Flattening Filter Free Mode Research Articles

25 Dosimetric impacts of FFF large-field beams for lung cancer VMAT treatment

Introduction Volumetric modulated arc therapy (VMAT) using 6 MV (X6) flattening filter free (FFF) beams is generally used for stereotactic body radiation therapy (SBRT) in the treatment of lesions less than 4 cm diameter. FFF beams provide a higher dose rate and a reduced treatment delivery time compared to flattened beams (FF) [1] . This study offers an analysis of the dosimetric impacts caused by large field X6 FFF beams as part of the VMAT treatment for lung cancer. Methods A retrospective cohort of 30 patients with lung cancer who received VMAT X6 FF radiotherapy with wild-field going from 10 × 10 cm to 30 × 30 cm for a prescribed dose ranging from 50 to 66 Gy between 2016 and 2017 was included in this study. Planned target volume (PTV) ranges from 50.0 to 1850.0 cm3 with a margin of 5–7 mm around the clinical target volume (CTV). Determination of the X6 FFF dose distribution was made possible by the anisotropic analytical algorithm (AAA version 13.6) implemented in the Eclipse® treatment planning system (Varian Medical Systems) with a new objective function (Varian PRO/PO). Standardization of the treatment plans was implemented (95% of the dose covers 98% of the PTV) without exceeding 110% of the prescribed dose (1% of the PTV). The number of monitor unit (MU) and the treatment time of each plan were recorded as well as the 2 cm distance dose of PTV and organ-at-risk (OAR). Results A treatment delivery time reduction was found but limited by the linac gantry rotation speed and multi-leaf collimator (MLC) speed. A relative deviation of less than 1% was observed for the mean PTV dose between the X 6 FFF and X 6 FF beams. A 4,0% ± 0,17% reduction of the dose at 2 cm near from the PTV and 15,2% ± 0,25% to OAR was obtained using X 6 FFF . A relative dosimetric discrepancy of 7,1% ± 0,13% between X 6 FFF and X 6 FF beams for the average lung dose was found. However, the MU increased to 10.0% ± 0,21%. for the use of X 6 FFF . Conclusions Large field VMAT treatment with FFF mode provides a highly homogenous dose distribution to target volumes while generating high dose gradients that allows to spare surrounding healthy tissues.

Advantages and disadvantages of removed flattening filter Linac: A Monte Carlo study on full quantitative dosimetric data of 18 MV-Varian Linac

Introduction: During intensity modulated radiation therapy (IMRT) technique, theoretically, presence of flattening filter (FF) across the beamline of clinical linear accelerator (linac) is not essential to obtain uniform lateral profiles due to intensity modulation of photon beams by multileaf collimators (MLCs). The aim of this study was to investigate the dosimetrical properties of 18 MV photon Beam-Varian linac with and without FF. Materials and Methods: All dose measurements were performed on 18 MV, FF Mode-Varian 2100C/D linac. The FF and flattening filter free (FFF) modes of linac were modeled by MCNPX 2.4. code. The photon and contaminant electrons spectra, dose rate, present depth doses (PDD), lateral dose profiles, total and collimator scatter factors and out of field doses were calculated and compared with and without FF. Results: Removing the FF increased the photon and contaminant electron fluences by factors of 5.48 and 3.94 for a 5 × 5 cm2 field size, respectively. The surface dose increased up to 155%. The flatness of dose profile was disturbed and deteriorated with increase of field size. Despite the dependence of the total scattering factor on field size, the variation of collimator scattering factors was neglected. The out-of-field dose decreased about 81.5 % for a 5 × 5 cm2 field size. Conclusion: Removing FF from the linac’s head increases the dose rate and decreases the out-of-field dose, but the increased skin dose and deteriorated flatness of lateral dose profile are the main

Comparison of Flattening Filter and Flattening Filter-Free Volumetric Modulated Arc Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma.

BackgroundThis study aimed to investigate the therapeutic role of flattening filter-free (FFF) mode in volumetric modulated arc therapy (VMAT) compared with flattening filter (FF) mode in patients with locally advanced nasopharyngeal carcinoma (NPC).Material/MethodsTen previously treated patients with NPC underwent treatment re-planning with FFF and FF VMAT. Radiotherapy dose distribution on planning target volume (PTV), organs at risk (OAR), target conformity index (CI), total monitor units (MUs), and therapeutic time were compared.ResultsMaximum and mean radiotherapy dose in PTV and PGTV (primary lesions of NPC and cervical lymph node metastases) in FFF VMAT planning were significantly increased compared with FF VMAT planning, but PTV and OAR showed no significant differences. The CI value of PTV in FFF VMAT planning was significantly reduced compared with FF planning (P<0.05). No differences were found for the maximum radiotherapy dose in the spinal cord and left and right optic nerve, and the mean radiotherapy dose in the brainstem, left and right parotid gland (P>0.05). The maximum dose in the brainstem in the FFF planning was significantly higher compared with FF planning (P>0.05). The maximum radiotherapy dose in left and right crystalline lens (P<0.05) in FFF planning was significantly reduced compared with FF planning. The total hop count in FFF planning was significantly increased compared with FF planning (P<0.05).ConclusionsBoth 6 MV X-ray FFF mode and FF mode in the treatment of patients with NPC showed that FFF VMAT planning provided improved protection for OAR.

Flattening filter free in intensity-modulated radiotherapy (IMRT) - Theoretical modeling with delivery efficiency analysis.

The flattening filter (FF) has traditionally been used to flatten beams or create uniform fields in conformal and intensity-modulated radiation therapy (IMRT) but reduces the dose rate. Many studies have confirmed improvement in delivery efficiency by removing the FF, also known as flattening filter free (FFF), especially for small field stereotactic body radiation therapy (SBRT); but it is unclear if large treatment fields still favor the FFF beam. We propose a novel, unified approach to quantify delivery efficiency of the FFF and flattened beams. We modeled the FF effect by inverse conical filters and systematically studied delivery efficiency (beam-on time, BOT) by varying the filter thickness, including the FF and FFF mode. We formulated the BOT of different beams for any arbitrary fluence map in linear programming to solve the optimal inverse conical filter that minimizes the BOT. One-dimensional optimal filters of minimum BOT were also derived in closed form for conical fluence to gain insight for arbitrary clinical fluence maps. We evaluated the BOT of the FFF beam and flattened beam for conformal treatment fields of various dimensions ranging from 5cm×5cm to 25cm×25cm. We also analyzed the BOT for 698 clinical IMRT prostate fluence maps of field size 10cm×10cm, 17 head-and-neck fluence maps of field size 15cm×15cm, and additional realistic test data from 90° rotation and up to 40cm×40cm enlargement of these clinical fluence maps, which were all initially generated with flattened beams. The FFF beam minimized the BOT for A field size less than 20cm in single leaf pair cases and for conformal fields of dimension less than 20cm×20cm. The FFF beam also minimized the BOT for all tested prostate and head-and-neck cases. The median BOT ratios of the FFF beam to the flattened beam were 0.56 and 0.61 for prostate and head-and-neck cases, respectively. The FFF beam minimized the BOT for field size up to 30cm×30cm and had similar BOTs to those of the flattened beam for field size greater than 30cm×30cm in those clinically realistic test data. The filter modeling and BOT calculation enable us to quantify delivery efficiency of the FFF beam and flattened beam in a unified approach. The FFF beam minimized the BOT both theoretically and in simulations for all clinically relevant field sizes and fluence maps in IMRT. The results for conformal fields imply that the FFF beam requires less BOT than the flattened beam for volumetric modulated arc therapy (VMAT) treatments. The delivery efficiency consideration favors the FFF beam in intensity-modulated treatments and may eventually lead to removal of the FF in all future linear accelerator head designs.

Dosimetric Impact of Automated Non-Coplanar Treatment Planning Using Stereotactic Radiosurgery for Multiple Cranial Metastases: Comparison between Hyperarc and Cyberknife Dose Distributions

The purpose of this study was to clarify the dosimetric impact of automated non-coplanar treatment planning, HyperArc-based stereotactic radiosurgery (SRS) for multiple cranial metastases by comparing with Robotic Radiosurgery System, Cyberknife planning. Eleven cancer patients with multiple cranial metastases (range of the number of tumors: 3 to 5) treated by CyberKnife M6 (Accuray Inc, Sunnyvale, USA) in the period from January 2016 to December 2017 were selected for this study. MultiPlan (Accuray Inc, Sunnyvale, USA) with Ray-tracking dose calculation algorithm was used as a radiation treatment planning system. The GTV was defined as the area of contrast enhancement on MRI and PTV was equal to GTV. The treatment dose of 23 Gy per single fraction was prescribed to the 70%-80% isodose line covering 95% of PTV. The dose to the surrounding healthy brain and organs at risk (e.g., brain stem) were optimized to minimize complications for all plans generated using our institutional protocol; PTV Dmin > 20 Gy, brainstem Dmax < 15 Gy, spinal cord Dmax < 14 Gy, optic nerve Dmax < 10 Gy, and optic chiasm Dmax < 10 Gy. These plans were replanned with an automated non-coplanar treatment planning solution implemented in a treatment planning system, named HyperArc. HyperArc plan were designed using four non-coplanar arc single-isocenter VMAT in 6MV flattening filter free mode for simulated delivery with the True beam STx (Varian) in high intensity with HD-MLC. One isocenter was used for all targets and was placed at the center of mass of all targets. Dose calculation algorithm was Acuros XB. Dosimetric differences were evaluated using target conformity/homogeneity (Paddick CI and HI), dose fall-off (gradient index) and radiation necrosis indicator (total brain V12-GTV). Summary of DVH paramters for two plans are shown in table 1. PTV CI, HI and GI were 0.60±0.11, 1.72±0.22, and 3.94±0.74, respectively, for Cyberknife plan, whereas they were 0.87±0.08, 1.44±0.05, and 5.31±1.42, respectively, for HyperArc plan (p <0.05). Total brain V12-GTV for Cyberknife and HyperArc were 5.26±2.83 cm3 and 4.02±1.71, respectively. These results showed that HyperArc plan had better CI, HI and total lung V12-GTV, although Cyberknife had better GI.Abstract TU_31_3204; Table 1DVH parameterHyperArcCyberknifePTV maximum29.78±0.9233.60±2.61 minimum20.73±0.2719.71±1.34 mean25.88±0.2727.53±1.01 HI1.44±0.051.72±0.22 CI0.87±0.080.60±0.11 GI5.31±1.423.94±0.74Total brain mean1.15±0.290.97±0.35 V12-GTV4.02±1.715.26±2.83 V2163.00±78.84151.20±89.73 Open table in a new tab We compared HyperArc and Cyberknife plans for brain multiple metastases SRS for the first time. Our results demonstrated that HyperArc-based SRS planning had the great potential for being suitable tool for brain multiple metastases SRS.

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.

Quantitative comparison of volumetric-modulated arc therapy plans for liver cancer using flattening filter-free and flattening filter modes based on plan quality metric (PQM)

Objective To quantitatively assess the plan quality of conventional fractionated volumetric modulated arc therapy (VMAT) plans for liver cancer treatments using flattening filter-free (FFF) and flattening filter (FF) modes based on plan quality metric (PQM). Methods A total of ten patients with liver tumor were selected. The patients were planned in FF and FFF mode of 6 MV X-ray respectively. The dose distribution of the planning target volume (PTV) and the organ at risk (OAR) were evaluated, and the monitor units (MUs) and the beam on time (BoT) were compared. According to the clinical requirements, a PQM with 16 submetrics was defined to evaluate the plan quality of the two modes. Results The maximal dose of FFF plan was less than that of FF plan (t=3.828, P<0.05). The normalized volume of 5 Gy (V5) and mean dose (Dmean) of the normal liver of FFF plan were lower than those of the FF plan (t=2.716, 3.007, P<0.05). The average MU of FFF plan [(574±130) MU] was higher than that of FF plan [(518±81) MU](t=-2.782, P<0.05), while the average BoT of FFF plan [(108±36)s] was significantly lower than that of FF plan [(160±29) s](t=6.767, P<0.05). The score of FFF plan was higher than that of FF plan (t=-2.746, P<0.05). Conclusions FFF mode can better protect the OAR at low dose levels. The MU of FFF mode plan were higher, but the beam on time was significantly lower than FF mode plan. With reference to the PQM evaluation criteria, the overall plan quality of the FFF mode was slightly higher. Key words: Plan quality metric; Flattening filter-free; Liver cancer; Volumetric modulated arc therapy; Dosimetry

Second cancer risk after radiation therapy of ependymoma using the flattening filter free irradiation mode of a linear accelerator.

Pediatric patients suffering from ependymoma are usually treated with cranial or craniospinal three‐dimensional (3D) conformal radiotherapy (3DCRT). Intensity‐modulated techniques spare dose to the surrounding tissue, but the risk for second malignancies may be increased due to the increase in low‐dose volume. The aim of this study is to investigate if the flattening filter free (FFF) mode allows reducing the risk for second malignancies compared to the mode with flattening filter (FF) for intensity‐modulated techniques and to 3DCRT. A reduction of the risk would be advantageous for treating pediatric ependymoma. 3DCRT was compared to intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) with and without flattening filter. Dose–volume histograms (DVHs) were compared to evaluate the plan quality and used to calculate the excess absolute risk (EAR) to develop second cancer in the brain. Dose verification was performed with a two‐dimensional (2D) ionization chamber array and the out‐of‐field dose was measured with an ionization chamber to determine the EAR in peripheral organs. Delivery times were measured. Both VMAT and IMRT achieved similar plan quality in terms of dose sparing in the OAR and higher PTV coverage as compared to 3DCRT. Peripheral dose in low‐dose region, which is proportional to the EAR in organs located in this region, for example, gonads, bladder, or bowel, could be significantly reduced using FFF. The lowest peripheral EAR and lowest delivery times were hereby achieved with VMATFFF. The EAR calculated based on DVH in the brain could not be reduced using FFF mode. VMATFFF improved the target coverage and homogeneity and kept the dose in the OAR similar compared to 3DCRT. In addition, delivery times were significantly reduced using VMATFFF. Therefore, for radiotherapy of ependymoma patients, VMATFFF may be considered advantageous for the combination of Elekta Synergy linac and Oncentra External Beam planning system used in this study.

OA150] Influence of extrafocal dose in the out-of-field dose distribution in a paediatric anthropomorphic phantom, irradiated with a FFF beam and a 120 HD MLC – Monte Carlo simulations and gafchromic EBT3 dose distribution

Purpose Since paediatric patients have higher life expectancy than adults and tend to receive higher secondary organ doses due to geometrical factors, secondary induced cancer due to out-of-field dose among paediatric patients is thus an increased concern. This work focusses on the influence of extrafocal dose in the out-of-field dose distribution using a paediatric anthropomorphic phantom. Monte Carlo simulations (Penelope) and Gafchromic EBT3 2D dose distribution measurement will be assessed. Methods A 5-year pediatric phantom (hypothetical brain lesion) was irradiated by Varian TrueBeam 120HD MLC linac, using four 6MV Flattening Filter Free (FFF) static fields. A cranio-caudal beam was employed. The plan was simulated by Monte-Carlo (MC) simulation using PRIMO software. The 2D dose distributions at several distances from the isocenter were measured using Gafchromic EBT3 films. The results were evaluated using 2D Gamma index on PTW-Verisoft software. MC simulations were previously validated by gamma index method (98% of points 1; 2mm, 2%). Results Dose distributions were evaluated at 10, 15 and 17.5 cm away from the isocenter. The maximum local measured doses were 3.5, 1.6 and 1cGy per Gy at the isocenter respectively. A well-defined rectangular shaped dose distribution was noticed in the films located at 15 and 17.5 cm from the isocenter. The same shape was also detected in all the MC simulations of the same planes. The rectangular shape was not observed in the TPS (Acuros nor AAA). The center of this rectangular shape is aligned with the isocenter, and the width is consistent with the X-jaws opening direction. The dose values inside this rectangle are approximately 150% of the dose values in the surrounding regions. Conclusions The rectangular shaped dose distribution was linked to the extrafocal dose due to the cranio-caudal beam, which propagates significantly in the out-of-field regions (lungs, heart, etc.), even after applying known methodologies to lower it such as using FFF modes; choosing lower-energy beams; and limiting the Y-field size. The authors acknowledge support from the project NORTE-01-0145-FEDER-000027, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).

P278] The use of flattening filter free irradiation mode in normo-fractionated treatments

Purpose The aim of this study was to identify general advantages and disadvantages of the flattening filter free irradiation mode in normo-fractionated intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). Methods Various studies have been performed at our department to identify specific advantages and disadvantages of the use of the flattening filter free irradiation mode for the normo-fractionated treatment of different tumor sites. Volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) plans were created with and without flattening filter for at least 10 patients for each tumor site. All treatment plans were verified by a 2D-ionization-chamber-array and delivery times were measured from first beam on to last beam off. Peripheral doses were determined with an ionization chamber as a measure of radiation induced second cancer risk. Differences were evaluated in terms of plan quality, irradiation time, and radiation induced second cancer risk. The results of these studies were reviewed to assess general advantages or disadvantages independent on the specific tumor entity or treatment technique. Results Differences in plan quality and irradiation time were dependent on the tumor entity and treatment technique. The peripheral dose as a measure of radiation induced second cancer risk, however, was significantly reduced in the flattening filter free mode for all tumor sites and treatment techniques. The reduction ranged from 17% to 33%. Conclusion The only general advantage of the flattening filter free irradiation mode observed for all tumor sites and treatment techniques was a reduction in dose to peripheral organs corresponding to a reduction in radiation induced second cancer risk in these organs. No general advantage or disadvantage could be observed in terms of plan quality or treatment time.

54. Stereotactic plans quality assurance with Delta 4PT phantom

Introduction As treatment plans of intra and extracranial stereotactic radiotherapy do not benefit in vivo dosimetry, it is a common practice to perform a pretreatment quality control of the dose calculated by the treatment planning system (TPS). In the midst of the different solutions proposed in terms of detectors, we wanted to evaluate the performances of the Delta 4PT (Scandidos) phantom for the measurement of dynamic conformal arctherapy fields. We were then able to define evaluation criteria which proved to be suitable for the whole range of field dimensions in stereotactic radiotherapy (fields of 10 × 10 mm and more). Methods At the Jean PERRIN center, stereotactic radiotherapy treatment plans are mainly calculated with the iPlan TPS from Brainlab (v4.5). A dedicated module, “Phantom Mapping 4.5.4”, allows to calculate the plan on the Delta 4PT phantom CT image. The treatments were then delivered by a Novalis TrueBeam STX linear accelerator (Varian) for 6 MV photon beams in FFF (Flattening Filter Free) mode. The Delta 4PT used for the measurement is a cylindrical PMMA phantom containing 2 planes of orthogonal p-Si detectors. These very small volume detectors are spaced 5 mm apart on a surface of 6 × 6 cm in the center then every 10 mm on the perimeter. The Delta 4PT has been previously calibrated for energy 6 FFF to take into account the energy response. We also took advantage of the “High Resolution” option of the Delta 4PT phantom to double the longitudinal resolution (2.5 mm). Several sets of parameters were tested for the studied treatment plans. For the analysis, we choosed four parameters: the gamma score, the mean gamma, the number of detectors taken into account in the statistics and the median dose. A comparison with the results obtained by VARIAN’s Portal Dose Image Prediction (PDIP) solution was also carried out to indicate the limits of these two pretreatment control methods. Results Statistical and rational analyzes show that an optimal parameter set makes it possible to verify that the Delta 4PT can be used for all types of dynamic conformal arctherapy treatment plans. Initial results show that the absolute dose measured with The Delta 4PT is correct with an average median dose of −0.4% and a standard deviation of 0.75%. The gamma score (2%/2 mm) systematically passes the criteria of the 95% of points having a gamma of less than 1. Moreover, the results obtained for the PDIP are similar. Conclusions A threshold of 30% with a 2%/2 mm criteria on the local gamma seems to be a good compromise for all field sizes. The physical and dosimetric characteristics of the Delta 4PT phantom allow it to be integrated into a quality assurance plan, whether in classic VMAT or stereotactic radiotherapy.

Simultaneous integrated boost therapy of carcinoma of the hypopharynx/larynx with and without flattening filter - a treatment planning and dosimetry study

BackgroundThe aim of this study was to investigate if the flattening filter free (FFF) irradiation mode of a linear accelerator (linac) is advantageous as compared to the flat beam (FF) irradiation mode in intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for carcinoma of the hypopharynx / larynx.MethodsFour treatment plans were created for each of 10 patients for an Elekta Synergy linac with Agility collimating device, a dual arc VMAT and a nine field step and shoot IMRT each with and without flattening filter. Plan quality was compared considering target coverage and dose to the organs at risk. All plans were verified by a 2D–ionization-chamber-array and delivery times were compared. Peripheral point doses were determined as a measure of second cancer risk. The Wilcoxon test was used for statistical analysis with a significance level of 0.05.ResultsPlan quality was similar for all four treatment plans without statistically significant differences of clinical relevance. The clinical goals were met in all plans for the PTV-SIB (V95% > 95%), the spinal cord (D1ccm < 45 Gy) and the brain stem (D1ccm < 48 Gy). For the parotids, the goal of D50% < 30 Gy was met in 70% and 60% of the plans for the left and right parotid respectively, and the V95% of the SIB reached an average of 94%. Delivery times were similar for FF and FFF and significantly decreased by around 70% for VMAT as compared to IMRT. Peripheral doses were significantly reduced by 18% in FFF mode as compared to FF and by 26% for VMAT as compared to IMRT. Lowest peripheral doses were found for VMAT FFF, followed by VMAT FF.ConclusionsThe FFF mode of a linear accelerator is advantageous for the treatment of hypopharynx/larynx carcinoma only with respect to reduction of second cancer induction in peripheral organs for the combination of Elekta Synergy linacs and Oncentra® External Beam v4.5 treatment planning system. This might be of interest in a therapy with curative intent.

Stereotactic ablative radiation therapy for brain metastases with volumetric modulated arc therapy and flattening filter free delivery: feasibility and early clinical results.

For selected patients with brain metastases (BMs), the role of stereotactic radiosurgery (SRS) or fractionated stereotactic radiotherapy (SFRT) is well recognized. The recent introduction of flattening filter free (FFF) delivery during linac-based SRS or SFRT allows shorter beam-on-time, improving patients' comfort and facility workflow. Nevertheless, limited experiences evaluated the impact of FFF linac-based SRS and SFRT in BMs treatment. Aim of the current study was to analyze SRS/SFRT linac-based FFF delivery for BMs in terms of dosimetric and early clinical results. Patients with life expectancy >3months, number of BMs <5, diameter <3cm, and controlled or synchronous primary tumor received SRS/SFRT. The prescribed total dose and fractionation, based on BMs size and proximity to organs at risk, ranged from 15Gy in 1 fraction to 30Gy in 5 fractions. A FFF volumetric modulated arc therapy (VMAT) plan was generated with one or two coplanar partial arcs. Toxicity was assessed according to CTCAE v4.0. From April 2014 to February 2016, 45 patients (89 BMs) were treated with SRS/SFRT linac-based FFF delivery. The mean beam-on-time was 140s for each lesion (range 90-290s) and the average brain Dmean was 1Gy (range 0.1-4.8Gy). At the time of analysis, local control was reported in 93.2% (83/89BMs). With a median follow-up time of 12months (range 1-27months), the median overall survival was 14months and the 6-month overall survival was 77%. Finally, the median intracranial disease control was 11months. Acute and late toxicities were acceptable without severe events (no adverse events ≥G2 were recorded). These preliminary results highlighted the feasibility and safety of linac-based SRS/SFRT with FFF mode for BMs patients. A longer follow-up is necessary to confirm the efficacy of this treatment modality in BM patients.

Optimal Patient Selection for Stereotactic Body Radiation Therapy: A Practical Approach in a High-Volume Community-Based Private Setting

Establishment of an objective and documentable selection process of patients for free-breathing stereotactic body radiation therapy (SBRT) in a high-volume community-based private setting. A vendor-based respiratory gating system and wide-bore multi-slice CT simulation acquire 4-dimensional image data sets throughout full respiratory cycles. SBRT is delivered on modern linear accelerators including SBRT-dedicated systems, with respiratory gating control and multiple volumetric modulated arc therapy (VMAT) techniques. Flattening Filter Free (FFF) mode of RT delivery is available as needed. Use of cone-beam CT image guidance and 6 degrees of freedom patient repositioning are also available to optimize target alignment. Patients were observed closely at time of CT simulation to select those with optimal and stable respiratory patterns with or without the need for supplemental oxygen. Selection of appropriate patients for SBRT is needed to assure optimal CT simulation image capture, prediction for subsequent treatment deliverability, and reproducibility of high dose SBRT. Typical SBRT is up to 10 Gy per fraction for 5 fractions. Through real-time evaluation of respiratory patterns, appropriate patients can be selected for SBRT. Compliance rate of selected patients during simulation and subsequent SBRT is 100% without significant interruption during SBRT delivery. Use of respiratory gating control effectively prevents out-of-range SBRT delivery during fits of coughing and dyspnea. Use of FFF mode and VMAT via multiple arcs significantly reduces SBRT delivery time and optimizes normal tissue protection. The selection ”process“ is also recorded via screen capture of the respiratory cycles with a corresponding graphical representation of steady breathing patterns. These graphics can then be inserted into the CT simulation documentation as part of the permanent electronic health record. Appropriate selection of patients for SBRT via objective analysis and recorded respiratory patterns ensures optimal conditions for clinical setup, tumor targeting, and reproducible delivery of SBRT. This process is also recorded in the electronic health records of each patient for permanent documentation, future reference, and chart review.

Structural Shielding Design of a 6 MV Flattening Filter Free Linear Accelerator: Indian Scenario.

Detailed structural shielding of primary and secondary barriers for a 6 MV medical linear accelerator (LINAC) operated with flattening filter (FF) and flattening filter free (FFF) modes are calculated. The calculations have been carried out by two methods, one using the approach given in National Council on Radiation Protection (NCRP) Report No. 151 and the other based on the monitor units (MUs) delivered in clinical practice. Radiation survey of the installations was also carried out. NCRP approach suggests that the primary and secondary barrier thicknesses are higher by 24% and 26%. respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes with an assumption that only 20% of the workload is shared in FFF mode. Primary and secondary barrier thicknesses calculated from MUs delivered on clinical practice method also show the same trend and are higher by 20% and 19%, respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes. Overall, the barrier thickness for a LINAC operated in FF mode is higher about 20% to that of a LINAC operated in both FF and FFF modes.

Second Cancer Risk after simultaneous integrated boost radiation therapy of right sided breast cancer with and without flattening filter.

The aim of this study was to investigate if the flattening filter free mode (FFF) of alinear accelerator reduces the excess absolute risk (EAR) for second cancer as compared to the flat beam mode (FF) in simultaneous integrated boost (SIB) radiation therapy of right-sided breast cancer. Six plans were generated treating the whole breast to 50.4Gy and aSIB volume to 63Gy on CT data of 10patients: intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and atangential arc VMAT (tVMAT), each with flattening filter and without. The EAR was calculated for the contralateral breast and the lungs from dose-volume histograms (DVH) based on the linear-exponential, the plateau, and the full mechanistic dose-response model. Peripheral low-dose measurements were performed to compare the EAR in more distant regions as the thyroids and the uterus. FFF reduces the EAR significantly in the contralateral and peripheral organs for tVMAT and in the peripheral organs for VMAT. No reduction was found for IMRT. The lowest EAR for the contralateral breast and lung was achieved with tVMAT FFF, reducing the EAR by 25% and 29% as compared to tVMAT FF, and by 44% to 58% as compared to VMAT and IMRT in both irradiation modes. tVMAT FFF showed also the lowest peripheral dose corresponding to the lowest EAR in the thyroids and the uterus. The use of FFF mode allows reducing the EAR significantly when tVMAT is used as the treatment technique. When second cancer risk is amajor concern, tVMAT FFF is considered the preferred treatment option in SIB irradiation of right-sided breast cancer.

On flattening filter-free portal dosimetry.

Varian introduced (in 2010) the option of removing the flattening filter (FF) in their C‐Arm linacs for intensity‐modulated treatments. This mode, called flattening filter‐free (FFF), offers the advantage of a greater dose rate. Varian's “Portal Dosimetry” is an electronic portal imager device (EPID)‐based tool for IMRT verification. This tool lacks the capability of verifying flattening filter‐free (FFF) modes due to saturation and lack of an image prediction algorithm. (Note: the latest versions of this software and EPID correct these issues.) The objective of the present study is to research the feasibility of said verifications (with the older versions of the software and EPID). By placing the EPID at a greater distance, the images can be acquired without saturation, yielding a linearity similar to the flattened mode. For the image prediction, a method was optimized based on the clinically used algorithm (analytical anisotropic algorithm (AAA)) over a homogeneous phantom. The depth inside the phantom and its electronic density were tailored. An application was developed to allow the conversion of a dose plane (in DICOM format) to Varian's custom format for Portal Dosimetry. The proposed method was used for the verification of test and clinical fields for the three qualities used in our institution for IMRT: 6X, 6FFF and 10FFF. The method developed yielded a positive verification (more than 95% of the points pass a 2%/2 mm gamma) for both the clinical and test fields. This method was also capable of “predicting” static and wedged fields. A workflow for the verification of FFF fields was developed. This method relies on the clinical algorithm used for dose calculation and is able to verify the FFF modes, as well as being useful for machine quality assurance. The procedure described does not require new hardware. This method could be used as a verification of Varian's Portal Dose Image Prediction.PACS number(s): 87.53.Kn, 87.55.T‐, 87.56.bd, 87.59.‐e

SU‐F‐T‐545: Dosimetric and Radiobiological Evaluation of Dose Calculation Algorithms On Prostate Stereotactic Body Radiotherapy Using Conventional Flattened and Flattening‐Filter‐Free Beam

Purpose:The purpose of this study is to evaluate the dosimetric and radiobiological impact of Acuros XB (AXB) and Anisotropic Analytic Algorithm (AAA) dose calculation algorithms on prostate stereotactic body radiation therapy plans with both conventional flattened (FF) and flattening‐filter free (FFF) modes.Methods:For thirteen patients with prostate cancer, SBRT planning was performed using 10‐MV photon beam with FF and FFF modes. The total dose prescribed to the PTV was 42.7 Gy in 7 fractions. All plans were initially calculated using AAA algorithm in Eclipse treatment planning system (11.0.34), and then were re‐calculated using AXB with the same MUs and MLC files. The four types of plans for different algorithms and beam energies were compared in terms of homogeneity and conformity. To evaluate the radiobiological impact, the tumor control probability (TCP) and normal tissue complication probability (NTCP) calculations were performed.Results:For PTV, both calculation algorithms and beam modes lead to comparable homogeneity and conformity. However, the averaged TCP values in AXB plans were always lower than in AAA plans with an average difference of 5.3% and 6.1% for 10‐MV FFF and FF beam, respectively. In addition, the averaged NTCP values for organs at risk (OARs) were comparable.Conclusion:This study showed that prostate SBRT plan were comparable dosimetric results with different dose calculation algorithms as well as delivery beam modes. For biological results, even though NTCP values for both calculation algorithms and beam modes were similar, AXB plans produced slightly lower TCP compared to the AAA plans.

SU-G-BRB-07: Developing a QA Procedure for Gated VMAT SABR Treatments Using 10 MV Beam in Flattening-Filter Free Mode

Purpose: To develop a QA procedure for gated VMAT stereotactic ablative radiotherapy (SABR) treatments. Methods: An interface was constructed to attach the translational stage of a Quasar respiratory motion phantom to a pinpoint ion chamber insert and move the ion chamber inside an ArcCheck diode array. The Quasar phantom controller used a patient specific breathing pattern to translate the ion chamber in a superior-inferior direction inside the ArcCheck. An amplitude-based RPM tracking system was specified to turn the beam on during the exhale phase of the breathing pattern. SABR plans were developed using Eclipse for liver PTVs ranging in size from 3-12 cm in diameter using a 2-arc VMAT technique. Dose was measured in the middle of the penumbra region, where the high dose gradient allowed for sensitive detection of any inaccuracies in gated dose delivery. The overall fidelity of the dose distribution was confirmed using ArcCheck. The sensitivity of the gating QA procedure was investigated with respect to the following four parameters: PTV size, duration of exhale, baseline drift, and gating window size. Results: The difference between the measured dose to a point in the penumbra and the Eclipse calculated dose was under 2% for small residual motions. The QA procedure was independent of PTV size and duration of exhale. Baseline drift and gating window size, however, significantly affected the penumbral dose measurement, with differences of up to 30% compared to Eclipse. Conclusion: This study described a highly sensitive QA procedure for gated VMAT SABR treatments. The QA outcome was dependent on the gating window size and baseline drift. Analysis of additional patient breathing patterns will be required to determine a clinically relevant gating window size and an appropriate tolerance level for this procedure.

Modeling of flattening filter free photon beams with analytical and Monte Carlo TPS

Recent conventional medical linear particle accelerators (linacs) come with the capacity to deliver flattening filter free (FFF) beams, allowing higher dose rates. Intensity modulated radiation therapy (IMRT) and small-fields stereotactic techniques no longer seem to require flattened beams. To implement such beams, it is important to correctly measure and model the dose distribution they deliver. The objective of this work was to model the recent VERSA-HD (Elekta®) FFF beams using RayStation® TPS, as done externally on Monaco TPS. There exists no publication on the modeling of this machine in FFF mode. Also, no published work relates to the use of RayStation® to model unflattened beams. Flattened X6 and unflattened 6FFF and 10FFF beams have been modeled. Lateral and depth dose profiles as well as output factors were measured using ionization chambers and diodes detectors. Models were created in RayStation® and Monaco® treatment planning systems (TPS), offering analytically- and Monte Carlo-algorithms, respectively. Surface dose, out-of-field dose and collimator transmission were also measured to improve models. All steps and parameters used during the modeling process are presented, along with measured and calculated dose profiles. Agreement between measurements and models was assessed by one-dimensional gamma analysis. Stereotactic and VMAT (volumetric modulated arc therapy) treatment plans were controlled by measuring isocenter absolute dose and dose distribution with the ArcCheck device. The impacts of the model parameters on each region of the depth and lateral dose profiles were explained. FFF beams models exhibited different energy spectra, smaller source sizes and lower electron contamination compared to flattened beams, in both TPS. Monaco® and RayStation® exhibit good agreement with depth and lateral dose profile measurements, 1% and 1 mm criteria being respected in the high dose regions. Stereotactic and VMAT treatment plans measured led to good agreement with calculation.