Source-to-surface Distance Research Articles

Comparison of directly and indirectly estimated entrance skin dose (ESD) for diagnostic radiation qualities (RQR, RQA and RQT) using water phantom and shadowfree diagnostic chamber (SFD)

BackgroundThe aim of this study was to find the entrance skin dose (ESD) for diagnostic radiation qualities RQRs, RQAs and RQTs given in IAEA technical report series No. 457 using direct and indirect methods of measurement. Measurements were done for 5 × 5, 10 × 10, 15 × 15, 20 × 20 and 25 × 25 cm2 field sizes and 70, 80, 90 and 100 cm source to surface distance (SSD) using shadow-free diagnostic (SFD) chamber and water phantom having dimension 30 × 30 × 30 cm3. ESD direct measurements were done by placing SFD chamber on the surface of water phantom, while in the case of indirect measurements, air kerma values were obtained.ResultsESD values for different selected radiation qualities RQR2, RQR5, RQR8, RQR10, RQA2, RQA5, RQA8, RQA10, RQT8, RQT9 and RQT10 were found to be in the range of 0.0045–5.11 mGy per examination.ConclusionsResults obtained were found to be comparable with ESD values published in the literature. The obtained results in this research would help in establishing the national diagnostic reference levels (DRLs) which would help in the optimization of diagnostic imaging procedures. It would also help the radiographers to optimize field sizes and SSDs in order to reduce dose to the patients thereby ensuring good radiological practices, and this would reduce the stochastic risk to the patients caused by the ionizing radiations.

Validation of the Elekta Synergy Platform Linac at 6 MV Photon Beam using PRIMO Monte Carlo Software

Aims: The objective was to validate the initial beam parameters of the Davao Doctors Hospital’s 6 MV Elekta Synergy Platform linac, which performs to the specification of the commissioning data per our records using the gamma-index analysis toolkit integrated inside PRIMO software. Materials and Methods: In PRIMO, a sequence of optimization processes is performed, in which the measured and simulated percent depth dose (PDD) and lateral beam profiles at various depths are compared, using the stringent gamma-index passing rate at 1%/1 mm criteria (GPR11). Using four fields of sizes 3 cm × 3 cm, 4 cm × 4 cm, 5 cm × 5 cm, and 10 cm × 10 cm, the dose is calculated on a water phantom measuring 16.2 cm × 16.2 cm × 30.0 cm. In addition, one field of size 20 cm × 20 cm is used on a 50.0 cm × 50.0 cm × 30.0 cm water phantom with a bin size of 0.2 cm × 0.2 cm × 0.2 cm at a source-surface distance of 100.0 cm. Results: For PDD and beam profiles comparison at the largest field size, the 6.5 MeV initial electron beam energy, 0.25 MeV full-width-half-maximum energy, 0.20 cm focal spot size, and 3° beam divergence tuned configuration yield GPR11 values of 94.0% and 97.7% (PRIMO PDD and lateral beam profile at 200 mm scan depth, respectively) with a statistical uncertainty of 2.9%. For lower field sizes, the GPR11 values are consistent at more than 90% for the PDD, whereas GPR11 values of 80.3% and 70.6% for the lateral beam profiles (at 15 mm and 200 mm scan depths) at 10 cm × 10 cm and 5 cm × 5 cm, respectively. The percentage difference between the measured and simulated PDD 20,10 ratios of not more than 2.45% is observed in all field settings. Conclusions: These tuned beam parameters are remarkably in agreement with the suggested beam parameters listed on the PRIMO website for the 6 MV Elekta linac which was optimized with a different set of measurements.

Assessment of peripheral dose as a function of distance and depth from cobalt-60 beam in water phantom using TLD-100

BackgroundInnovations in cancer treatment have contributed to the improved survival rate of cancer patients. The cancer survival rates have been growing and nearly two third of those survivors have been exposed to clinical radiation during their treatment. The study of long-term radiation effects, especially secondary cancer induction, has become increasingly important. An accurate assessment of out-of-field/peripheral dose (PDs) is necessary to estimate the risk of second cancer after radiotherapy and the damage to the organs at risk surrounding the planning target volume. This study was designed to measure the PDs as a function of dose, distances, and depths from Telecobalt-60 (Co-60) beam in water phantom using thermoluminescent dosimeter-100 (TLD-100).MethodsThe PDs were measured for Co-60 beam at specified depths of 0 cm (surface), 5 cm, 10 cm, and 15 cm outside the radiation beam at distances of 5, 10, and 13 cm away from the radiation field edge using TLD-100 (G1 cards) as detectors. These calibrated cards were placed on the acrylic disc in circular tracks. The radiation dose of 2000 mGy of Co-60 beam was applied inside 10 × 10 cm2 field size at constant source to surface distance (SSD) of 80 cm.ResultsThe results showed maximum and minimum PDs at surface and 5 cm depth respectively at all distances from the radiation field edge. Dose distributions out of the field edge with respect to distance were isotropic. The decrease in PDs at 5 cm depth was due to dominant forward scattering of Co-60 gamma rays. The increase in PDs beyond 5 cm depth was due to increase in the irradiated volume, increase in penumbra, increase in source to axis distance (SAD), and increase in field size due to inverse square factor.ConclusionIt is concluded that the PDs depends upon depth and distance from the radiation field edge. All the measurements show PDs in the homogenous medium (water); therefore, it estimates absorbed dose to the organ at risk (OAR) adjacent to cancer tissues/planning target volume (PTV). It is suggested that PDs can be minimized by using the SAD technique, as this technique controls sources of scattered radiation like inverse square factor and effect of penumbra up-to some extent.

A Comparison Study of the Solid Phantom and Water Phantom Using 6 MV and 15 MV Photon Energies, Depending on the Depth

Water is such an environmental element that is considered the best human body tissue equivalent. In the field of dosimetry studies, water is frequently used. This comparison study is conducted by a solid phantom and a water phantom with 6 MV and 15 MV photon energies, respectively. A cylindrical-type ionization chamber is used to collect charge when beams are on. The distance between the ray source and the surface of the phantom was fixed at 100 cm i.e. to SSD (Source to Surface Distance) of during the experiment. Chamber travels 1 cm to 20 cm in both phantoms and an electrometer is attached in the experimental set-up to measure the charge. The field size was 10x10 cm2. The relative deviation ratio of the solid phantom to the water phantom was calculated. In the result, the maximum deviation was 0.64%, while the minimum deviation was 0%, corresponding to the depths of 1 cm and 2.5 cm, respectively, for 6 MV and at 15 MV, maximum deviation and minimum deviation were 1.90% and 0.167% respectively, corresponding to the depths of 1.5 cm and 13 cm. Therefore, it can be said that the solid phantom can overcome the disadvantages of installation time required for the water phantom and problems while water level changing for depth measurement, simultaneously can used to measure the radiological dose precisely. Bangladesh J. Nuclear Med. 27(1): 34-38, 2024

A scattering-foil free electron beam to increase dose rate for total skin electron therapy (TSET).

Electron beams are used at extended distances ranging between 300 to 700cm to uniformly cover the entirety of the patient's skin for total skin electron therapy (TSET). Even with electron beams utilizing the high dose rate total skin electron (HDTSe) mode from the Varian 23iX or TrueBeam accelerators, the dose rate is only 2500 cGy/min at source-to-surface distance (SSD)=100cm. At extended distances, the decrease in dose rate leads to long beam delivery times that can limit or even prevent the use of the treatment for patients who, in their weakened condition, may be unable to stand on their own for extended periods of time. Previously, to increase dose rate, a customized 6 MeV electron beam was created by removing the x-ray target, flattening filter, beam monitor chamber, and so forth. from the beam path (Chen, et at IJROBP 59, 2004) for TSET. Using this scattering-foil free (SFF) electron beam requires the treatment distance be extended to 700cm to achieve dose uniformity from the single beam. This room size requirement has limited the widespread use of the 6 MeV-SFF beam. This study explores an application of a dual-field technique with a 6 MeV-SFF beam to provide broad and uniform electron fields to reduce the treatment distances in order to overcome treatment room size limitations. The EGSnrc system was used to generate incident beams. Gantry angles between 6 MeV-SFF dual-fields were optimized to achieve the similar patient skin dose distribution resulting from a standard 6 MeV-HDTSe dual-field configuration. The patient skin dose comparisons were performed based on the patient treatment setup geometries using dose-volume-histograms. Similar dose coverage can be achieved between 6 MeV-SFF and 6 MeV-HDTSe beams by reducing gantry angles between dual-field geometries by 8° and 7° at treatment distances of 400 and 500cm, respectively. To achieve 95% mean dose to the first 5mm of skin depth in the torso area, the mean dose to depths of 5-10mm and 10-15mm below the skin surface was 74% (74%) and 49% (50%) of the prescribed dose when using 6 MeV-SFF (6 MeV-HDTSe) beam, respectively. The 6 MeV-SFF electron beam is feasible to provide similar TSET skin dose coverage at SSD ≥ 400cm using a dual-field technique. The dose rate of the 6 MeV-SFF beam is about 4 times that of current available 6 MeV-HDTSe beams at treatment distances of 400-500cm, which significantly shortens the treatment beam-on time and makes TSET available to patients in weakened conditions.

Dosimetric characterization for GRID collimator-based spatially fractionated radiation therapy: Dosimetric parameter acquisition and machine interchangeability investigation.

The purpose of this study is to characterize the dosimetric properties of a commercial brass GRID collimator for high energy photon beams including 15 and 10 MV. Then, the difference in dosimetric parameters of GRID beams among different energies and linacs was evaluated. A water tank scanning system was used to acquire the dosimetric parameters, including the percentage depth dose (PDD), beam profiles, peak to valley dose ratios (PVDRs), and output factors (OFs). The profiles at various depths were measured at 100cm source to surface distance (SSD), and field sizes of 10×10 cm2 and 20×20 cm2 on three linacs. The PVDRs and OFs were measured and compared with the treatment planning system (TPS) calculations. Compared with the open beam data, there were noticeable changes in PDDs of GRID fields across all the energies. The GRID fields demonstrated a maximal of 3mm shift in dmax (Truebeam STX, 15MV, 10×10 cm2). The PVDR decreased as beam energy increases. The difference in PVDRs between Trilogy and Truebeam STx using 6MV and 15MV was 1.5%±4.0% and 2.1%±4.3%, respectively. However, two Truebeam linacs demonstrated less than 2% difference in PVDRs. The OF of the GRID field was dependent on the energy and field size. The measured PDDs, PVDRs, and OFs agreed with the TPS calculations within 3% difference. The TPS calculations agreed with the measurements when using 1mm calculation resolution. The dosimetric characteristics of high-energy GRID fields, especially PVDR, significantly differ from those of low-energy GRID fields. Two Truebeam machines are interchangeable for GRID therapy, while a pronounced difference was observed between Truebeam and Trilogy. A series of empirical equations and reference look-up tables for GRID therapy can be generated to facilitate clinical applications.

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

Objective. Beam current transformers (BCT) are promising detectors for real-time beam monitoring in ultra-high dose rate (UHDR) electron radiotherapy. However, previous studies have reported a significant sensitivity of the BCT signal to changes in source-to-surface distance (SSD), field size, and phantom material which have until now been attributed to the fluctuating levels of electrons backscattered within the BCT. The purpose of this study is to evaluate this hypothesis, with the goal of understanding and mitigating the variations in BCT signal due to changes in irradiation conditions. Approach. Monte Carlo simulations and experimental measurements were conducted with a UHDR-capable intra-operative electron linear accelerator to analyze the impact of backscattered electrons on BCT signal. The potential influence of charge accumulation in media as a mechanism affecting BCT signal perturbation was further investigated by examining the effects of phantom conductivity and electrical grounding. Finally, the effectiveness of Faraday shielding to mitigate BCT signal variations is evaluated. Main Results. Monte Carlo simulations indicated that the fraction of electrons backscattered in water and on the collimator plastic at 6 and 9 MeV is lower than 1%, suggesting that backscattered electrons alone cannot account for the observed BCT signal variations. However, our experimental measurements confirmed previous findings of BCT response variation up to 15% for different field diameters. A significant impact of phantom type on BCT response was also observed, with variations in BCT signal as high as 14.1% when comparing measurements in water and solid water. The introduction of a Faraday shield to our applicators effectively mitigated the dependencies of BCT signal on SSD, field size, and phantom material. Significance. Our results indicate that variations in BCT signal as a function of SSD, field size, and phantom material are likely driven by an electric field originating in dielectric materials exposed to the UHDR electron beam. Strategies such as Faraday shielding were shown to effectively prevent these electric fields from affecting BCT signal, enabling reliable BCT-based electron UHDR beam monitoring.

Implementation of a Stereoscopic Camera System for Clinical Electron Simulation and Treatment Planning

PurposeA 3D stereoscopic camera system developed by .decimal was commissioned and implemented into the clinic to improve the efficiency of clinical electron simulations. Capabilities of the camera allowed simulations to be moved from the treatment vault into any room with a flat surface that could accommodate patient positioning devices, eliminating the need for clinical patient setup timeslots on the treatment machine. This work describes the process used for these simulations and compares the treatment parameters determined by the system to those used in delivery. MethodsThe Decimal3D scanner workflow consisted of: scanning the patient surface; contouring the treatment area; determining gantry, couch, collimator, and source-to-surface distance (SSD) parameters for en face entry of the beam with sufficient clearance at the machine; and ordering custom electron cutouts when needed. Transparencies showing the projection of in-house library cutouts at various clinical SSDs were created to assist in choosing an appropriate library cutout. Data from 73 treatment sites were analyzed to evaluate the accuracy of the scanner-determined beam parameters for each treatment delivery. ResultsClinical electron simulations for 73 treatment sites, predominately keloids, were transitioned out of the LINAC vault using the new workflow. For all patients, gantry, collimator, and couch parameters along with SSD and cone size were determined using the Decimal3D scanner with 57% of simulations using library cutouts. Tolerance tables for patient setup were updated to allow differences of 10, 20 and 5 degrees for gantry, collimator and couch, respectively. Approximately 7% of fractions (N=181 total fractions) were setup outside of the tolerance table based on physician-direction during treatment. This reflects physician preference to adjust the LINAC rather than patient position during treatment setup. No scanner-derived plan was untreatable due to cutout shape inaccuracy or clearance issues. ConclusionClinical electron simulations were successfully transitioned out of the LINAC vault using the Decimal3D scanner without loss of setup accuracy as measured through machine parameter determination and electron cutout shape.

Evaluating the performance of automated external contouring tool on dose calculation of treatment planning system

Abstract Introduction The main goal of radiation therapy is to eradicate all cancer cells and minimize the damage to healthy tissues around the tumour. Treatment planning systems are used to predict the outcome of the treatment in terms of dose distribution prior to the treatment. One of the most reliable dose calculation algorithms is Monte Carlo. The aim of this study is to evaluate the performance of automated external contouring tool on dose calculation using Monte Carlo algorithm. Materials and methods The external contour of thorax phantom was created by automated tools of Monaco treatment planning system, and then, the IAEA-TECDOC-1583 quality assurance tests were created. Then, the treatment plans were delivered to the phantom, and the dose was measured by the Farmer ionization chamber at specific points. The external contour was corrected according to the source surface distances (SSD) which are mentioned in TECDOC-1583, and the dose was re-calculated. Finally, a comparison was made between the results. Results Dosimetric tests of TECDOC-1583 showed the errors ranged from −2·8% to +2·5%. In case of editing external contour and omitting fluctuations, the errors were decreased. The comparisons indicated that the most significant variation occurred in test 4 and the least changes were related to the tests 1 and 3. Conclusions The results of the study showed that the fluctuations of the external contour affect the calculated volume of the phantom and thus the dose. In order to obtain correct results, automated external contouring tools should be used with the correct instructions and re-checked before treatment planning.

Measurement of field match lines in electron beam radiotherapy using carbon quantum dot sheeting enhanced Cherenkov imaging

The aim of this study is to test the utility of optical imaging technique that contains a flexible carbon quantum dot (cQD) sheet and optical camera to measure matching of adjacent radiation fields due to introduced distances delivered during electron radiotherapy treatments. Aqueous solution of cQD is prepared with distilled water at the concentration of 0.1 g/L and the cQD sheet is prepared by spin coating and UV curing with the same concentration. All experiments are performed on the Varian VitalBeam system and optical emission is captured at source to surface distance (SSD) of 100 cm using a tripod mounted camera (Andor iXon ultra-888). The results demonstrate that the captured optical intensity is linear with delivered dose, almost independent of energy and dose rate. The profile in x and y axis show agreement with defined field sizes using maximum slope profile estimate measurement of <3.5 mm. The results of random tests also show that the differences between the measured field match distances calculated by match amplitude of optical imaging and the actual settings are all less than 1.3 mm. This work demonstrated that the optical imaging technique which consist of flexible cQD sheet and optical camera has potential to detect and quantify the field match line in surface tissue during electron beam radiotherapy.

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.

Impact of Measurement Conditions and Design Features of IBA Detectors on the Small Radiation Fields Output Factors

The use of modern diagnostic equipment in oncology has made it possible to detect small tumor foci, as&#x0D; well as to detect distant metastases of the disease at an early stage of their formation. In these cases, it&#x0D; becomes possible to refuse surgical interventions, replacing them with stereotactic radiosurgery using&#x0D; radiation therapy devices. In this case, small irradiation fields can be used (from fields less than 1 cm to&#x0D; 4 cm). In such cases, there is a problem of delivering large doses to small foci. It consists in the accuracy of determining the absorbed doses delivered to small targets. The complexity of monitoring the quality of dosimetric support of irradiation with the use of various radiotherapy devices is increasing. Increased requirements are also imposed on the detectors used for measurements.&#x0D; This work has several goals:&#x0D; 1) to study the results of measurements of the output factors using five different detectors of small&#x0D; sizes;&#x0D; 2) to determine the changes in the output factors when using two types of beam collimation (collimator&#x0D; shutters - JAWS and multileaf collimator - MLC) of linear electron accelerators;&#x0D; 3) to study the dependence of the output factors on the source-to-surface distance (SSD), on the depth&#x0D; of measurements and on the type of detectors.&#x0D; The measurements were carried out in 6 MV photon beams with a flattening filter on a TrueBeam linear&#x0D; accelerator. Five IBA detectors were used: two diode detectors (PFD and SFD) and three ionization&#x0D; chambers (CC01, CC04 and CC13). The field sizes varied from 1010 cm to 11 cm. The studies were&#x0D; carried out for the following combinations of parameters: SSD is 100 cm and the measurement depth is&#x0D; 10 cm; SSD is 95 cm and the measurement depth is 5 cm. The fields were formed by JAWS and MLC.&#x0D; The analysis of the obtained results made it possible to compare the data obtained from each detector&#x0D; in similar combinations of measurements. Based on the analysis of the output factors measured by all&#x0D; five detectors, conclusions were drawn about the parameters that affect the change in the output factors.

Investigation of Surface Dose Accuracy of Two Dose Calculation Algorithms Using Thermoluminescent Dosimeters

Accurate estimation of the surface dose in radiotherapy is very important in reducing skin reactions. This study aims to evaluate the accuracy of two different treatment planning algorithms in calculating the surface dose in a specially designed phantom using thermoluminescent dosimetry (TLD). In this study, a special phantom was designed for surface dose measurement. The phantom surface consisted of an adhesive bolus for the adhesion of TLDs. 121 TLDs were placed 1 cm apart on the bolus surface. In TPS, irradiation plans were created at different fields and source-surface distances (SSD). Dose calculations were made with Anisotropic Algorithm algorithms (AAA) and Pencil Beam Convolution (PBC) algorithms for all plans. The mean dose was measured for each point. For each of the 4x4, 6x6, 8x8, 10x10, and 12x12 cm2 domains, the TLDs within the domain were approximately 1 cm inward from the edge. To measure the effect of SSD on surface dose, the isocenter point was located at depths of 0 cm, 2.5 cm and 5.0 cm, respectively. The surface dose at each depth was measured with TLDs. The doses calculated by the AAA and PBC algorithms were compared with the doses measured by TLDs. The AAA algorithm overestimates the surface dose by 4% compared to the TLD measurement for the 4x4 field. The surface dose calculation of the PBC algorithm was found to be high when compared to TLD measurements for all SSDs and fields. There was a significant difference between the PBC algorithm dose calculation and TLD measurements in all fields and SSDs (p&amp;lt;0.001). It was observed that the AAA algorithm performed better in calculating the surface dose than the PBC algorithm. AAA and PBC algorithm users are advised to be more careful about surface dose calculation.

Evaluating the effect of low-density inhomogeneities: A comparative study of lithium fluoride detectors, radiochromic films, and collapsed cone algorithm in 6 MV photon beams

In recent years, there has been significant advancement in the development of physical simulators for dose evaluation. Many dosimetric studies employ solid materials, equivalent to human tissues, to evaluate dose distribution. This study aims to evaluate the effectiveness of inhomogeneity correction carried out by the Monaco/Elekta radiotherapy planning software. To achieve this goal, a physical simulator was created using cork boards to simulate lung tissue and solid water to represent other tissues. This simulator was combined with a dosimetric system that utilized lithium fluoride thermoluminescent detectors - RADOS MTS-N (LiF:Mg,Ti). The thermoluminescent detectors were positioned at various depths using a precisely drilled 2.0 mm thick acrylic plate, and they were placed at predefined positions. The irradiation of the simulator was conducted using an Elekta Synergy® Platform accelerator, employing a 6 MV photon beam with a field size of 15 × 15 cm2 and a source-surface distance (SSD) of 97.5 cm. A radiation dose of 200 cGy was applied for this study. In addition to the dosimetric assessment using thermoluminescent detectors, GAFCHROMIC™ EBT-3 Dosimetry Films were utilized to evaluate the dose at the same measurement points. The dose distribution data measured with the detectors were compared with the values provided by the planning system (TPS) and the inhomogeneity correction was verified. The results support the hypothesis that there is a lack of precision in the analytical simulations performed by the TPS, particularly in cases involving dose distribution at interfaces with varying densities.

Mean dose rate in ultra-high dose rate electron irradiation is a significant predictor for O2 consumption and H2O2 yield

Objective. The objective of this study was to investigate the impact of mean and instantaneous dose rates on the production of reactive oxygen species (ROS) during ultra-high dose rate (UHDR) radiotherapy. The study aimed to determine whether either dose rate type plays a role in driving the FLASH effect, a phenomenon where UHDR radiotherapy reduces damage to normal tissues while maintaining tumor control. Approach. Assays of hydrogen peroxide (H2O2) production and oxygen consumption (ΔpO2) were conducted using UHDR electron irradiation. Aqueous solutions of 4% albumin were utilized as the experimental medium. The study compared the effects of varying mean dose rates and instantaneous dose rates on ROS yields. Instantaneous dose rate was varied by changing the source-to-surface distance (SSD), resulting in instantaneous dose rates ranging from 102 to 106 Gy s−1. Mean dose rate was manipulated by altering the pulse frequency of the linear accelerator (linac) and by changing the SSD, ranging from 0.14 to 1500 Gy s−1. Main results. The study found that both ΔH2O2 and ΔpO2 decreased as the mean dose rate increased. Multivariate analysis indicated that instantaneous dose rates also contributed to this effect. The variation in ΔpO2 was dependent on the initial oxygen concentration in the solution. Based on the analysis of dose rate variation, the study estimated that 7.51 moles of H2O2 were produced for every mole of O2 consumed. Significance. The results highlight the significance of mean dose rate as a predictor of ROS production during UHDR radiotherapy. As the mean dose rate increased, there was a decrease in oxygen consumption and in H2O2 production. These findings have implications for understanding the FLASH effect and its potential optimization. The study sheds light on the role of dose rate parameters and their impact on radiochemical outcomes, contributing to the advancement of UHDR radiotherapy techniques.

Enabling ultra-high dose rate electron beams at a clinical linear accelerator for isocentric treatments

Background and PurposeRadiotherapy delivery with ultra-high dose rates (UHDR) has consistently produced normal tissue sparing while maintaining efficacy for tumour control in preclinical studies, known as the FLASH effect. Modified clinical electron linacs have been used for pre-clinical studies at reduced source-surface distance (SSD) and novel intra-operative devices are becoming available. In this context, we modified a clinical linac to deliver 16 MeV UHDR electron beams with an isocentric setup. Materials and MethodsThe first Varian TrueBeam (SN 1001) was clinically operative between 2009–2022, it was then decommissioned and converted into a research platform. The 18 MeV electron beam was converted into the experimental 16 MeV UHDR. Modifications were performed by Varian and included a software patch, thinner scattering foil and beam tuning. The dose rate, beam characteristics and reproducibility were measured with electron applicators at SSD = 100 cm. ResultsThe dose per pulse at isocenter was up to 1.28 Gy/pulse, corresponding to average and instantaneous dose rates up to 256 Gy/s and 3⋅105 Gy/s, respectively. Beam characteristics were equivalent between 16 MeV UHDR and conventional for field sizes up to 10x10cm2 and an overall beam reproducibility within ± 2.5% was measured. ConclusionsWe report on the first technical conversion of a Varian TrueBeam to produce 16 MeV UHDR electron beams. This research platform will allow isocenter experiments and deliveries with conventional setups up to field sizes of 10x10 cm2 within a hospital environment, reducing the gap between preclinical and clinical electron FLASH investigations.

Surface guided electron FLASH radiotherapy for canine cancer patients.

During recent years FLASH radiotherapy (FLASH-RT) has shown promising results in radiation oncology, with the potential to spare normal tissue while maintaining the antitumor effects. The high speed of the FLASH-RT delivery increases the need for fast and precise motion monitoring to avoid underdosing the target. Surface guided radiotherapy (SGRT) uses surface imaging (SI) to render a 3D surface of the patient. SI provides real-time motion monitoring and has a large scanning field of view, covering off-isocentric positions. However, SI has so far only been used for human patients with conventional setup and treatment. The aim of this study was to investigate the performance of SI as a motion management tool during electron FLASH-RT of canine cancer patients. To evaluate the SI system's ability to render surfaces of fur, three fur-like blankets in white, grey, and black were used to imitate the surface of canine patients and the camera settings were optimized for each blanket. Phantom measurements using the fur blankets were carried out, simulating respiratory motion and sudden shift. Respiratory motion was simulated using the QUASAR Respiratory Motion Phantom with the fur blankets placed on the phantom platform, which moved 10mm vertically with a simulated respiratory period of 4 s. Sudden motion was simulated with an in-house developed phantom, consisting of a platform which was moved vertically in a stepwise motion at a chosen frequency. For sudden measurements, 1, 2, 3, 4, 5, 6, 7, and 10Hz were measured. All measurements were both carried out at the conventional source-to-surface distance (SSD) of 100cm, and in the locally used FLASH-RT setup at SSD=70cm. The capability of the SI system to reproduce the simulated motion and the sampling time were evaluated. As an initial step towards clinical implementation, the feasibility of SI for surface guided FLASH-RT was evaluated for 11 canine cancer patients. The SI camera was capable of rendering surfaces for all blankets. The deviation between simulated and measured mean peak-to-peak breathing amplitude was within 0.6mm for all blankets. The sampling time was generally higher for the black fur than for the white and grey fur, for the measurement of both respiratory and sudden motion. The SI system could measure sudden motion within 62.5ms and detect motion with a frequency of 10Hz. The feasibility study of the canine patients showed that the SI system could be an important tool to ensure patient safety. By using this system we could ensure and document that 10 out of 11 canine patients had a total vector offset from the reference setup position<2mm immediately before and after irradiation. We have shown that SI can be used for surface guided FLASH-RT of canine patients. The SI system is currently not fast enough to interrupt a FLASH-RT beam while irradiating but with the short sampling time sudden motion can be detected. The beam can therefore be held just prior to irradiation, preventing treatment errors such as underdosing the target.

Quantification of the role of lead foil in flattening filter free beam reference dosimetry

To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG-51, we examined differences in measurements of the beam quality conversion factor kQ when using or not using lead foil. Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG-51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose-to-water calibrations. In determining kQ , the percentage depth-dose at 10cm [PDD(10)] was measured with 10×10 cm2 field size at 100cm source-to-surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the kQ factors determined by using the empirical fit equation in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate kQ for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in kQ factors were compared for with lead foil vs. without lead foil. Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9±0.2% for the 6 MV FFF beam and 0.6±0.1% for the 10 MV FFF beam. Differences in kQ values with lead foil and with omission of lead foil were -0.1±0.02% for the 6 MV FFF and -0.1±0.01% for the 10 MV FFF beams. With evaluation of the lead foil role in determination of the kQ factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

Auto-commissioning of a Monte Carlo electron beam model with application to photon MLC shaped electron fields

Objective. Presently electron beam treatments are delivered using dedicated applicators. An alternative is the usage of the already installed photon multileaf collimator (pMLC) enabling efficient electron treatments. Currently, the commissioning of beam models is a manual and time-consuming process. In this work an auto-commissioning procedure for the Monte Carlo (MC) beam model part representing the beam above the pMLC is developed for TrueBeam systems with electron energies from 6 to 22 MeV. Approach. The analytical part of the electron beam model includes a main source representing the primary beam and a jaw source representing the head scatter contribution each consisting of an electron and a photon component, while MC radiation transport is performed for the pMLC. The auto-commissioning of this analytical part relies on information pre-determined from MC simulations, in-air dose profiles and absolute dose measurements in water for different field sizes and source to surface distances (SSDs). For validation calculated and measured dose distributions in water were compared for different field sizes, SSDs and beam energies for eight TrueBeam systems. Furthermore, a sternum case in an anthropomorphic phantom was considered and calculated and measured dose distributions were compared at different SSDs. Main results. Instead of the manual commissioning taking up to several days of calculation time and several hours of user time, the auto-commissioning is carried out in a few minutes. Measured and calculated dose distributions agree generally within 3% of maximum dose or 2 mm. The gamma passing rates for the sternum case ranged from 96% to 99% (3% (global)/2 mm criteria, 10% threshold). Significance. The auto-commissioning procedure was successfully implemented and applied to eight TrueBeam systems. The newly developed user-friendly auto-commissioning procedure allows an efficient commissioning of an MC electron beam model and eases the usage of advanced electron radiotherapy utilizing the pMLC for beam shaping.

Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications.

In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms. A 1.65m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse. The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80Gy/s at an SSD of 50cm and 45Gy/s at an SSD of 67.9cm, both at a 2.1cm depth of the water phantom. The FFF radiation fields at 50cm and 67.9cm SSD at a 2.1cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20cm full-width at half-maximum (FWHM) values, respectively. This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.