leg, and scalp. We used 3 cm and 10 cm circular field sizes at 80 cm SSD with the gantry angled en-face to the targets. Tissue-equivalent bolus and flexible tungsten-silicone skin collimators (made from silicone sheets with tungsten powder cut into donut shapes of varying diameters) were used to mimic clinical treatment standards. Measurements were done using radiographic film and we evaluated the beam width, percent depth dose, and penumbra width. <h3>Results</h3> All results reported include 1 cm bolus and skin collimators unless otherwise specified. The values for R80 (depth at 80% dose), beam width of 80% dose (at R95), and penumbra width (at R85) for the 10 cm chest/breast field are 5.2, 8.7, and 0.9 cm, respectively. Without skin collimators, the values are 5.4, 9.0, and 1.2 cm, respectively. The values for the 3 cm chest/breast field are 2.9, 1.9, and 0.6 cm, respectively. Without skin collimators, the values are 3.2, 1.9, and 0.7 cm, respectively. The values for the 10 cm leg field are 4.0, 6.6, and 0.9 cm, respectively. The values for the 3 cm scalp field (with 4 cm bolus to minimize dose penetration to the brain) are 1.1, 1.6, and 0.9 cm, respectively. The steep curvature of the leg and scalp sites led to expected decrease in beam flatness but retained a treatment area that allows for a reasonable range of tumor sizes and depths. <h3>Conclusion</h3> Our configuration is able to deliver electron UHDR RT with clinically applicable beam profiles to various anatomic sites under realistic clinical conditions. Tungsten-silicone skin collimators are an easy-to-use tool to narrow the penumbra. Overall, these results show potential for treatment of superficial indications, including cutaneous lymphoma, extremity sarcoma, and partial breast irradiation, and place us one step closer toward the clinical translation of FLASH.

Clinical LINAC-Based Electron FLASH: Pathway for Practical Translation to Trials of FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Studies show ultra-high dose rate (UHDR) radiotherapy (RT), at rates of ≥40 Gy per second, can produce the FLASH effect: reduced toxicity with equal tumor control when compared to conventional RT. Leveraging these benefits for clinical translation can profoundly impact RT. Early clinical trials exhibited safe delivery of UHDR-RT using specialized RT machines. Our prior study showed that a reversible configuration of a standard linear accelerator (LINAC) can be optimized for UHDR output. Here, we explored the components necessary for clinically practical delivery of LINAC-based electron FLASH RT. <h3>Materials/Methods</h3> Using a standard, clinically operational system for image-guided radiation, we configured the program board for a decommissioned electron beam energy for UHDR delivery. No LINAC hardware manipulation or exchange was performed. Key changes included adjustment of the dose per pulse and pulse delivery control using the gating interface. Of note, the scattering foil remained in place within the beam so that field sizes and depth doses could approximate those of standard clinical operations. A commercial electron arc applicator was installed at the face of the gantry using the standard applicator mount to allow for short SSD electron collimation. Copper electron blocks were fabricated with apertures ranging from 3.0 to 20.0 cm. Dose rates were measured at dmax for 100 to 59 cm source-to-surface distances (SSD) (level of machine head) and beam profile measurements were captured at 80 cm SSD using an IC profiler and radiographic film. <h3>Results</h3> UHDR rates ranging from 36.8 to 112.8 Gy/s were measured at 100 to 59 cm SSD, respectively. An 80 cm SSD was selected for treatment arrangement measurements to maintain UHDR and clinical utility, allowing for 13.5 cm of clearance. UHDR ≥60 Gy/s was observed at 80 cm SSD with all field sizes, using a standard, scattered electron beam, with an R80 depth of 6.1 cm, approximating to a 16-18 MeV energy. Average dose per pulse was measured at 0.33 Gy/pulse. Circular block apertures of 3, 10, and 20 cm exhibited beam flatness <5.0 % variability and symmetry of 2.2 to 6.2%. Penumbra width between the 80 and 20% isodose lines (IDL) measured at 0.7, 1.1, and 1.4 cm, respectively. <h3>Conclusion</h3> Optimizing a reversible setup for UHDR irradiation on a standard LINAC, electron delivery at dose rates expected to produce the FLASH effect was achieved. Flat, homogenous field sizes, ranging from 3 to 20 cm of coverage, were obtained at a clinically practical SSD of 80 cm with a treatment depth of 6.1 cm to the 80% IDL. This setup would enable UHDR irradiation at useful depths for a number of different clinical indications, bringing us a step closer to practical UHDR RT using a standard LINAC in future clinical trials.

Intensity Modulation in Electron FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Combining methods of conformal dose delivery with ultra-high dose rate (UHDR) beams is advantageous for realizing the benefits of the FLASH effect in clinical treatments. We hypothesize that deployment of intensity modulation in passive electron FLASH radiotherapy through treatment plan optimization is feasible. <h3>Materials/Methods</h3> A beam model of an electron FLASH irradiator (delivering ∼300Gy/s at the isocenter) in the decimal ElectronRT treatment planning system (TPS) was validated with film measured lateral and percent-depth-dose (PDD) profiles. Plans were developed comparing collimated open fields and intensity modulated electron beams achieved with cylindrical passive metal compensators that vary in spacing and size. Homogeneity and conformity were quantified for plans developed in a water phantom and anonymized patient cases considering constraints in prescribed dose to the target volume and minimizing dose to organs-at-risk (OAR). <h3>Results</h3> The film measured profiles and TPS beam model agreed on average to within 1% and 2% for lateral profile and PDD, respectively. For a large 15 × 15 cm² field in a water phantom the intensity modulation improved the beam flatness (∼30% to <5%) and penumbra (35 mm to 15mm) at 3 cm depth while retaining UHDR in the treatment field with a 38% reduction in central axis output while still retaining the UHDR conditions (∼180Gy/sec). The PDD exhibited <2% difference along the central axis and minimal changes to symmetry, shift, and practical range. In the rib metastasis case, the unmodulated and modulated plans treat the tumor volume with a homogeneity index (HI), improvement by 0.2. In the facial orbital plan, the conformity index improved by 8% with an improvement in HI by 0.05 and comparable dose to OARs. <h3>Conclusion</h3> Intensity modulation of electrons FLASH is achievable and improves homogeneity of prescribed dose with proper treatment constraints while reducing hot spots for clinical plans. Depending on the treatment case intensity modulation can also generate superior dose distributions while retaining the UHDR conditions to best exploit the FLASH effect. Future study will focus on demonstrating dosimetric benefits quantified by tumor control probability and normal tissue complication probability models in a larger number of relevant cases.

Characterization of the PTB ultra-high pulse dose rate reference electron beam

Purpose. This investigation aims to present the characterisation and optimisation of an ultra-high pulse dose rate (UHPDR) electron beam at the PTB facility in Germany. A Monte Carlo beam model has been developed for dosimetry study for future investigation in FLASH radiotherapy and will be presented. Material and methods. The 20 MeV electron beams generated by the research linear accelerator has been characterised both in-beamline with profile monitors and magnet spectrometer, and in-water with a diamond detector prototype. The Monte Carlo model has been used to investigate six different setups to enable different dose per pulse (DPP) ranges and beam sizes in water. The properties of the electron radiation field in water have also been characterised in terms of beam size, quality specifier R 50 and flatness. The beam stability has also been studied. Results. The difference between the Monte-Carlo simulated and measured R 50 was smaller than 0.5 mm. The simulated beam sizes agreed with the measured ones within 2 mm. Two suitable setups have been identified for delivering reference UHPDR electron beams. The first one is characterised by a SSD of 70 cm, while in the second one an SSD of 90 cm is used in combination with a 2 mm aluminium scattering plates. The two set-ups are quick and simple to install and enable an expected overall DPP range from 0.13 Gy up to 6.7 Gy per pulse. Conclusion. The electron beams generated by the PTB research accelerator have shown to be stable throughout the four-months length of this investigation. The Monte Carlo models have shown to be in good agreement for beam size and depth dose and within 1% for the beam flatness. The diamond detector prototype has shown to be a promising tool to be used for relative measurements in UHPDR electron beams.

Design and performance test of discharge chamber and grid for multi-mode ion thrusters

In view of the application requirements of electric propulsion system for China’s asteroid deep space exploration mission, the overall scheme is designed, an ion thruster prototype model is established by using a four-ring-cusp field discharge chamber, 30-cm beam current extraction diameter three-grid ion optics system. Reasonableness and compatibility of discharge chamber and grid design are verified experimentally and theoretically . The test results are shown below. The ion thruster can operate steadily over an input power envelope of 277–3120 W, thrust increases linearly from 9.9 to 117.2 mN, specific impulse rises from 1269 to 3492 s, the beam divergence angle drops from 30.7° to 26.8° and stabilizes above a certain power value, the thrust vector angle is less than 1.5° and beam flatness parameter is greater than 0.75 at different operating points. The maximum percentage reduction in grid gap aberration is 90% with the strain relief molybdenum mounting ring thermal design. This research provides a reference for multi-mode ion thruster design and in-orbit engineering applications.

Validation of a Novel Cone-Less Set-up for Electron FLASH Radiation Delivery on a Clinical-Use Linear Accelerator

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (FLASH) radiotherapy (RT) at ≥40 Gray per second has been shown preclinically to achieve improved normal tissue sparing while maintaining tumor control similar to that of conventional dose-rate RT. Many published FLASH-RT delivery models are inaccessible for widespread use and/or not applicable for clinical translation. We previously described a model for reversible configuration of a clinical linear accelerator (LINAC) that can deliver ∼16-18 MeV electron FLASH-RT at source-to-surface distances (SSDs) < 95 cm. However, it precludes the use of a standard electron cone. This proof-of-concept study will characterize the beam profiles from a novel cone-less set-up, to verify its feasibility as a widely accessible method of electron FLASH-RT delivery for the intention of clinical treatment. <h3>Materials/Methods</h3> This novel set-up employs a flat 25 × 25 cm applicator with custom cut-outs that lies within the accessory mount at 66.6 cm SSD of a standard treatment delivery system. We tested a 10 × 10 cm circular field size at a target distance of 80 cm SSD (selected to maintain FLASH dose rates). Radiographic film and an IC Profiler were used to evaluate penumbra width, beam flatness, symmetry, and percentage depth dose (PDD). FLASH-RT was delivered in 20 pulses with 3 independent measurements each using film and the profiler. This was compared to a conventional 16 MeV beam using a standard cone with a 10 × 10 cm field size at 100 cm SSD. <h3>Results</h3> Our novel cone-less FLASH set-up achieved a beam profile that was similar to the conventional electron set-up. Respectively, for the cone-less and conventional set-up, measured at a standardized depth of half R85, penumbra width was 1.13 cm and 1.15 cm, and uniformity index was 0.72 and 0.74 (defined as the ratio of the areas inside the 90% and 50% isodose lines); measured at a standardized depth of R95, flatness (defined as the maximum dose deviation from the central axis at a specific depth over an area confined within lines 2 cm inside the geometric edge of fields) was 1.5% and 4.1%, beam diameter containing dose > 80% of the central axis was 10.6 cm and 9.6 cm, and symmetry (defined as the maximum dose difference between any two symmetric points about the central axis in the flat portion of the beam, as a percentage of central dose) was 2.24% and 2.20%. PDD values for the cone-less set-up at Dmax, R95, R85, and R50 were, respectively, 3.1, 4.1, 5.3, and 6.8 cm. <h3>Conclusion</h3> We have developed a novel cone-less set-up that allows for the delivery of electron FLASH-RT utilizing a clinical LINAC at practical SSDs and verified that its beam profiles are clinically feasible. Further work will be done to characterize additional SSDs and field sizes, along with the addition of a custom skin collimator to further shape the beam. This will pave the way for practical treatment using electron FLASH-RT to a host of oncologic indications, including cutaneous malignancies and partial breast irradiation in the future.

Parametric Analysis of Linear Periodic Arrays Generating Flat-Top Beams

Several synthesis techniques are available to optimize amplitude and phase excitations of periodic linear arrays to generate flat-top beams. Clearly, the optimal tapering depends on design parameters such as the array length, the number of array elements, the beam flatness, the beam width, the side lobe levels, and others. In this paper, in order to derive useful guidelines and rule of thumb for the synthesis of periodic array antennas, relations between these parameters are derived employing linear programming techniques, which guarantee optimality of the solutions. Such relations are then plotted and used in some design examples.

Investigation of variable aperture on the performance and lifetime of ion thruster

Beam flatness is an important parameter that determines the performance and the lifetime of a gridded ion thruster. To improve the beam flatness of the 30 cm (LIPS-300) ion thruster, variable aperture ion optics that adapts to the decreasing ion density as the radius increases is proposed. It is the ion optics that the screen grid surface is divided into several zones, where the aperture diameter in each zone is determined by the ion density and the electron temperature upstream of the screen grid. The beam current density in the central area is artificially reduced. A particle in cell-Monte Carlo collision model is applied in this work to investigating the effect of variable aperture on the perveance and the maximum beam current per aperture by simulating the extraction, focusing and acceleration processes of ions. Taking into account the engineering implementability, the screen grid surface is divided into four zones. The hole diameter in each zone is decreased from 1.95 mm to 1.8 mm, 1.9 mm, 1.8 mm and 1.7 mm, respectively. The simulation results show that the maximum ion density in the center area of grid is decreased by 10.6% and 6.99%, while it is increased by 6.49% and 22.3% in the edge region, respectively. The beam flatness of the variable aperture ion optics is improved from 0.69 to 0.88. The erosion rate is decreased by 31.9%, but the total beam current is also decreased by 7.15%. The simulation results can provide a valuable reference of the development of the ion thruster.

Performance of the Secondary Electrons Profile Monitor for the scanning gantry in the Heavy Ion Medical Machine

A Secondary Electrons Profile Monitor (SEPM) is developed as part of the beam dose delivery system in a clinical gantry of the Heavy Ion Medical Machine (HIMM). With the purpose of less disturbance to the beam, a thin foil with a thickness of 0.7μm and an accelerating grid with a diameter of 15μm are employed to produce and accelerate the secondary electrons, and two microchannel plates (MCPs) are used to multiply the electrons. Providing the real-time information of beam size, position, flatness, and symmetry in front of the scanning magnet, the beam monitor features an active area of 989 mm2, beam intensities between 1.0 ×103 to 1.0 ×108 particles per pulse (ppp) and a wide range of beam energies. A prototype is tested at the horizontal treatment room of the HIMM. Using different beam positions, sizes, and energies of 12C6+ beams, test experiments show that the beam monitor maintains good consistency with a Multi-Strip Ionization Chamber (MSIC). Compared to the MSIC, the SEPM shows a better signal-to-noise ratio (SNR) and higher sensitivity to the variation of the beam.

Development and validation of an optimal GATE model for double scattering proton beam delivery

Proton therapy (PT) is an emerging external beam radiation therapy characterized by superior dose distribution compared to conventional modalities. In the present study, an optimal GATE model was developed and then validated for a double scattering proton nozzle based on the previously constructed model. To this aim, a double scattering treatment nozzle was modeled in the GATE platform. To accelerate the GATE simulations, a virtual range modulation wheel (vRMW) and a variance reduction technique (VRT) were implemented. Proton beam flatness, symmetry, and delivery efficiency, secondary neutron dose, and dosimetric performance were characterized through a set of GATE simulations. The findings show that range cutoff value of 0.075 mm provides the best compromise between the simulation accuracy and speed for the simulated geometry. A proton beam flatness of 98.6% was observed at the downstream of the aperture for a 7 × 7 cm2 field size. The beam flatness deteriorates at the edge of the treatment field for the single scattering model while it remains approximately constant for the double scattering one. In comparison to the single scattering delivery, the second scattering model results in a 1.28 times increase in neutron dose for the nickel, as the optimal collimator/aperture material. Furthermore, a flat beam modulation width of 3.50 cm is formed with a distal edge at 7.86 cm in water using both GATE and MCNPX codes. The GATE model agreed with the MCNPX results with a maximum difference of ±6.3% in absorbed dose estimation. The findings demonstrate that the constructed GATE model of double scattering proton nozzle results in a fast and accurate simulation of passive scattering PT.

Electron FLASH Delivery at Treatment Room Isocenter for Efficient Reversible Conversion of a Clinical LINAC

In this study, procedures were developed to achieve efficient reversible conversion of a clinical linear accelerator (LINAC) and deliver ultrahigh-dose-rate (UHDR) electron or conventional beams to the treatment room isocenter for FLASH radiation therapy. The LINAC was converted to deliver UHDR beam within 20 minutes by retracting the x-ray target from the beam's path, positioning the carousel on an empty port, and selecting 10 MV photon beam energy in the treatment console. Dose rate surface and depth dose profiles were measured in solid water phantom at different field sizes with Gafchromic film and an optically stimulated luminescent dosimeter (OSLD). A pulse controller counted the pulses via scattered radiation signal and gated the delivery for a preset pulse count. A fast photomultiplier tube-based Cherenkov detector measured the per pulse beam output at a 2-ns sampling rate. After conversion back to clinical mode, conventional beam output, flatness, symmetry, field size, and energy were measured for all clinically commissioned energies. The surface average dose rates at the isocenter for 1-cm diameter and 1.5-in diameter circular fields and for a jaws-wide-open field were 238 ± 5 Gy/s, 262 ± 5 Gy/s, and 290 ± 5 Gy/s, respectively. The radial symmetry of the beams was within 2.4%, 0.5%, and 0.2%, respectively. The doses from simultaneous irradiation of film and OSLD were within 1%. The photomultiplier tube showed the LINAC required ramp up time in the first 4 to 6 pulses before the output stabilized, after which its stability was within 3%. At the isocenter of the treatment room, 10 MeV UHDR beams were achieved. The beam output was reproducible but requires further investigation of the ramp up time, equivalent to ∼1 Gy, requiring dose monitoring. The UHDR beam can irradiate both small and large subjects to investigate potential FLASH radiobiological effects in minimally modified clinical settings, and the dose rate can be further increased by reducing the source-to-surface distance.

Evaluation of the effects of implementing a diode transmission device into the clinical workflow.

This work investigated effects of implementing the Delta4 Discover diode transmission detector into the clinical workflow. PDD and profile scans were completed with and without the Discover for a number of photon beam energies. Transmission factors were determined for all beam energies and included in Eclipse TPS to account for the attenuation of the Discover. A variety of IMRT plans were delivered to a Delta4 Phantom+ with and without the Discover to evaluate the Discover's effects on IMRT QA. An imaging QA phantom was used to assess the detector's effects on MV image quality. OSLDs placed on the Phantom+ were used to determine the detector's effects on superficial dose. The largest effect on PDDs after dmax was 0.5%. The largest change in beam profile symmetry and flatness was 0.2% and 0.1%, respectively. An average difference in gamma passing rates (2%/2mm) of 0.2% was observed between plans that did not include the Discover in the measurement and calculation to plans that did include the Discover in the measurement and calculation. The Discover did not significantly change the MV image quality, and the largest observed increase in the relative superficial dose when the Discover was present was 1%. The effects the Discover has on the linac beam were found to be minimal. The device can be implemented into the clinic without the need to alter the TPS beam modeling, other than accounting for the device's attenuation. However, a careful workflow review to implement the Discover should be completed.

Quality Assurance of LINAC by Analyzing the Profile of 6-MV and 10-MV Photon Beams Using Star Track Device

Introduction: According to the American Society of Radiation Oncology, all patients receive radiation therapy during their illness, where radiation is delivered by the medical linear accelerator (Linac). The aim of this study was to evaluate the quality assurance (QA) of the Linac in analyzing the used dose profile in the treatment of cancer tumors. Material and Methods: This experimental study was performed using Linac (synergy device type) at Baghdad Radiotherapy and Nuclear Medicine laboratories, Baghdad, Iraq. The Star Track device was used for the routine quality assurance of the Linac, using photon beam for the reference Dmax and source to surface distance of 100 cm. The Star Track consists of 453 vented parallel plate ionization chambers. Results: The flatness and symmetry of beams for the reference field size did not exceed from ±2%, as they were within the allowed range. Moreover, the penumbra region showed a change in value that did not exceed from ±0.2 cm. using the Star Track method; maximum differences in beam symmetry and beam flatness were measured at 0.76%±2% and 1.17%±2%, respectively. Moreover, the maximum difference in the penumbra region was estimated at 0.12±0.2 cm. Conclusion: The results indicated, the Star Track could successfully calculate the characteristics of dose profile during a time period of 2,500 ms, showing the superiority of this instrument over other verification devices.

Propagation of fully and partially coherent flat-topped multi-Gaussian beams through axicons.

Theoretically, we have investigated the propagation of fully and partially coherent flat-topped multi-Gaussian beams through classical and logarithmic axicons. The stationary phase method applied to the Fresnel diffraction integral for intensity distribution has led to applicable relationships. The influences of axicon base angle, beam width, beam flatness, and coherence length with intensity distribution were studied. A number of results have been analyzed and discussed revealing some important conclusions. During this investigation, a specific interest has been given toward producing uniform intensity and long focal depth.