Percentage Depth Dose Curves Research Articles

KV reference dosimetry in Australia and New Zealand: Survey results and trends.

To assess the number of radiotherapy kilovoltage (kV) units in service, their clinical utilization, and methodology and equipment used for absorbed dose determination across Australia and New Zealand. A survey was sent to 61 Australian and New Zealand radiotherapy providers in the second half of 2023. Fifty-seven responses were received, with 43 departments having kV units and providing beam quality data for 185 therapeutic kV beams 20-300kVp. Percentage depth dose curves were compared between five clinical beams with 100 kVp and 2.13-6.28mm Aluminum half value layers (HVLs), demonstrating large differences that can occur between beams with the same kVp. Eighteen departments provided clinical utilization data for their kV units, with a total of 4458 treatment courses and their corresponding kVp reported. All departments complied with national and international recommendations with respect to the equipment used for reference dosimetry of kV beams; 77% of ionization chambers used for absorbed dose determination were of Farmer-type, with the remaining 23% being plane parallel soft x-ray chambers. Methods of derivation of air-kerma calibration factors varied, with 73% of respondents using a draft document disseminated by the Australian Primary Standards laboratory, 23% using HVL alone, and 6% using other methods. The results of this survey provide a snapshot of kilovoltage radiation therapy use and the number of kV units across Australia and New Zealand. This data can be used as a point of reference for future investigations into clinical utilization and reference dosimetry methods across Australia and New Zealand or for comparisons with other countries, facilitating standardization of reference dosimetry practice for kilovoltage units.

Fitting Monte Carlo simulation results with an empirical model of megavoltage x-ray beams for rapid depth dose calculations in water

Objective. The purpose of this study was to assess a method of accelerating Monte Carlo simulations for modeling depth dose distributions from megavoltage x-ray beams by fitting them to an empirically-derived function. Approach. Using Geant4, multiple simulations of a typical medical linear accelerator beam in water and in water with an air cavity were conducted with varying numbers of initial electrons. The resulting percent depth dose curves were compared to published data from actual linear accelerator measurements. Two methods were employed to reduce computation time for this modeling process. First, an empirical function derived from measurements at a particular linear accelerator energy, source-to-surface distance, and field size was used to directly fit the simulated data. Second, a linear regression was performed to predict the empirical function’s parameters for simulations with more initial electrons. Main results. Fitting simulated depth dose curves with the empirical function yielded significant improvements in either accuracy or computation time, corresponding to the two methods described. When compared to published measurements, the maximum error for the largest simulation was 5.58%, which was reduced to 2.01% with the best fit of the function. Fitting the empirical function around the air cavity heterogeneity resulted in errors less than 2.5% at the interfaces. The linear regression prediction modestly improved the same simulation with a maximum error of 4.22%, while reducing the required computation time from 66.53 h to 43.75 h. Significance. This study demonstrates the effective use of empirical functions to expedite Monte Carlo simulations for a range of applications from radiation protection to food sterilization. These results are particularly impactful in radiation therapy treatment planning, where time and accuracy are especially valuable. Employing these methods may improve patient outcomes by ensuring that dose delivery more accurately matches the prescription or by shortening the preparation time before treatment in Monte Carlo-based treatment planning systems.

Effect of photons and electrons on the over-response of optical fiber X-ray sensors

Optical fiber X-ray sensors are prone to over-response compared to ionization chambers (ICs) when used to measure the percentage depth dose (PDD) curves and off axis ratio (OAR) curves. Investigating this phenomenon is crucial for the successful adoption of these sensors in clinical radiotherapy. Compared with ion chambers and other water equivalent dosimeters, the fiber X-ray sensors using high Zeff scintillators (Gd2O2S:Tb) have generally higher mass attenuation coefficients. In this case, the response of scintillators to photons needs to be considered. In this article, a Monte-Carlo (MC) software package BEAMnrc was used to simulate the distribution of photon and electron fluxes in a clinical based water phantom, besides the energy deposition of air and Gd2O2S:Tb in varying energy was also simulated. The distribution of the photon and electron fluxes in the horizontal (radial) direction at a fixed 10 cm water depth was studied, as well as the fluxes in the vertical (depth) direction. An embedded-structure fiber X-ray sensor was used for the experimental measurements. The simulation results indicate that, in the horizontal direction, there are many photons remain outside the radiation field. In the vertical direction, as the size of the radiation field increases, the maximum point of the photon flux shifts towards a deeper water depth. These behavior of photon flux is consistent with the fiber X-ray sensors measurement. However, the maximum point of the electron flux remains unchanged in the vertical direction. In the horizontal direction, the electron flux decreases rapidly, and this scenario is similar to the case when using the ion chamber. Results of analyzing the spectra inside and outside the radiation field show that there is a large number of photons with energy below 0.2 MeV outside the radiation field, and the proportion of these low-energy photons is greater than 60 %. The findings indicate that the fiber X-ray sensors' over-response can be primarily attributed to the lower energy scattered photons.

Determination of kilovoltage x-ray therapy depth doses with open-ended applicators.

The purpose of this work was to determine percentage depth dose (PDD) curves for kilovoltage x-rays from the WOmed-T105 unit, with open-ended steel applicators and beam qualities ranging from 0.5 to 4.2mm Al. Measurements were made with parallel plate chambers in a water phantom, with extrapolation based on a fifth order polynomial used to estimate the surface dose. Measurements were also made with parallel plate chambers in a plastic water phantom, with thin plastic sheets used to obtain detailed measurements at shallow depths (less than 1mm). Monte Carlo simulations were performed using the EGSnrc package, with two different sources as input: a SpekPy simulation of the x-ray beam and a full simulation of the x-ray tube, treatment head and applicators. Results showed that all four methods (two measurements and two simulations) agreed within the measurement uncertainty at depths greater than 2mm. At shallow depths, significant differences were noted. At depths less than 0.1mm, the full Monte Carlo simulation and the solid water measurements showed a sharp spike in surface dose which is attributed to electron contamination, which was not seen in the SpekPy Monte Carlo simulation or the extrapolated water measurements. At depths between 0.1mm and 2mm, beyond the range of contaminant electrons, the extrapolated water measurements underestimate the dose by up to 13% compared to the full Monte Carlo simulation and the solid water measurements, attributed to fluorescent photons generated in the applicators. This work demonstrates that for open-ended applicators, measurement of depth doses in water with extrapolation of surface dose has the potential to significantly underestimate the dose at shallow depths between the surface and 2mm, even after eliminating electron contamination from the beam.

TOPAS simulation of photoneutrons in radiotherapy: accuracy and speed with variance reduction

Objective. We provide optimal particle split numbers for speeding up TOPAS Monte Carlo simulations of linear accelerator (linac) treatment heads while maintaining accuracy. In addition, we provide a new TOPAS physics module for simulating photoneutron production and transport. Approach. TOPAS simulation of a Siemens Oncor linac was used to determine the optimal number of splits for directional bremsstrahlung splitting as a function of the field size for 6 MV and 18 MV x-ray beams. The linac simulation was validated against published data of lateral dose profiles and percentage depth-dose curves (PDD) for the largest square field (40 cm side). In separate simulations, neutron particle split and the custom TOPAS physics module was used to generate and transport photoneutrons, called ‘TsPhotoNeutron’. Verification of accuracy was performed by comparing simulations with published measurements of: (1) neutron yields as a function of beam energy for thick targets of Al, Cu, Ta, W, Pb and concrete; and (2) photoneutron energy spectrum at 40 cm laterally from the isocenter of the Oncor linac from an 18 MV beam with closed jaws and MLC. Main results. The optimal number of splits obtained for directional bremsstrahlung splitting enhanced the computational efficiency by two orders of magnitude. The efficiency decreased with increasing beam energy and field size. Calculated lateral profiles in the central region agreed within 1 mm/2% from measured data, PDD curves within 1 mm/1%. For the TOPAS physics module, at a split number of 146, the efficiency of computing photoneutron yields was enhanced by a factor of 27.6, whereas it improved the accuracy over existing Geant4 physics modules. Significance. This work provides simulation parameters and a new TOPAS physics module to improve the efficiency and accuracy of TOPAS simulations that involve photonuclear processes occurring in high-Z materials found in linac components, patient devices, and treatment rooms, as well as to explore new therapeutic modalities such as very-high energy electron therapy.

Planar dose calculation of electron therapy

Purpose. The aim of this study is to determine the planar dose distribution of irregularly-shaped electron beams at their maximum dose depth (z max) using the modied lateral build-up ratio (LBR) and curve-fitting methods. Methods. Circular and irregular cutouts were created using Cerrobend alloy for a 14 × 14 cm2 applicator. Percentage depth dose (PDD) at the standard source-surface-distance (SSD = 100 cm) and point dose at different SSD were measured for each cutout. Orthogonal profiles of the cutouts were measured at z max. Data were collected for 6, 9, 12, and 15 MeV electron beam energies on a VERSA HDTM LINAC using the IBA Blue Phantom2 3D water phantom system. The planar dose distributions of the cutouts were also measured at z max in solid water using EDR2 films. Results. The measured PDD curves were normalized to a normalization depth (d 0) of 1 mm. The lateral-buildup-ratio (LBR), lateral spread parameter (σ R (z)), and effective SSD (SSD eff ) for each cutout were calculated using the PDD of the open applicator as the reference field. The modified LBR method was then employed to calculate the planar dose distribution of the irregular cutouts within the field at least 5 mm from the edge. A simple curve-fitting model was developed based on the profile shapes of the circular cutouts around the field edge. This model was used to calculate the planar dose distribution of the irregular cutouts in the region from 3 mm outside to 5 mm inside the field edge. Finally, the calculated planar dose distribution was compared with the film measurement. Conclusions. The planar dose distribution of electron therapy for irregular cutouts at z max was calculated using the improved LBR method and a simple curve-fitting model. The calculated profiles were within 3% of the measured values. The gamma passing rate with a 3%/3 mm and 10% dose threshold was more than 96%.

Plastic scintillator-based dosimeters for ultra-high dose rate (UHDR) electron radiotherapy

This paper reports the development of dosimeters based on plastic scintillating fibers imaged by a charge-coupled device camera, and their performance evaluation through irradiations with the electron Flash research accelerator located at the Centro Pisano Flash Radiotherapy. The dosimeter prototypes were composed of a piece of plastic scintillating fiber optically coupled to a clear optical fiber which transported the scintillation signal to the readout systems (an imaging system and a photodiode). The following properties were tested: linearity, capability to reconstruct the percentage depth dose curve in solid water and to sample in time the single beam pulse. The stem effect contribution was evaluated with three methods, and a proof-of-concept one-dimensional array was developed and tested for online beam profiling. Results show linearity up to 10 Gy per pulse, and good capability to reconstruct both the timing and spatial profiles of the beam, thus suggesting that plastic scintillating fibers may be good candidates for low-energy electron Flash dosimetry.

Time-gated single-pixel imaging of Cherenkov emission from a medical linear accelerator.

Cherenkov imaging is an ideal tool for real-time in vivo verification of a radiation therapy dose. Given that radiation is pulsed from a medical linear accelerator (LINAC) together with weak Cherenkov emissions, time-gated high-sensitivity imaging is required for robust measurements. Instead of using an expensive camera system with limited efficiency of detection in each pixel, a single-pixel imaging (SPI) approach that maintains promising sensitivity over the entire spectral band could be used to provide a low-cost and viable alternative. A prototype SPI system was developed and demonstrated here in Cherenkov imaging of LINAC dose delivery to a water tank. Validation experiments were performed using four regular fields and an intensity-modulated radiotherapy (IMRT) delivery plan. The Cherenkov image-based projection percent depth dose curves (pPDDs) were compared to pPDDs simulated by the treatment planning system (TPS), with an overall average error of 0.48, 0.42, 0.65, and 1.08% for the 3, 5, 7, and 9 cm square beams, respectively. The composite image of the IMRT plan achieved a 85.9% pass rate using 3%/3 mm gamma index criteria, in comparing Cherenkov intensity and TPS dose. This study validates the feasibility of applying SPI to the Cherenkov imaging of radiotherapy dose for the first time to our knowledge.

Determination of output factor for CyberKnife using scintillation dosimetry and deep learning

Objective. Small-field dosimetry is an ongoing challenge in radiotherapy quality assurance (QA) especially for radiosurgery systems such as CyberKnifeTM. The objective of this work is to demonstrate the use of a plastic scintillator imaged with a commercial camera to measure the output factor of a CyberKnife system. The output factor describes the dose on the central axis as a function of collimator size, and is a fundamental part of CyberKnife QA and integral to the data used in the treatment planning system. Approach. A self-contained device consisting of a solid plastic scintillator and a camera was build in a portable Pelicase. Photographs were analysed using classical methods and with convolutional neural networks (CNN) to predict beam parameters which were then compared to measurements. Main results. Initial results using classical image processing to determine standard QA parameters such as percentage depth dose (PDD) were unsuccessful, with 34% of points failing to meet the Gamma criterion (which measures the distance between corresponding points and the relative difference in dose) of 2 mm/2%. However, when images were processed using a CNN trained on simulated data and a green scintillator sheet, 92% of PDD curves agreed with measurements with a microdiamond detector to within 2 mm/2% and 78% to 1%/1 mm. The mean difference between the output factors measured using this system and a microdiamond detector was 1.1%. Confidence in the results was enhanced by using the algorithm to predict the known collimator sizes from the photographs which it was able to do with an accuracy of less than 1 mm. Significance. With refinement, a full output factor curve could be measured in less than an hour, offering a new approach for rapid, convenient small-field dosimetry.

An iterative algorithm to estimate the energy spectrum of an electron beam from PDD curves

Electron beam central-axis percentage depth dose (PDD) curves in water phantom are routinely employed to evaluate the electron beam energy at the phantom surface, in particular the mean and most probable energies from the values of R 50 (half-value range) and Rp (practical range). However, these two quantities are not enough to evaluate important details of the energy distribution, such as the FWHM (Full Width Half Maximum) and the possible presence of a low-energy tail. This paper presents a numerical method that allows estimating the shape of the energy spectrum from a PDD curve. The algorithm uses a database consisting of a set of depth dose curves for monochromatic beams computed by FLUKA in the range 0.1-7.0 MeV by steps of 0.1 MeV and, using an adaptive iterative Monte Carlo process, reconstructs the incident energy spectrum by minimizing the distance between the measured PDD and the computed one. Applications of a MATLAB code based on this algorithm to simulated and real measurements of electron beams done at APAM lab (ENEA Frascati) are presented. This approach represents a strong simplification with respect to energy analysis based on the use of a magnetic spectrometer.

Photon beam modeling: A comparative study of primo and gate simulation toolkits for the TrueBeam STx Linac

This study compares the PRIMO and GATE Monte Carlo simulation toolkits for modeling photon beams from a TrueBeam STx Linac used in radiation therapy. Various beam configurations were evaluated against Varian's Golden Beam Data using the Gamma Index method. Both toolkits demonstrated good agreement overall, with GATE generally achieving higher gamma pass rates for percent depth dose curves than PRIMO.

FLASH radiotherapy using high-energy X-rays: Current status of PARTER platform in FLASH research

PurposeRecent studies indicated that ultrahigh dose rate (FLASH) radiation can reduce damage to normal tissue while maintaining anti-tumour activity compared to conventional dose rate (CONV) radiation. This paper provides a comprehensive description of the current status of the Platform for Advanced Radiotherapy Research (PARTER), which serves as the first experimental FLASH platform utilizing megavoltage X-rays and has facilitated numerous experiments. Methods and MaterialsPARTER was established in 2019 based on a superconducting linac to support experimental FLASH studies using megavoltage X-rays. Continuous upgrades have been made to the accelerator, collimators, flattening filters, monitors, other auxiliary devices, and irradiation process in order to achieve optimal results. Passive and active dosimeters are employed for measuring dose distribution and to ensure traceability of radiation doses. ResultsThe dose monitors and dosimeters demonstrate reliable performance with acceptable stability. At PARTER, the maximum mean dose rate is approximately 400 Gy/s at a surface-source distance of 20 cm (over 1000 Gy/s at smaller distances), with an instantaneous dose rate of approximately 8E5 Gy/s. Both passive and active dosimeters exhibit good linearity and agreement during FLASH X-ray irradiation. The monitors show good linearity to dose rate, with short-term fluctuations within 1.5 % for the diamond monitor. The discrepancy between measured absorbed dose and dose protocol is typically less than 4 %. The X-ray energy spectra on PARTER are comparable to those for megavoltage CONV linacs operating in flattening filter-free mode. The maximum field size of the FLASH beam is 4.5 cm × 4.5 cm. The FLASH dose profile demonstrates satisfactory flatness (1.04) and similar penumbra compared to clinical CONV linac, while the percentage depth dose curve of FLASH X-rays is steeper than that of the clinical megavoltage CONV X-ray. ConclusionsPARTER represents a pioneering platform for conducting megavolts FLASH X-ray irradiation in biological experiments. It effectively fulfills the requirements of preclinical research on megavoltage X-ray FLASH and undergoes continuous upgrades to meet increasingly demanding performance criteria.

Evaluation of the water-equivalent characteristics of the SP34 plastic phantom for film dosimetry in a clinical linear accelerator.

In this study, some confusing points about electron film dosimetry using white polystyrene suggested by international protocols were verified using a clinical linear accelerator (LINAC). According to international protocol recommendations, ionometric measurements and film dosimetry were performed on an SP34 slab phantom at various electron energies. Scaling factor analysis using ionometric measurements yielded a depth scaling factor of 0.923 and a fluence scaling factor of 1.019 at an electron beam energy of <10 MeV (i.e., R50 < 4.0 g/cm2). It was confirmed that the water-equivalent characteristics were similar because they have values similar to white polystyrene (i.e., depth scaling factor of 0.922 and fluence scaling factor of 1.019) presented in international protocols. Furthermore, percentage depth dose (PDD) curve analysis using film dosimetry showed that when the density thickness of the SP34 slab phantom was assumed to be water-equivalent, it was found to be most similar to the PDD curve measured using an ionization chamber in water as a reference medium. Therefore, we proved that the international protocol recommendation that no correction for measured depth dose is required means that no scaling factor correction for the plastic phantom is necessary. This study confirmed two confusing points that could occur while determining beam characteristics using electron film dosimetry, and it is expected to be used as basic data for future research on clinical LINACs.

Dosimetric characterization of a new surface-conforming electron MLC prototype.

The purpose is toreduce normal tissue radiation toxicity for electron therapy through the creation of a surface-conforming electron multileaf collimator (SCEM). The SCEM combines the benefits of skin collimation, electron conformal radiotherapy, and modulated electron radiotherapy. An early concept for the SCEM was constructed. It consists of leaves that protrude towards the patient, allowing the leaves to conform closely to irregular patient surfaces. The leaves are made of acrylic to decrease bremsstrahlung, thereby decreasing the out-of-field dose. Water tank scans were performed with the SCEM in place for various field sizes for all available electron energies (6, 9, 12, and 15 MeV) with a 0.5cm air gap to the water surface at 100cm source-to-surface distance (SSD). These measurements were compared with Cerrobend cutouts with the field size-matched at 100 and 110cm SSD. Output factor measurements were taken in solid water for each energy at dmax for both the cerrobend cutouts and SCEM at 100cm SSD. Percent depth dose (PDD) curves for the SCEM shifted shallower for all energies and field sizes. The SCEM also produced a higher surface dose relative to Cerrobend cutouts, with the maximum being a 9.8% increase for the 3cm × 9cm field at 9 MeV. When compared to the Cerrobend cutouts at 110cm SSD, the SCEM showed a significant decrease in the penumbra, particularly for lower energies (i.e., 6 and 9 MeV). The SCEM also showed reduced out-of-field dose and lower bremsstrahlung production than the Cerrobend cutouts. The SCEM provides significant improvement in the penumbra and out-of-field dose by allowing collimation close to the skin surface compared to Cerrobend cutouts. However, the added scatter from the SCEM increases shallow PDD values. Future work will focus on reducing this scatter while maintaining the penumbra and out-of-field benefits the SCEM has over conventional collimation.

3D Dosimetry for Electron Flash Radiotherapy: Assessment of Radiochromic Dosimeter Phantoms with Optical CT Scanning as a 3D Dosimetry System

Many of the dosimeters used in conventional radiation therapy exhibit dose rate dependence which prohibits their use in ultra-high-dose-rate (FLASH) radiation therapy. Radiochromic plastic dosimeter PRESAGE® has been used for 3D dosimetry for many years. We hypothesized that these phantoms would show dose-rate independence throughout both the conventional and FLASH RT regimes, indicating these phantoms exhibit qualities useful for relative 3D dosimetry in FLASH electron beams. FLASH experiments were performed using a commercially available linear accelerator, converted to deliver an ultra-high-dose-rate 10 MeV electron beam. The LINAC delivered approximately 0.7 Gy/pulse for FLASH irradiations. Dose rate was varied from about 40 Gy/s to 240 Gy/s by changing the repetition rate. PRESAGE phantoms were irradiated en face at six FLASH dose rates: 40 Gy/s, 80 Gy/s, 120 Gy/s, 160 Gy/s, 200 Gy/s, and 240 Gy/s. EBT film and scintillator measurements were used to verify dose delivered. The optical response of the PESAGE phantom versus delivered dose was evaluated with various known doses. A novel parallel-beam optical CT scanner, utilizing fiber optic taper for collimated images, was developed for fast, high resolution, and accurate readout of 3D dosimeters. Percent depth dose curves for various FLASH dose rates and conventional dose rate beams were generated and compared based on the optical response versus dose measurements. Percent depth dose curves from Monte Carlo calculation of the presage phantom were also compared. As shown in Table 1, the percent depth dose as a function of depth for six FLASH dose rates (240-40 Gy/s) are nearly identical, indicating that optical response of PRESAGE is dose-rate independent. The optical density of PRESAGE phantom was confirmed to be linear with absorbed dose for all FLASH dose rates, consistent with the observation at regular treatment dose rates. PRESAGE phantoms show dose-rate independence in electron beams for a wide range of dose rates from conventional to ultra-high-dose-rates, indicating these phantoms can be useful for relative 3D dose measurements in FLASH electron beams. Future experiments will be undertaken as part of the commissioning of a commercially available FLASH radiotherapy unit.

Commissioning and Initial Validation of Commercial Treatment Planning System for the Electron FLASH Research Extension

The aim of this study was to investigate the feasibility of commissioning the 16 MeV electron FLASH beam in a commercial treatment planning system (TPS) for pre-clinical research purposes. The delivery system consisted of a new commercial solution for which a linear accelerator was modified into a FLASH Research Extension platform. Additionally, preliminary radiation biology results were highlighted to showcase the future use of this system. To commission a commercial electron Monte Carlo (MC) for dose calculation of a 16 MeV FLASH beam in the TPS, radiochromic film was used to measure the vendor-required beam data, e.g., profiles and percent depth dose (PDD) curves for cone sizes of 6 × 6 cm2, 10 × 10 cm2, and 15 × 15 cm2 as well as an in-air profile for a 40 × 40 cm2 open field (no cone). Once the electron MC beam model was generated, additional measurements were collected for validation and compared against the calculated dose from the TPS. A treatment planning comparison between the newly commissioned FLASH beam and the conventional electron beam was conducted. Specifically, the dose-volume histograms (DVHs) for target volumes and organs at risk were investigated for skin cancer cases previously treated with conventional electron beams. Lastly, the FLASH dose distribution predicted by the electron MC for an in vitro cell study setup was validated with radiochromic film measurements, and initial radiobiology tests were conducted using FLASH and conventional dose-rate electron beams. The electron MC calculated dose for the 16 MeV electron FLASH beam agreed with measured PDDs within 1% for all field sizes. The beam profile characteristics, such as penumbra, shape, and full width at half maximum, demonstrated good agreement with less than 0.5 mm difference between the TPS and measurements. There were noticeable differences in the profiles of large fields between the FLASH and conventional dose-rate beam models due to the more forward-peaked FLASH beam. For treatment planning, Regarding DVH, the FLASH dose-rate plan provided comparable plan quality to the conventional dose-rate plan, achieving adequate coverage for the target volumes and sparing the healthy organs and tissues. The electron MC dose prediction for the FLASH beam was also found to be in good agreement with the film measurements of the in vitro cell study setup. Furthermore, the FLASH beam was observed to be more effective with a 20 % increase in killing pancreatic cancer cells compared to the conventional dose rate. The study successfully incorporated the 16 MeV electron FLASH Research Extension into the commercial TPS using electron Monte Carlo for dose calculation. This will be valuable for pre-clinical cell and animal studies. This research also enables FLASH treatment planning studies, a key component for the future implementation of FLASH into clinical care. Further research is necessary to incorporate the radiation biology effect of FLASH into the treatment planning system.

Can a 10 MV-FFF Photon Beam be Used in Stereotactic Thalamotomy to Reduce Beam Delivery Times: A Dosimetric Evaluation

Stereotactic radiosurgery (SRS) relies on small fields to ablate lesions. Treating functional diseases, such as essential tremor, requires a significantly higher dose (150 Gy) to a small target in a single fraction. Currently, Linac based treatment is delivered via pinhole-shaped fields formed by a 4 mm circular cone using a 6 MV photon beam. Utilizing a higher energy, high dose rate beam such as a 10 MV flattening-filter-free (FFF) beam with dose rates up to 2400 cGy/min could reduce long radiation delivery times by a factor of 4 comparing to the standard 6 MV beam with dose rate up to 600 cGy/min. This study aims to assess the benefit and feasibility of using a 10 MV-FFF beam by evaluating the dosimetric effects between the proposed 10 MV-FFF and commonly used 6 MV beam and comparing the dose to organ-at-risk (OARs) based on a typical real patient tremor treatment plan. The planning tool codes BEAMnrc/DOSXYZnrc were used to generate beams of 6 MV and 10 MV-FFF for a medical linear accelerator. Calculated patient dose distributions were compared between two energy beams based on a realistic treatment plan for thalamotomy. Additionally, dose to OARs were evaluated by using the dose-volume histograms (DVH) for the same target dose coverage. The mean energies of photons within the 4 mm cone projected area were 1.73 MeV and 2.09 MeV and the values of percentage depth-dose curve (%dd) at d = 5 cm and SSD = 95 cm were 75.5% and 78.3%, for the 6 MV and 10 MV-FFF beams, respectively. A patient delivered treatment plan with a prescribed dose of 15,000 cGy utilizing the 6MV-SRS beam was developed which used a 4 mm cone with a 21-arc delivery. Using that plan as the template, doses to OARs were evaluated comparing the 6MV to the 10MV-FFF energies. The maximum doses to the nearest OAR, brainstem, was 733 cGy and 822 cGy for the 6 MV and 10 MV-FFF beams, respectively. The details of maximum and mean doses to OARs are presented in Table 1 between the two energies. Compared to the 6 MV delivery, there is only a 12% increase in the maximum dose to the brain stem when using the 10 MV-FFF delivery based on the patient treatment plan. If the mild dose increase to OAR is acceptable for using 10 MV-FFF, the treatment time with 10 MV-FFF beam can be reduced to less than 1 hour from 3 hours with the 6 MV beam. The shorter treatment time can increase the accuracy of treatment target positioning as well patient comfort. Table 1 Calculated doses to OARs based on the real patient plan to deliver the same 15,000 cGy target dose between using 6 MV and 10 MV-FFF beams.

Initial experience with an electron FLASH research extension (FLEX) for the Clinac system.

Radiotherapy delivered at ultra-high-dose-rates (≥40Gy/s), that is, FLASH, has the potential to effectively widen the therapeutic window and considerably improve the care of cancer patients. The underlying mechanism of the FLASH effect is not well understood, and commercial systems capable of delivering such dose rates are scarce. The purpose of this study was to perform the initial acceptance and commissioning tests of an electron FLASH research product for preclinical studies. A linear accelerator (Clinac 23EX) was modified to include a non-clinical FLASH research extension (the Clinac-FLEX system) by Varian, a Siemens Healthineers company (Palo Alto, CA) capable of delivering a 16MeV electron beam with FLASH and conventional dose rates. The acceptance, commissioning, and dosimetric characterization of the FLEX system was performed using radiochromic film, optically stimulated luminescent dosimeters, and a plane-parallel ionization chamber. A radiation survey was conducted for which the shielding of the pre-existing vault was deemed sufficient. The Clinac-FLEX system is capable of delivering a 16MeV electron FLASH beam of approximately 1Gy/pulse at isocenter and reached a maximum dose rate >3.8Gy/pulse near the upper accessory mount on the linac gantry. The percent depth dose curves of the 16MeV FLASH and conventional modes for the 10×10cm2 applicator agreed within 0.5mm at a range of 50% of the maximum dose. Their respective profiles agreed well in terms of flatness but deviated for field sizes >10×10cm2 . The output stability of the FLASH system exhibited a dose deviation of <1%. Preliminary cell studies showed that the FLASH dose rate (180Gy/s) had much less impact on the cell morphology of 76N breast normal cells compared to the non-FLASH dose rate (18Gy/s), which induced large-size cells. Our studies characterized the non-clinical Clinac-FLEX system as a viable solution to conduct FLASH research that could substantially increase access to ultra-high-dose-rate capabilities for scientists.

Regression fitting megavoltage depth dose curves to determine material relative electron density in radiotherapy.

The relative electron density (RED) parameter is ubiquitous throughout radiotherapy for clinical dosimetry and treatment planning purposes as it provides a more accurate description of the relevant radiological properties over mass density alone. RED is in practice determined indirectly from calibrated CT Hounsfield Units (HU). While CT images provide useful 3D information, the spectral differences between CT and clinical LINAC beams may impact the validity of the CT-ED calibration, especially in the context of novel tissue-mimicking materials where deviations from biologically typical atomic number to atomic weight ratios 〈Z/A〉 occur and/or high-Z materials are present. A theoretical basis for determining material properties directly in a clinical beam spectrum via an electron-density equivalent pathlength (eEPL) method has been previously established. An experimental implementation of this approach is introduced whereby material-specific measured percentage depth dose curves (PDDs) are regressed to a PDD measured in a reference material (water), providing an inference of 〈Z/A〉, which when combined with the physical density provides a determination of RED. This method is validated over a range of tissue-mimicking materials and compared against the standard CT output, as well as compositional information obtained from the manufacturer's specifications. The measured PDD regression method shows consistent results against both manufacturer-provided and CT-derived values between 0.9 and 1.15 RED. Outside of this soft-tissue range a trend was observed whereby the 〈Z/A〉 determined becomes unrealistic indicating the method is no longer reporting RED alone and the assumptions around the eEPL model are constrained. Within the soft-tissue RED range of validity, the regression method provides a practical and robust characterisation for unknown materials in the clinical setting and may be used to improve on the CT derived RED where high Z material components are suspected.

Monte Carlo dosimetry of a novel Yttrium-90 disc source for episcleral brachytherapy.

To calculate the dose distribution using Monte Carlo simulations for a novel high-dose-rate Yttrium-90 (Y-90) disc source recently developed for episcleral brachytherapy and provide a lookup table for treatment planning. Monte Carlo simulations were performed to calculate the in-water dose distribution of the Y-90 disc source using the "GATE", a software based on the "Geant4" Monte Carlo simulation toolkit developed by the international OpenGATE collaboration. The geometry of this novel beta source, its capsule, and the surrounding water medium were accurately modeled in the simulation input files. The standard Y-90 element beta spectrum from ICRU 72 was used, and the physics processes for beta and photon interactions with matters were all included. The dose distribution of this Y-90 disc source was measured in a separate study using Gafchromic EBT-3 films and the results were reported elsewhere. To match the setup of the experiment, a Gafchromic EBT-3 film was also included in the simulation geometry. The simulated dose profiles were exported from the 3D dose distribution results and compared with the measured dose profiles. Transverse dose profiles at different distances from the seed surface were also obtained to study the lateral coverage of the source. The measured percent depth dose (PDD) curves along the central axis perpendicular to the surface of the Y-90 disc were constructed from the experimental and simulated data, and normalized to the reference point at 1mm from the source capsule. Both PDD curves agreed well up to 4mm from the source surface (maximum difference±10%) but deviated from each other beyond 4mm. The deviation might be caused by the increased measurement uncertainty in the low-dose region. The dose rate at the reference point calculated from the Monte Carlo simulation was 1.09cGy/mCi-s and agreed very well with the measured dose rate of 1.05cGy/mCi-s. If the 80% isodose line is selected as the lateral coverage, the lateral dose coverage is maximal (∼4.5mm) at the plane next to the source surface, and gradually decreases with the increasing distance, approaching 3.5mm when the plane is 5mm from the 6-mm diameter source surface. Monte Carlo simulations were successfully performed to confirm the measured PDD curve of the novel Y-90 disc source. This simulation work laid a solid foundation for characterizing the full dosimetry parameters of this source for episcleral brachytherapy applications.