MV Flattening Filter Free Beams Research Articles

Dosimetric Comparison between 6MV Flattened Filter and Flattening Filter Free Photon Beams in the Treatment of Glioblastoma with IMRT Technique: A Treatment Planning Study

Introduction: The present study evaluated the dosimetric comparison between 6MV flattened filter (FF) and flattening filter-free (FFF) photon beams in intensity-modulated radiation therapy (IMRT) technique for the treatment of glioblastoma (GBM) patients. Material and Methods: The present study was conducted on 10 patients with GBM previously planned and treated with 6MV FF photon beam by IMRT technique. Additional IMRT plans were retrospectively created using 6MV FFF photon beam for each patient plan. The dose prescription, beam parameters, and planning objective were kept same in both plans. The plans were evaluated using cumulative dose-volume histogram (c-DVH). Both types of plans were compared on the basis of homogeneity index (HI), conformity index (CI), beam-on time (BOT), monitor unit (MU), and doses to organs at risk (OARs). Results: Dose received by 95% (D95%) of planning target volume (PTV) coverage was observed significantly higher in 6MV_FF_IMRT plan than 6MV_FFF_IMRT plan (P<0.05). No significant dose differences were noticed for HI, CI, D98%, and D2% between both plans. Significantly lower Dmax for the brainstem, eyes, and eye lens was observed in 6MV_FFF_IMRT plan. For the brain, less than 2% mean dose was observed in 6MV_FFF_IMRT plan than 6MV_FF_IMRT plan (P=0.017). In 6MV_FFF_IMRT plan, mean BOT decreased by 39% in comparison to that in 6MV_FF_IMRT plans. Conclusion: The 6MV FFF beam provides a desirable and clinically acceptable IMRT plan for the treatment of GBM than 6MV FF beam. In addition, 6MV FFF beam provides higher MUs, better OARs sparing, lower scattered dose, and lower beam delivery time.

Dosimetric characteristics of a 2D silicon diode array for stereotactic radiotherapy end-to-end patient-specific QA

The measurement accuracy of stereotactic radiotherapy (SRT) patient-specific quality assurance (QA) is challenging as small-field dosimetry contains lateral charge disequilibrium, partial blocking of the beam source, and limitations based on the detector size. The aim of this study was to investigate the dosimetric characteristics of a two-dimensional high-resolution silicon diode array for SRT end-to-end patient-specific QA.The dosimetric characteristics of the diode array, delivered to an end-to-end QA phantom, were evaluated for 6 MV, with a flattening filter (FF) and flattening filter-free (FFF), and 10 MV FFF photon beams. Dose linearity, dose rate dependence, dose-per-pulse dependence, field size dependence, angular dependence, and beam directivity were investigated for all photon beams. A total of 27 clinical plans were selected to evaluate the SRT end-to-end patient-specific QA for treatment delivery using volumetric modulated arc therapy (VMAT).For all beam qualities, the differences of dose and dose rate linearity of the diode array were below 0.7% (10–1400 MU range) and 0.6% (100–2400 MU/min range), respectively. The diodes had a decrease in sensitivity of about 4% for the 21-fold decrease in dose-per-pulse. Output factors were in good agreement for field size down to 2 × 2 cm2 (<0.6% for both 6 MV FF and FFF beams, <1.6% for the 10 MV FFF beam), while the field size down to 0.6 × 0.6 cm2 had an under-response (<4% for all beam qualities). Angular dependence was within ±2% and ±4%, for coplanar and non-coplanar beams, respectively. However, this was not the case for the beam direction nearly parallel to the diode array plane. Beam directivity response on both sides of the array was in excellent agreement. For the end-to-end patient-specific QA, the average gamma passing rate (GPR) with a 2%/1 mm criterion was 96.8% and the minimum value was 93.0%. There was no significant difference in the GPRs between coplanar and non-coplanar VMAT plans.The diode array, with several corrections, demonstrated good dosimetric characteristics for SRT QA. The GPRs of end-to-end patient-specific QA for non-coplanar and multiple metastases were as high as that for coplanar and single-target, and a gamma criterion of 2%/1 mm would be suitable for SRT QA when using the array.

Impact of Flattening Filter Free Photon Beam on Rapidarc Radiotherapy for Gynaecological Malignancies: A Comparative Study

Introduction: To compare the dosimetric outcomes of 6 and 10 MV flattening filter free beam (FFFB) energies in gynaecological malignancies RapidArc (RA) planning. Material and Methods: The RA plans were generated for a cohort of 20 patients using 6 and 10 MV FFFBs. The plans aimed to deliver a dose of 50.4Gy in 28 fractions to planning target volume (PTV); moreover, planning objectives were kept as low as reasonably achievable for organs at risk (OARs). Dosimetric analysis was performed in terms of PTV coverage, conformity index (CI), homogeneity index (HI), dose to OAR’s, integral dose to normal tissue (NTID), and total number of monitor units (MU’s). Results: According to the results, volumes of PTV receiving prescription dose and CI values were 95.03±0.10% and 95.02±0.18%, as well as 1.018±0.028 and 1.024±0.027, respectively. Moreover, HI values were estimated at 1.063±0.008 and 1.068±0.010. Additionally, the corresponding values of mean NTID and MUs were 280.3±42.5 and 267.9±39.1 (liter-Gy), as well as 610.3±30.3 and 630.6±39.7 for FFFB using 6 and 10 MV, respectively. The 6 and 10 MV FFFBs were statistically similar in terms of mean dose to bladder, rectum and both femoral heads, while comparison yielded significant difference (p <0.05) in terms of HI, CI, MUs and NTID. Conclusion: The FFFB of 6MV was found superior, compared to 10MV, for RA planning in case of gynaecological malignancies. Moreover, it offers better HI and CI values, as well as fewer numbers of MUs (3.33%). In addition, it delivers more NTID (4.42%) for similar target coverage and OAR’s sparing.

Clinical evaluation of photon optimizer (PO) MLC algorithm for stereotactic, single-dose of VMAT lung SBRT

Recently implemented photon optimizer (PO) MLC optimization algorithm is mandatory for RapidPlan modeling in Eclipse. This report quantifies and compares the dosimetry and treatment delivery parameters of PO vs its predecessor progressive resolution optimizer (PRO) algorithm for a single-dose of volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT). Clinical SBRT treatment plans for 12 early-stage non–small-cell lung cancer patients receiving 30 Gy in 1 fraction using PRO-VMAT were re-optimized using the PO-VMAT MLC algorithm with identical planning parameters and objectives. Average planning target volume derived from the 4D CT scans was 13.6 ± 12.0 cc (range: 4.3 to 41.1 cc) Patients were treated with 6 MV flattening filter free beam using Acuros-based calculations and 2.5 mm calculation grid-size (CGS). Both treatment plans were normalized to receive same target coverage and identical CGS to isolate effects of MLC positioning optimizers. Original PRO and re-optimized PO plans were compared via RTOG–0915 protocol compliance criteria for target conformity, gradient indices, dose to organs at risks and delivery efficiency. Additionally, PO-VMAT plans with a 1.25 mm CGS were evaluated. Both plans met RTOG protocol requirements. Conformity indices showed no statistical difference between PO 2.5 mm CGS and PRO 2.5 mm CGS plans. Gradient index (p = 0.03), maximum dose to 2 cm away from planning target volume in any direction (D2cm) (p < 0.05), and gradient distance (p < 0.05) presented statistically significant differences for both plans with 2.5 mm CGS. Some organs at risks showed statistically significant differences for both plans calculated with 2.5 mm CGS; however, no clinically significant dose differences were observed between the plans. Beam modulation factor was statistically significant for both PO 1.25 mm CGS (p = 0.001) and PO 2.5 mm CGS (p < 0.001) compared to clinical PRO 2.5 mm CGS plans. PO-VMAT plans provided decreased beam-on time by an average of 0.2 ± 0.1 minutes (up to 1.0 minutes) with PO 2.5 mm and 1.2 ± 0.39 minutes (maximum up to 3.22 minutes) with PO 1.25 mm plans compared to PRO 2.5 mm plans. PO-VMAT single-dose of VMAT lung SBRT plans showed slightly increased intermediate-dose spillage but boasted overall similar plan quality with less beam modulation and hence shorter beam-on time. However, PO 1.25 mm CGS had less intermediate-dose spillage and analogous plan quality compared to clinical PRO-VMAT plans with no additional cost of plan optimization. Further investigation into peripheral targets with PO-MLC algorithm is warranted. This study indicates that PO 1.25 mm CGS plans can be used for RapidPlan modeling for a single dose of lung SBRT patients. PO-MLC 1.25 mm algorithm is recommended for future clinical single-dose lung SBRT plan optimization.

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 .

Initial commissioning measurements of respiratory gated liver VMAT stereotactic ablative body radiotherapy

A dosimetric evaluation of a respiratory gated VMAT SABR technique was performed at two different beam energies. Dynamic ion chamber, EBT3 film and Fricke-xylenol orange-gelatin (FXG) gel measurements were acquired using a motion phantom with custom inserts for each dosimeter. Ion chamber and gel dosimeter measurements show good agreement between the measured and calculated plan dose within the planning target volume (PTV). Lower agreement than expected was observed between calculated plan dose and measured film dose, particularly for the 10 MV flattening filter free beam plan, a result that warrants further investigation.

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.

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.

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.

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.

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.

Evaluation of a 2D Diode Array Corresponding to Flattening Filter Free X-ray Beams

Recently, a medical linear accelerator with a flattening filter free (FFF) mode has led to the use of FFF X-ray beams at clinical sites. The usefulness of FFF X-ray beams in high-precision radiation therapy has been reported. Therefore, the quality assurance and quality control for FFF X-ray beams have become necessary. In this study, the characteristics of the detectors of a newly developed 2-D diode array (MapCHECK2, Sun Nuclear Corporation) for FFF X-ray beams, i.e., dose reproducibility, dose rate dependence, dose linearity, and output factor, were evaluated. For the measurements, 6 and 10 MV FFF beams were used. The results showed that the coefficient of variation for dose reproducibility was within 0.08%, the dose rate dependence was less than 1.0%, the coefficient of determination of dose linearity was found to be R2=1.0, which was high, and the output factor agreed within 2.5% as compared with the farmer ion chamber, diode E, and pinpoint ion chamber for field sizes greater than 2×2 cm2. The results suggested that MapCHECK2 could be a useful tool for quality assurance and quality control for FFF X-ray beams.

The effect of beam interruption during FFF-VMAT plans for SBRT.

To investigate the dosimetric effect of intended beam interruption during volumetric modulated arc therapy (VMAT) with flattening filter free (FFF) beam for exploring the possibility of deep inspiration breath hold stereotactic body radiation therapy (SBRT). A total of ten SBRT plans with 6 and 10 MV FFF beams were retrospectively selected. All plans consisted of four partial arcs, except one plan with six partial arcs. We delivered the plans using a Varian Truebeam™ with three different scenarios; without interruption (0int), with one intentional interruption (1int), or with two intentional interruptions (2int), per each partial arc. The treatment log files were exported from the treatment console, and the variations in delivered MU were evaluated at the beam interruption angles. The dose distributions were also measured using a 3D cylindrical diode array detector, ArcCHECK™. The 2D global gamma evaluations were performed, compared to the planned dose distribution, with 3%/3 and 4%/2mm passing criterion. The dose difference (DD) was also determined between uninterrupted and interrupted data with 3, 2, 1, and 0.5% of global maximum dose. The interruption caused a total increase of 0.14 ± 0.05% and 0.25 ± 0.08% of the total planned MU, ranging from 1746 to 3261 MU, at the interrupted angles in 1int and 2int, respectively. All global gamma passing rates satisfied our clinical threshold of 90%, and the differences of passing rates were less than 0.3% on average with both criterions. All measured 1int and 2int data were within 3% DD from 0int measured data. For 6 MV FFF beams, the average passing rate with 2, 1, and 0.5% DD were 99.9 ± 0.2%, 92.3 ± 12.0%, and 81.9 ± 24.9%, respectively, between 0int and 1int, and 99.8 ± 0.4%, 92.1%12.4%, and 80.7 ± 26.5%, respectively, between 0int and 2int. For 10 MV FFF beams, the average passing rate with 2, 1, and 0.5% DD were 100.0 ± 0.2%, 95.4 ± 9.4% and 87.0 ± 19.8%, respectively, between 0int and 1int, and 99.9 ± 0.3%, 95.4 ± 9.7%, and 87.2 ± 21.3% between 0int and 2int. The dosimetric impact of beam interruption was investigated with small field and high dose rate FFF-VMAT SBRT plans. The delivered dose distributions with up to 12 interruptions per plan were still clinically acceptable. Only minimal changes were observed in Gamma, DD, and log file analysis.