MV Flattening Filter Free Research Articles

Low-dose megavoltage cone-beam computed tomography using a novel multi-layer imager (MLI).

The feasibility of low-dose megavoltage cone-beam acquisition (MVCBCT) using a novel, high detective quantum efficiency (DQE) multi-layer imager (MLI) was investigated. The aim of this work was to reconstruct MVCBCT images using the MLI at different total dose levels, and assess Hounsfield Unit (HU) accuracy, noise and contrast-to-noise ratio (CNR) for low-dose megavoltage cone-beam acquisition. The MLI has four stacked layers; each layer contains a combination of copper filter/converter, gadolinium oxysulfide (GOS) scintillator and a-Si detector array. In total, 720 projections of a CATPHAN® phantom were acquired over 360° at 2.5, 6, and 6MV flattening filter free (FFF) beam energies on a Varian TrueBeam LINAC. The dose per projection was 0.01, 0.0167, and 0.05MU for 2.5, 6, and 6MV FFF, respectively. MVCBCT images were reconstructed with varying numbers of projections to provide a range of doses for evaluation. Hounsfield Unit uniformity, accuracy, noise and CNR were estimated. Improvements were quantified relative to the standard AS1200 single-layer imager. Average HU uniformity for the MLI reconstructions was within a range of 95%-99% for all of the energies studied. Relative electron density estimation from HU values was within 0.4%±1.8% from nominal values. The CNR for MVCBCT based on MLI projections was 2-4× greater than from AS1200 projections. The 2.5MV beam acquisition with the MLI exhibited the lowest noise and the best balance between CNR and dose for low-dose reconstructions. Megavoltage cone-beam acquisition imaging with a novel MLI prototype mounted on a clinical linear accelerator demonstrated substantial improvement over the standard AS1200 EPID. Further optimization of MVCBCT reconstruction, particularly for 2.5MV acquisitions, will improve image metrics. Overall, the MLI improves CNR at substantially lower doses than currently required by conventional detectors. This new high DQE detector could provide high-quality MVCBCT at clinically acceptable doses.

PERIPHERAL SURFACE DOSE FROM A LINEAR ACCELERATOR: RADIOCHROMIC FILM EXPERIMENTAL MEASUREMENTS OF FLATTENING FILTER FREE VERSUS FLATTENED BEAMS

There is a growing interest in the use of flattening filter free (FFF) beams due to the shorter treatment times. The reduction of head scatter suggests a better radiation protection to radiotherapy patients, considering the expected decrease in peripheral surface dose (PSD). In this work, PSD of flattened (FF) and FFF-photon beams was compared. A radiochromic film calibration method to reduce energy dependence was used. PSD was measured at distances from 2 to 50cm to the field border for different square field sizes, modifying relevant clinical parameters. Also, clinical breast and prostate stereotactic body radiotherapy (SBRT) plans were studied. For square beams, FFF PSD is lower compared with FF PSD (differences ranging from 3 to 64%) and 10 MV FFF yields to the lowest value, for distances greater than 5 cm. For SBRT plans, near and far away from the field border, there is a reduction of PSD for FFF-beams, but the behavior at intermediate distances should be checked depending on the case.

Flattening filter free beam energy selection and its impact in multitarget intracranial stereotactic radiosurgery treatments

Extensive work has been done on the characteristics, dosimetry, and efficacy of flattening filter free (FFF) beams in radiosurgery. However, no study has addressed the dosimetric impact of FFF beam energy selection on treatment plan quality. This study aims to present a systematic dosimetric comparison of plan quality between 10 FFF vs 6 FFF beams in intracranial stereotactic radiosurgery (SRS) treatments using volumetric modulated arc therapy (VMAT). The dosimetric evaluation is based on radiation therapy oncology group (RTOG) dose conformity (CIRTOG) and gradient (GIRTOG) indices, and irradiated normal brain tissue volume. Thirty-five VMAT-based intracranial SRS treatments to multiple brain metastases using a 2.5 mm multileaf collimator (MLC) and 10 MV FFF beam were replanned with a 6 MV FFF and same MLC. The replans incorporated the same arc arrangement, planning target volume (PTV) and organs at risk structures, PTV coverage and prescription isodose normalization. The 6 MV FFF had a sharper dose fall-off compared to the 10 MV FFF (GIRTOG-10 FFF = 4.70, GIRTOG-6 FFF = 4.56; p < 0.05) and comparable conformity index (CIRTOG-10 FFF = 1.11, CIRTOG-6 FFF = 1.10; p = 0.9). On average, the irradiated normal brain tissue volume was 11% lower with 6 MV FFF compared to 10 MV FFF (p < 0.05). However, this difference was diminished for large target volumes and increased number of targets treated. The main dosimetric improvement of a 6 MV FFF over a 10 MV FFF beam is the sharper dose fall-off which directly correlates with less normal brain tissue volume irradiation.

Optical imaging method to quantify spatial dose variation due to the electron return effect in an MR-linac.

Treatment planning systems (TPSs) for MR-linacs must employ Monte Carlo-based simulations of dose deposition to model the effects of the primary magnetic field on dose. However, the accuracy of these simulations, especially for areas of tissue-air interfaces where the electron return effect (ERE) is expected, is difficult to validate due to physical constraints and magnetic field compatibility of available detectors. This study employs a novel dosimetric method based on remotely captured, real-time optical Cherenkov and scintillation imaging to visualize and quantify the ERE. An intensified CMOS camera was used to image two phantoms with designed ERE cavities. Phantom A was a 40cm×10cm×10cm clear acrylic block drilled with five holes of increasing diameters (0.5, 1, 2, 3, 4cm). Phantom B was a clear acrylic block (25cm×20cm×5cm) with three cavities of increasing diameter (3, 2, 1cm) split into two halves in the transverse plane to accommodate radiochromic film. Both phantoms were imaged while being irradiated by 6MV flattening filter free (FFF) beams within a MRIdian Viewray (Viewray, Cleveland, OH) MR-linac (0.34T primary field). Phantom A was imaged while being irradiated by 6MV FFF beams on a conventional linac (TrueBeam, Varian Medical Systems, San Jose, CA) to serve as a control. Images were post processed in Matlab (Mathworks Inc., Natick, MA) and compared to TPS dose volumes. Control imaging of Phantom A without the presence of a magnetic field supports the validity of the optical image data to a depth of 6cm. In the presence of the magnetic field, the optical data shows deviations from the commissioned TPS dose in both intensity and localization. The largest air cavity examined (3cm) indicated the largest dose differences, which were above 20% at some locations. Experiments with Phantom B illustrated similar agreement between optical and film dosimetry comparisons with TPS data in areas not affected by ERE. There are some appreciable differences in dose intensity and spatial dose distribution observed between the novel experimental data set and the dose models produced by the current clinically implemented MR-IGRT TPS.

82 Output ratios measurements in stereotactic radiosurgery using cones with a 6 MV FFF beam: comparison between 8 detectors

Introduction. The purpose of this work is to assess 8 detectors performances for output ratio (OR) determination, in clinical 6 MV flattening filter free (FFF) beam of the Truebeam STx Novalis stereotactic radiosurgery linear accelerator for cone irradiation using EBT3 radiochromic films (Ashland) as reference 1 Morales JE. et al. A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements. Australas. Phys Eng Sci Med 2014;37,303–9. Google Scholar , 2 Garnier N, Amblard R, Villeneuve R. et al. Detectors assessment for stereotactic radiosurgery with Cones. J Appl Clin Med Phys 2018;19(6), 88–98. Google Scholar , 3 Girardi A. et al. Small field correction factors determination for the IBA razor nano chamber and the IBA razor chamber. Phys Med 2018;52,151–2. Google Scholar .

Feasibility of a GATE Monte Carlo platform in a clinical pretreatment QA system for VMAT treatment plans using TrueBeam with an HD120 multileaf collimator.

PurposeTo evaluate the quality of patient‐specific complicated treatment plans, including commercialized treatment planning systems (TPS) and commissioned beam data, we developed a process of quality assurance (QA) using a Monte Carlo (MC) platform. Specifically, we constructed an interface system that automatically converts treatment plan and dose matrix data in digital imaging and communications in medicine to an MC dose‐calculation engine. The clinical feasibility of the system was evaluated.Materials and MethodsA dose‐calculation engine based on GATE v8.1 was embedded in our QA system and in a parallel computing system to significantly reduce the computation time. The QA system automatically converts parameters in volumetric‐modulated arc therapy (VMAT) plans to files for dose calculation using GATE. The system then calculates dose maps. Energies of 6 MV, 10 MV, 6 MV flattening filter free (FFF), and 10 MV FFF from a TrueBeam with HD120 were modeled and commissioned. To evaluate the beam models, percentage depth dose (PDD) values, MC calculation profiles, and measured beam data were compared at various depths (Dmax, 5 cm, 10 cm, and 20 cm), field sizes, and energies. To evaluate the feasibility of the QA system for clinical use, doses measured for clinical VMAT plans using films were compared to dose maps calculated using our MC‐based QA system.ResultsA LINAC QA system was analyzed by PDD and profile according to the secondary collimator and multileaf collimator (MLC). Values for MC calculations and TPS beam data obtained using CC13 ion chamber (IBA Dosimetry, Germany) were consistent within 1.0%. Clinical validation using a gamma index was performed for VMAT treatment plans using a solid water phantom and arbitrary patient data. The gamma evaluation results (with criteria of 3%/3 mm) were 98.1%, 99.1%, 99.2%, and 97.1% for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF, respectively.ConclusionsWe constructed an MC‐based QA system for evaluating patient treatment plans and evaluated its feasibility in clinical practice. We observed robust agreement between dose calculations from our QA system and measurements for VMAT plans. Our QA system could be useful in other clinical settings, such as small‐field SRS procedures or analyses of secondary cancer risk, for which dose calculations using TPS are difficult to verify.

A relative comparison of neutron production between conventional and energy-matched flattening-filter-free (FFF) 10MV modes for an elekta linear accelerator

Flattening Filter Free (FFF) Intensity Modulated Radiotherapy (IMRT) has become a widespread modality in radiotherapy treatments. Linac vendors have taken different approaches to implementing this technique. Elekta match the depth dose characteristics of FFF beams to their Conventional Flattening Filter (cFF) beams, increasing the maximum photon energy . At higher energies, greater than 10MV, there is the potential for increased unwanted neutron production . This study investigated the neutron production from an Elekta linac for 10MV FFF and cFF beams for various machine geometries.

Study of detectors response on the scatter radiation of 6 MV flattening filter free beams

Small field dosimetry which related to the use of Flattening Filter Free (FFF) beams is a great challenge because of the perturbation effects caused by the size of the detector’s active volume or detector’s materials. This phenomenon encourages researchers to do a further study about detector response. In this study, scatter and primary radiation from 6 MV FFF photon beams were studied to evaluate the detector response in small fields for a better understanding of the contribution of every phenomenon. Ion chamber and Gafchromic films were used in three measurement configurations representing open field measurement and approximations of both primary and scatter part of the beam, described as follows: (i) detectors positioned under steel block, exposing the detectors only to scatter part of radiation field, (ii) detectors positioned inside mini phantom, approximating the detector response to primary radiation with minimum scatter, (iii) detectors positioned in the standard open field, which was the superposition of the primary and scatter radiation. The results show that detector responses were heavily depended on its design (i.e. active volume) especially in small beams. The detector response in the open field could be reproduced from the blocked and primary beam set-up with a discrepancy ranging from 1.0% to 36%. Moreover, the volume averaging and other phenomenon affecting the detector response could be observed in the blocked beam.

Evaluating out-of-field doses during radiotherapy of paediatric brain tumours using lead shielding and flattening-filter free beams

Paediatric radiotherapy comes at the expense of increased risk of late effects due to out-of-field dose caused not only by the treatment itself but also by image guidance. This study examined how the out-of-field dose to selected radiosensitive organs was affected by applying a 1 mm lead shielding during delivery of volumetric-modulated arc therapy (VMAT) for paediatric brain cancer. The study also investigated how the out-of-field dose to the same organs was affected by the use of flattening-filter free (FFF) beams.Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms equivalent to a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams and delivered using a Varian TrueBeam linear accelerator, with and without lead shielding applied to the phantoms.The measured out-of-field doses were as large as 200,9 cGy for the whole treatment, with associated secondary cancer risk being as large as 1,1%. Shielding of the phantoms was found to decrease the out-of-field dose by up to 24%. The use of 6 MV FFF beams yielded a decrease in the dose to the testes by 21–42% compared to 6 MV, while in one case increasing the dose to the thyroid by 18%. The observation that only doses to organs distant to the primary irradiated volume were significantly decreased for FFF can be explained by an increase in internal scatter caused by the softer energy spectrum of the Varian FFF beam.

16 Dosimetry improvement of an EPID-based pre-treatment verification software by the implementation of a dual photon beam model from linear accelerators

Introduction Point-kernel convolution-based dose calculation algorithms require an accurate modelling of the incident energy. The contribution of diffused photons by the primary collimator and the flattening filter (extra-focal source) is added to the primary source. Deviations up to 5% can be observed (Alaei, 2010) compared to the measurement for certain settings of collimation elements without an explicit modelling of the extra-focal source according to the position of the various collimation elements. EPID-based pre-treatment verification solutions generally uses fluence-based convolution models with the impulsional response of the detector. To be perfectly reliable, these methods must accurately predict the portal image to be compared with the acquired image. The aim of this work concerns the implementation and validation of an extra-focal source model for the computation of the predicted EPID image for standard (FF) and FFF (Flattening Filter Free) beams. Methods The linear accelerator used in this study is a TrueBeam STx® equipped with MLC HD120 and EPID as1200 with 6MV FF and FFF photons. Square fields with swapped openings of jaws and MLC leafs of 5 to 22 cm are designed with Eclipse® (Varian). Reference dose measurements in water are carried out on the central axis, with a microDiamond detector at SSD = 95 cm and 5 cm deep. The predicted and acquired portal images are processed with the EPIbeam software (Dosisoft). The prediction model calculates the expected dose map at SSD = 95 cm and 5 cm depth in water from the DICOM RT plan file, which is then compared to the acquired portal image converted to dose. The accuracy of the TPS calculations, dose conversion and prediction models in EPIbeam, without and with implementation of an extra-focal source model including only the effects of the flattening filter, are confronted with the reference measures in water. Results The maximum discrepancy between the TPS calculations and the measurements is less than 1%. The dose values on the converted EPID image accurately match to microDiamond measurements (less than 1% of error). With the primary source model alone, the EPIbeam prediction model for FF photons shows dose differences up to 4% with the microDiamond detector with small MLC openings and retracted jaw positions. The observed deviations after the implementation of the extra-focal source model remain less than 1% for all collimations. For the FFF photons, deviations are in all cases less than 1%. Conclusions For FFF photons, the implementation of an extra-focal source model is optional since the contribution of the primary collimator appears negligible. However, this study confirms the importance of modelling the extra-focal source for FF photons to enhance the accuracy of the dosimetry calculations for pre-treatment checking.

26 Dosimetric comparison of dynamic conformal arc therapy with flattened beams and flattening filter free beams for SBRT lung cancer

Introduction The aim of this study was to compare dosimetric gain using dynamic conformal arc (DCA) with flattened filter (FF) and flattening filter free (FFF) beam for lung stereotactic body radiation therapy (SBRT). Methods and materials In our institution, SBRT lung cancer patients were treated with prescribed dose 60 Gy in 4 or 8 fractions. In Pinnacle 9.10 were planned two partial DCA’s for peripheral tumors and three to four partial DCA’s for central region tumors to help pull dose of the OARs (heart, esophagus, spinal cord and ribs). Treatments were delivered with 6 MV beams and dose rate 600 MU, for TrueBeam STx accelerator. Fifty patients who underwent lung SBRT treatment with DCA with FF were retrospectively analyzed using 6 MV FFF beams with dose rate of 1400 MU. We were observing the planning target volume (PTV) coverage, sparing of organ at risks (OARs) and time of treatment delivery. Results The PTV coverage (D98% > 95%) were similar, 98.3% vs 98% for FF and FFF, with 1887 MU’s vs 1860 MU’s, respectively. Dosimetric gain between FF and FFF for max dose to the heart was 1 Gy vs 0.87 Gy (p 0.0001). Max dose for spinal cord was 8.84 Gy vs 8.38 Gy (p 0.0001). Max dose for esophagus was 9.76 Gy vs 8.94 Gy (p 0.0001). Max dose for ribs was 34.16 Gy vs 31.89 Gy (p 0.0001). Time of treatment delivery was 4.50 min vs 2.43 min (p = 0.0006), for FF and FFF, respectively. Conclusion Treatment delivery with FFF DCA allows better sparing of OARs for SBRT of lung cancer patients. Shorter treatment time with FFF DCA for SBRT of lung cancer patients can reduce potentially discomfort or patient motion.

131 Performance analysis of new Delta4 Phantom+ using Flattening-Filter and Flattening Filter-Free beams

PurposeThere is a recent increase in the use of Flattening Filter-Free (FFF) beams with the aim of reducing treatment time and consequently intrafractional motions. The goal of this study was to evaluate the performances of the new pre-treatment system Delta4 Phantom+ (Scandidos) in terms of a stable and sensitive tool for achieving a reliable verification. MethodsDelta4+ was evaluated for verification with 6 MV and 6 MV FFF beams using static beam arrangement. The accelerator output was monitored with the ionisation chamber inserted into the Phantom. The device was tested for dose-rate dependence, linearity and stability using its daily output correction. The response of Delta4+ was evaluated for energy, measuring the gamma index of four Volumetric modulated arc therapy (VMAT) plan with different energies (6 MV, 6 FFF MV, 10 MV, 10 FFF MV). ResultsThe response of Delta4+ as a function of accelerator dose rate is in agreement with the ionisation chamber with a difference smaller than 0,1%.The Delta4+ output is constant for different MU, being the difference within 0,4%.The stability was measured during a 6 months period and the standard deviation obtained is 0,7% for 6 MV and 1% for 6 FFF MV.VMAT plan analysis show values higher of 99% for all energies with a threshold of 3%-3 mm, while with a threshold of 2%-2 mm the values are within 93,5%-97,5%. The plans calculated with FFF beams have the lowest value in the range. ConclusionsThis study shows that the Delta4+ is an accurate device, in fact, for all the measurements made, uncertainties below 1% were obtained.

Microstructure changes in radiochromic films due to magnetic field and radiation

To correlate the dose response and changes in microscopic structures of the radiochromic films exposed to the clinical magnetic field in the range 1.5-3 T with standard and flattening filter-free (FFF) photon beams. The radiochromic film was cut into 5 × 5 cm2 sheets/samples from one batch. These samples were exposed to a 1.5-T and/or 3-T B-fields from an MRI scanner using an abdominal sequence for 7 min before and after irradiation with 6 MV and/or 6 MV FFF beams. Films were placed in a reference condition at 5 cm depth in a solid water phantom and exposed up to 20 Gy. The sample orientation was maintained the same during exposure, readout, and scanning electron microscopic (SEM) analysis. The samples were scanned with an Epson Expression 11000XL in a 48-bit RGB color mode at 300 dpi with red channel. Scanned images were processed in Image J and red channel mean intensity values were recorded. The samples were then coated with 6 nm gold and imaged by SEM Teneo (5 kV, 13 pA) under 2000, 2500, and 3000 magnifications for texture analysis. The changes in the microstructure of the films in magnetic fields (1.5- and 3.0-T) were dose dependent. The orientation and granular size of samples at higher doses were altered compared to the controls. Needle-shaped structures of the active layer were longer and aligned for samples exposed to higher doses and magnetic field. However, no significant changes in optical density due to the presence of a magnetic field pre/postirradiation up to 20 Gy were observed. Fine structures of the film represent the polymerization characteristics that are affected by the radiation dose in the magnetic field. Upon exposure to radiation, diacetylene monomers undergo polymerization that forms longer chains with a temporal response. Even though this study did not notice any significant changes in optical density due to the presence of magnetic field, this should be studied in simultaneous application of the magnetic field during treatment in a dedicated MR-linac unit.

A simple method for determining dosimetric leaf gap with cross-field dose width for rounded leaf-end multileaf collimator systems

PurposeThe dosimetric leaf gap (DLG) and multileaf collimator (MLC) transmission are two important systematic parameters used to model the rounded MLC leaf ends effect when commissioning an Eclipse treatment planning system (TPS). Determining the optimal DLG is a time consuming process. This study develops a simple and reliable method for determining the DLG using the cross-field dose width.Methods and materialsA Varian TrueBeam linac with 6 MV, 10 MV, 6 MV flattening filter free (FFF) and 10 MV FFF photon beams and equipped with the 120 Millennium MLC and the Eclipse™ TPS was used in this study. Integral sliding fields and static slit MLC field doses with different gap widths were measured with an ionization chamber and GAFCHROMIC EBT3 films, respectively. Measurements were performed for different beam energies and at depths of 5 and 10 cm. DLGs were derived from a linear extrapolation to zero dose and intercepting at the gap width axis. In the ion chamber measurements method, the average MLC leaf transmission to the gap reading for each gap (RgT) were calculated with nominal and cross-field dose widths, respectively. The cross-field dose widths were determined according to the dose profile measured with EBT3 films. Additionally, the optimal DLG values were determined using plan dose measurements, as the value that produced the closest agreement between the planned and measured doses. DLGs derived from the nominal and cross-field dose width, the film measurements, and the optimal process, were obtained and compared.ResultsThe DLG values are insensitive to the variations in depth (within 0.07 mm). DLGs derived from nominal gap widths showed a significantly lower values (with difference about 0.5 mm) than that from cross-field dose widths and from film measurements and from plan optimal values. The method in deriving DLGs by correcting the nominal gap widths to the cross-field dose widths has shown good agreements to the plan optimal values (with difference within 0.21 mm).ConclusionsThe DLG values derived from the cross-field dose width method were consistent with the values derived from film measurements and from the plan optimal process. A simple and reliable method to determine DLG for rounded leaf-end MLC systems was established. This method provides a referable DLG value required during TPS commissioning.

Comparison of Spine SRS VMAT plans with Flattening Filter Free 6 MV or 10 MV beams

Spine stereotactic radiosurgery (SRS) with VMAT plans are usually complex and use highly modulated beams to spare the spinal cord and to generate a conformal plan. The flattening filter free (FFF) beams are ideal because the high dose rates reduce delivery time. The dedicated SRS/stereotactic body radiation therapy (SBRT) linacs commonly have 6 MV FFF (1400 MU/min) and 10 MV FFF (2400 MU/min) energies available. In this study, we investigated the differences in the quality of VMAT plans made with both FFF modes for the spine SRS. Twenty previously treated spine SRS patients (10 cervical (C) spine and 10 thoracic and lumbar spines(T,L) were planned using three VMAT arcs and treatment planning system’s auto-planning feature. Two plans were made for each patient with 18 Gy in one fraction using 6 MV FFF and 10 MV FFF beams. The same auto-planning template (objectives and settings) was used for both plans to reduce planner bias. 6 MV FFF plan (6FFF) was optimized using the auto-planning objectives and settings that were used for 10 MV FFF plan (10FFF). The template utilized varied across patients and determined iteratively by the planner until the spinal cord (or cauda equina) and plan quality objectives (a conformal plan with minimum of 90% coverage) were achieved. All 6FFF plans met planning constraints using 10FFF template except for 2 cases in which cord constraint was violated. These cases were replanned using a different template until a clinically acceptable plan was achieved. Mean values of maximum cord/cauda dose (Dmax), volume of cord/cauda equina receiving 10 Gy (V10Gy), minimum dose to target (Dmin), Paddick conformity index (CI), gradient index (GI), total monitor unit (MU) and total beam on time are shown in the table below. Values shown in bold were found to be statistically significant.Abstract TU_30_3191; Table 1C SpineT,L Spine6FFF10FFF6FFF10FFFCord Dmax (Gy)11.2 ± 1.011.9 ± 0.811.5 ± 1.111.6 ± 1.2 V10Gy (%)3.3 ± 2.24.9 ± 1.64.2 ± 2.54.4 ± 1.8Tumor Dmin (Gy)11.3 ± 2.411.7 ± 2.410.6 ± 1.310.8 ± 1.3 Conformity Index0.71 ± 0.100.70 ± 0.110.82 ± 0.060.82 ± 0.04 Gradient Index3.6 ± 0.33.7 ± 0.43.3 ± 0.33.2 ± 0.2 MU8283 ± 18707997 ± 160010368 ± 24629554 ± 1565 Time (min)5.9 ± 1.33.3 ± 0.77.4 ± 1.84.0 ± 0.7 Open table in a new tab The differences between 6 FFF and 10FFF plans were not statistically different except for cord/cauda equina Dmax and V10Gy for C-spine cases (p <0.05) and target Dmin for all cases (p<0.05). Total beam on time for 10FFF cases were significantly lower for all cases. Both energies provided similar plan qualities for all spine SRS cases. In general, 10 MV FFF would be better for spine SRS due to faster treatment delivery. 6 MV FFF would be preferable to achieve lower cord dose for C-Spine.

Profile- and output-determined Prp output corrections for a Varian TrueBeam and Elekta Versa HD

For calibration of flattening filter-free (FFF) linac beams, one needs to either use a chamber small enough to avoid volume averaging that occurs with the conventional Farmer-type chamber, or apply a gradient correction (Prp) to the Farmer chamber. Continued use of the Farmer chamber is attractive due to the desirable characteristics of the chamber. We measured Prp using two methods, the recommended profile gradient method, and an output method with a small chamber having negligible volume averaging, for both a Varian TrueBeam and Elekta Versa 6MV and 10MV FFF beams for 90 cm source-to-surface distance (SSD), 100 cm source-to-axis (SAD) calibrations. For profile-determined Prp, profiles were measured along the calibration plane (100 cm SAD, 90 cm SSD, 10 cm depth) using a diode having a sensitive diameter of 0.8mm. The profiles were integrated numerically along both the length and width of both chamber types, with the deviation from unity giving the magnitude of Prp. The Farmer chamber was 23 mm in length, 3.05 mm in radius, and had a volume of 0.6cc. The smaller chamber was 6.5 mm in length, 2.75 mm in radius, and a volume of 0.125cc. For the output-determined Prp, beam outputs were measured for 6MV conventional flattening filter (cFF), 6FFF, 10cFF, and 10FFF beams using the TG51 protocol for six Farmer and six 0.125cc chambers. Use of six chambers was thought to improve statistics. The 0.125cc chamber was not reference grade, and required cross calibration with the Farmer chamber using cFF beams. To validate the lack of a gradient for the cFF beams, profiles of the cFF beams were measured as well, with a corresponding profile-determined Prp. A ratio of the FFF beam output between Farmer and 0.125cc chambers gives the output-determined Prp. Values of profile-determined Prp for the TrueBeam were 1.003 and 1.007 for 6FFF and 10FFF beams, respectively. Values of profile-determined Prp for the Versa were 1.005 and 1.007 for 6FFF and 10FFF beams, respectively. Surprisingly, the Versa 10cFF beam used was not “flat”, having a central axis divot, giving a Prp 0.995. This beam was not included in the cross-calibration of the 0.125cc chambers. Values of the output-determined Prp were within 0.1% of the profile-determined Prp for the TrueBeam, and within 0.4% for the Versa. Repeated Versa measurements gave similar results. One reference for the 0.125cc chamber used here indicates an anomalous dose-per-pulse issue for cFF beams, which may be exacerbated for FFF beams. Values of Prp from other linacs available to us and from the literature indicate good agreement (within 0.1%, one 0.2% outlier) for the TrueBeam and profile-determined Prp for the Versa. Profile- and output-determined Prp values for the TrueBeam are consistent. A Prp correction for the Versa 10cFF may be warranted, but appears linac or energy dependent. The profile-determined Prp appears to be more reproducible.

Radiotherapy of lung cancers: FFF beams improve dose coverage at tumor periphery compromised by electronic disequilibrium

The purpose of this work was to investigate radiotherapy underdosing at the periphery of lung tumors, and differences in dose for treatments delivered with flattening filter-free (FFF) beams and with conventional flattened (FF) beams. The true differences between these delivery approaches, as assessed with Monte Carlo simulations, were compared to the apparent differences seen with clinical treatment planning algorithms AAA and Acuros XB.Dose was calculated in a phantom comprised of a chest wall, lung parenchyma, and a spherical tumor (tested diameters: 1, 3, and 5 cm). Three lung densities were considered: 0.26, 0.2, and 0.1 g cm−3, representing normal lung, lung at full inspiration, and emphysematous lung, respectively. The dose was normalized to 50 Gy to the tumor center and delivered with 7 coplanar, unmodulated 6 MV FFF or FF beams. Monte Carlo calculations used EGSnrc and phase space files for the TrueBeam accelerator provided by Varian Medical Systems. Voxel sizes were 0.5 mm for the 1 cm tumor and 1 mm for the larger tumors. AAA and Acuros XB dose calculations were performed in Eclipse™ with a 2.5 mm dose grid, the resolution normally used clinically.Monte Carlo dose distributions showed that traditional FF beams underdosed the periphery of the tumor by up to ~2 Gy as compared to FFF beams; the latter provided a more uniform dose throughout the tumor. In all cases, the underdosed region was a spherical shell about 5 mm thick around the tumor and extending into the tumor by 2–3 mm. The effect was most pronounced for smaller tumors and lower lung densities. The underdosing observed with conventional FF beams was not captured by the clinical treatment planning systems.We concluded that FFF beams mitigate dose loss at tumor periphery and current clinical practice fails to capture tumor periphery underdosing and possible ways to mitigate it.

P140] Flattening filter-free radiotherapy for paediatric brain cancer: Impact on the dose to selected out-of-field organs

Purpose Removal of the flattening filter has been shown to alter the out-of-field dose during radiotherapy. In this study, the dose to selected out-of-field organs is compared for conventional flattened and flattening filter free (FFF) volumetric-modulated arc therapy (VMAT) of paediatric brain cancer. Methods Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms resembling a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams, for two different PTV sizes (2 cm and 5 cm diameter), simulating post-operative radiotherapy of an ependymoma in the posterior fossa. The phantoms were subject to three VMAT fractions of 1,8 Gy each, delivered using a Varian TrueBeam linear accelerator. Some additional irradiations were performed for the 1-year phantom with the head of the phantom removed, to investigate the contribution from internal scatter to the total out-of-field dose. Results For the testes, measured doses showed a statistically significant decrease (Student’s t-test, significance level p 0,05) of 21–42% for FFF compared with conventional plans, for both phantoms as well as both PTV sizes. Doses to the breast showed a significant decrease of approximately 12% for the 5-year phantom for the small PTV, but other breast doses did not show a significant difference with and without the flattening filter. Doses to the thyroid showed a significant increase of approximately 18% for the 1-year phantom for the small PTV, with other dose differences being insignificant. However, doses to all three organs were significantly reduced (19–37%) when removing the head of the 1-year phantom, minimizing the contribution from internal scatter. Conclusions The results show that FFF VMAT of paediatric brain cancer leads to reduced doses to organs far from the field compared to VMAT with conventional flattened beams, while doses to organs close to the field seem to increase. The latter finding can be explained by an increase in internal scatter, which is possibly caused by the softer energy spectrum of the 6 MV FFF beam for the investigated linear accelerator model compared to conventional 6 MV.

Effect of secondary electron generation on dose enhancement in Lipiodol with and without a flattening filter.

PurposeLipiodol, which was used in transcatheter arterial chemoembolization before liver stereotactic body radiation therapy (SBRT), remains in SBRT. Previous we reported the dose enhancement in Lipiodol using 10 MV (10×) FFF beam. In this study, we compared the dose enhancement in Lipiodol and evaluated the probability of electron generation (PEG) for the dose enhancement using flattening filter (FF) and flattening filter free (FFF) beams.Methods FF and FFF for 6 MV (6×) and 10× beams were delivered by TrueBeam. The dose enhancement factor (DEF), energy spectrum, and PEG was calculated using Monte Carlo (MC) code BEAMnrc and heavy ion transport code system (PHITS).Results DEFs for FF and FFF 6× beams were 7.0% and 17.0% at the center of Lipiodol (depth, 6.5 cm). DEFs for FF and FFF 10× beams were 8.2% and 10.5% at the center of Lipiodol. Spectral analysis revealed that the FFF beams contained more low‐energy (0–0.3 MeV) electrons than the FF beams, and the FF beams contained more high‐energy (>0.3 MeV) electrons than the FFF beams in Lipiodol. The difference between FFF and FF beam DEFs was larger for 6× than for 10×. This occurred because the 10× beams contained more high‐energy electrons. The PEGs for photoelectric absorption and Compton scattering for the FFF beams were higher than those for the FF beams. The PEG for the photoelectric absorption was higher than that for Compton scattering.Conclusions FFF beam contained more low‐energy photons and it contributed to the dose enhancement. Energy spectra and PEGs are useful for analyzing the mechanisms of dose enhancement.

Comparison of Measured and Monte Carlo Calculated Dose Distributions from Indigenously Developed 6 MV Flattening Filter Free Medical Linear Accelerator.

Purpose:Monte Carlo simulation was carried out for a 6 MV flattening filter-free (FFF) indigenously developed linear accelerator (linac) using the BEAMnrc user-code of the EGSnrc code system. The model was benchmarked against the measurements. A Gaussian distributed electron beam of kinetic energy 6.2 MeV with full-width half maximum of 1 mm was used in this study.Methods:The simulation of indigenously developed linac unit has been carried out by using the Monte Carlo-based BEAMnrc user-code of the EGSnrc code system. Using the simulated model, depth and lateral dose profiles were studied using the DOSXYZnrc user-code. The calculated dose data were compared against the measurements using an RFA dosimertic system made by PTW, Germany (water tank MP3-M and 0.125 cm3 ion chamber).Results:The BEAMDP code was used to analyze photon fluence spectra, mean energy distribution, and electron contamination fluence spectra. Percentage depth dose (PDD) and beam profiles (along both X and Y directions) were calculated for the field sizes 5 cm × 5 cm - 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 3%.Conclusions:A Monte Carlo model of indigenous FFF linac (6 MV) has been developed and benchmarked against the measured data.