Flattening Filter Research Articles

Differentiation of Using Flattening Filter Free Energy in Vmat Plans for Prostate Cancer

It is aimed to compare the values of conformity index (CI), homogeneity index (HI), monitor unit (MU) of volumetrically adjusted arthritis therapy (VMAT) plans using 10 prostate cancer patients with flattened filter (FF) and without flattening filter (FFF). In the study, treatment plans were prepared using 6 FF and 6 FFF in the Eclipse (ver.13.6) treatment planning system with Varian Trilogy Linear Accelerator. When planning was completed, CI averaged 0.87, HI averaged 0.44 and MU values were found to be 591 ± 26.8, 650 ± 33.06, respectively. When the PTV coverage, CI, HI and MU comparisons were made as a result of planning, there was no significant difference when comparing VMAT plans in FFF and FF energies. When we compare the MU values, the MU increase is seen when the straightening filter is removed. In both energy modes, good homogeneity in PTV was achieved with conventional francitation and close dose rates. No significant advantages and disadvantages of the unfiltered energy mode were observed in the assessment of plan quality in terms of CI, HI.

Dose rate and energy dependence study of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with flattened and unflattened photon beams.

The aim of this study is to evaluate the dose rate and energy dependency of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride (MAGAT) gel in unflattened photon beam using X-ray computed tomography (CT) and ultraviolet (UV)-visible spectroscopy. MAGAT gel was prepared and it was exposed to 6 MV flattened and unflattened beams. The dose selected for irradiation was ranging from 3 to 15 Gy with an increment of 3 Gy. The dose rate dependency of the gel was investigated by exposing the gel to three different dose rates of 250, 500, and 1500 cGy/min for flattening filter free (FFF). To verify the energy dependency of the gel, it was exposed by both FFF and flattening filter (FF) for constant dose rate (250 cGy/min) and different energy (6 and 10 MV X-ray photons). The exposed gels were scanned by X-ray CT and UV-visible spectrophotometer. The change in dose sensitivity observed over the dose rate from 250 cGy/min to 1500 cGy/min was 58.00% and 57.89% using a UV-visible spectrophotometer and X-ray CT analysis method. Energy dependency was evaluated with respect to dose sensitivity and the variation between 6 MV FF and FFF photon beams was found to be 2.20% and 2.21% using UV-visible spectrophotometer analysis and X-ray CT, respectively. Similarly, the variation noticed with 10 MV FF and FFF was 2.30% using UV-visible spectrophotometer analysis and 2.22% using X-ray CT analysis. The results clearly show that the MAGAT gel was highly dose rate-dependent and less dependent on energy. The beam quality variation between FF and FFF was less. The similar results obtained using X-ray CT scanner and UV-visible spectrophotometer indicate that this study can be recommended for polymer gel scanning procedure.

Characterisation of flattening filter free (FFF) beam properties for initial beam set-up and routine QA, independent of flattened beams

Flattening filter free (FFF) beams have reached widespread use for clinical treatment deliveries. The usual methods for FFF beam characterisation for their quality assurance (QA) require the use of associated conventional flattened beams (cFF). Methods for QA of FFF without the need to use associated cFF beams are presented and evaluated against current methods for both FFF and cFF beams. Inflection point normalisation is evaluated against conventional methods for the determination of field size and penumbra for field sizes from 3 cm × 3 cm to 40 cm × 40cm at depths from dmax to 20 cm in water for matched and unmatched FFF beams and for cFF beams. A method for measuring symmetry in the cross plane direction is suggested and evaluated as FFF beams are insensitive to symmetry changes in this direction. Methods for characterising beam energy are evaluated and the impact of beam energy on profile shape compared to that of cFF beams. In-plane symmetry can be measured, as can cFF beams, using observed changes in profile, whereas cross-plane symmetry can be measured by acquiring profiles at collimator angles 0 and 180. Beam energy and ‘unflatness’ can be measured as with cFF beams from observed shifts in profile with changing beam energy. Normalising the inflection points of FFF beams to 55% results in an equivalent penumbra and field size measurement within 0.5 mm of conventional methods with the exception of 40 cm × 40 cm fields at a depth of 20 cm. New proposed methods are presented that make it possible to independently carry out set up and QA measurements on beam energy, flatness, symmetry and field size of an FFF beam without the need to reference to an equivalent flattened beam of the same energy. The methods proposed can also be used to carry out this QA for flattened beams, resulting in universal definitions and methods for MV beams. This is presented for beams produced by an Elekta linear accelerator, but is anticipated to also apply to other manufacturers’ beams.

A comparative study between flattening filter-free beams and flattening filter beams in radiotherapy treatment

Abstract Flattening filter-free (FFF) beams generated by medical linear particle accelerators (linacs) have recently been used in radiotherapy clinical practice. FFF beams have fundamental physical parameter differences with respect to standard flattening filter (FF) beams, such that the generally used dosimetric parameters and definitions are not always viable. This study investigates dosimetric parameters for use in the quality assurance of FFF beams generated by medical linacs in radiotherapy. The main characteristics of the photon beams are analyzed using specific data generated by a Varian TrueBeam linac having both FFF and FF beams of 6 and 10 MV (megavolt) energy, respectively. Definitions for dose profile parameters are suggested, starting from the renormalization of the FFF with respect to the corresponding FF beam. From this point, the flatness concept is translated into one of “un-flatness”, and other definitions are proposed, maintaining a strict parallelism between FFF and FF parameter concepts. The quality controls used in establishing a quality assurance program when introducing FFF beams into the clinical environment are given, maintaining similarity to those used for standard FF beams, and recommendations for the introduction of FFF beams into clinical radiotherapy application for breast cancer patients are provided as an example for comparison between FFF and FF for dose distribution and coverage for a target volume. Although there are many advantages of using a FFF beam, especially for advanced radiotherapy techniques, there are a few limitations (e.g., using a relatively higher energy photon beam for stereotactic radiotherapy (SRT), limited speed of current multileaf collimators (MLCs), and off-axis distance-dependent modulation in intensity-modulated radiation therapy (IMRT)) as well as challenges (e.g., criteria for beam quality evaluation and penumbra, establishment of dosimetry methods, and consequences of photon target burn-up) that need to be addressed for establishing the FFF beam as a viable alternative to the FF beam.

Relative Attenuation and Beam Softening Study with Flattening Filter Volume Reduction: Monte Carlo Study

The flattening filter (FF) volume reduction increases the clinical photons for deep tumor treatment. The beam softening determination is crucial for flattening filter improvement in geometry and materials. Determination and understanding the photon beam properties using material and geometry of a beam modifier is very important for dosimetry improvement in radiotherapy department and also for patient life quality development. This Monte Carlo study aims to investigate the relative attenuation and associated beam softening due to flattening filter volume reduction. The FF volume was reduced by 10, 20, and 30% of the initial volume data provided by the manufacturer. The relative attenuation and beam softening coefficients increased with FF volume reduction more near the beam central axis than the beam edge. We have illustrated that relative photon beam softening coefficient v was more stable than the coefficient u as a function of offaxis distance and with FF volume reduction. For increasing the photon fluence and dose delivered inside the phantom volume as mentioned in IAEA protocols, the FF volume should be reduced more near the FF top region than the FF edge region. Our work can be a basic investigation that will be used in improvement for the future linac configuration in terms of photon beam softening for material, geometry, and volume that were used in beam modifiers as a flattening filter.

Energetic properties’ investigation of removing flattening filter at phantom surface: Monte Carlo study using BEAMnrc code, DOSXYZnrc code and BEAMDP code

The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy and beam characterization investigation, in this study, the Varian Clinac 2100 medical linear accelerator with and without flattening filter (FF) was modelled. The objective of this study was to determine flattening filter impact on particles’ energy properties at phantom surface in terms of energy fluence, mean energy, and energy fluence distribution. The Monte Carlo codes used in this study were BEAMnrc code for simulating linac head, DOSXYZnrc code for simulating the absorbed dose in a water phantom, and BEAMDP for extracting energy properties. Field size was 10 × 10 cm2, simulated photon beam energy was 6 MV and SSD was 100 cm. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, gamma criteria was 3% for dose difference and 3mm for distance to agreement. In without-FF, the energetic properties was as following: electron contribution was increased by more than 300% in energy fluence, almost 14% in mean energy and 1900% in energy fluence distribution, however, photon contribution was increased 50% in energy fluence, and almost 18% in mean energy and almost 35% in energy fluence distribution. The removing flattening filter promotes the increasing of electron contamination energy versus photon energy; our study can contribute in the evolution of removing flattening filter configuration in future linac.

The Dosimetric Effect of Prostate Motion in Flattening Filter Free VMAT Treatments Using Extreme Hypofractionation

To evaluate the performance of two treatment planning systems (TPS), using radiotherapy beams both with and without flattening filter, for extreme hypo-fractionated prostate cancer treatment. The TPSs were evaluated regarding plan quality and robustness with respect to prostate motion. With hypofractionation, the treatment time per fraction is increased, making prostate motion more significant. Flattening filter free treatments can decrease the treatment time due to the higher dose rate, and therefore reduce the effect of prostate motion. Prostate cancer patients treated with extreme hypofractionation (7x6.1 Gy=42.7 Gy) in the experimental arm of the randomized Scandinavian HYPO-RT-PC trial were used in this study. The TPSs used were Varian Eclipse (Varian Medical Systems) and RayStation (RaySearch Laboratories). The treatment plan comparison was made using the Pareto front method. Several VMAT plans were optimized with varying importance assigned to rectal sparing and PTV coverage objectives. The rectal volume receiving more than 90%, (Rectum, V90%), of the prescribed dose was plotted as a function of the PTV under dosage, (PTV, V95%). Two different beam qualities were used on each TPS, 10MV with flattening filter (FF) and 10MV flattening filter free (FFF). All plans were made for a Varian TrueBeam accelerator. The Pareto optimal plans, where one objective cannot be improved without deteriorating the other, formed a front for each TPS and beam quality. Three plans for each Pareto front were selected for measurement from three different regions in the fronts. These were defined as follows: PTV more important than Rectum (PTV>>Rect), Rectum more important than PTV (Rect>>PTV) and the knee of the Pareto front. The latter is the point where the target coverage starts to drastically decrease without significant improvement in rectum dose. The measurements were performed using the Delta4 Phantom+ (ScandiDos) and motion was simulated using the HexaMotion device with realistic motion patterns. The measurements were evaluated with gamma analysis using the local clinical protocol at our clinic (90% pass rate at 3%/2mm). The Pareto front plan comparison showed very small differences between the two TPSs and beam qualities. Eclipse had slightly better target coverage in the PTV>>Rect region but RayStation had lower rectum doses in the Rect>>PTV region. All plans fulfilled the dose-volume constraints of the trial protocol. However, all fronts showed that the rectum dose could be decreased even more without losing target coverage. The static measurements showed 100% pass rate for all plans. For the in-motion measurements, the FF plans had a pass rate below the clinically acceptable 90% pass rate while the FFF plans were above 90%. The beam-on time was decreased from 2.5 min to 1 min. Both TPSs delivered plans with good quality for both FF and FFF beams. The FFF plans proved to be clinically acceptable even with prostate motion for both TPSs.

EP-1469: Flattening filter free beam profile analysis using two different normalization methods

EP-1469: Flattening filter free beam profile analysis using two different normalization methods

Surface doses of flattening filter free beams with volumetric modulated arc therapy dose delivery for breast cancer

Abstract Background and purpose Flattening filter free (FFF) beams enable high-dose rate irradiations and have the potential to speed up breast cancer radiotherapy (RT) treatments. In this work the surface doses of FFF beams were studied with various treatment plans for breast cancer RT. The near surface dose distributions were compared to the ones delivered with conventional flattening filter (FF) beams and to the calculated dose distributions. Materials and methods The study was executed with radiochromic films. In addition to one open field, four techniques were investigated: tangential open field, tangential IMRT (IMRT), tangential VMAT (tVMAT) and continuous VMAT (cVMAT) techniques, respectively. The dose distributions were calculated with commercial Monte Carlo (MC) algorithm with energies of 6 and 10MV with both FF and FFF. The surface areas investigated were divided into depths of 0–2, 2–5 and 5–8mm. Results The largest deviations (on average 5.9%) between the measured and calculated doses were recorded at the most shallowest depths (0–2mm). At deeper depths, the differences were on average 1.4% and always less than 6%. The measured near surface doses were slightly lower (4.6%) with modulated 6MVFFF beams than with the corresponding flattened beams and on the contrary with 10MV the surface doses by FFF were slightly higher (2.8%). Conclusions The studied MC calculation algorithm was accurate in determine the near surface doses. There was no significant difference in measured or calculated surface doses between FFF and FF beams. With respect to surface dose uniformity the VMAT techniques overall resulted in the most uniform dose distributions.

Efficient independent planar dose calculation for FFF IMRT QA with a bivariate Gaussian source model.

The aim of this study is to perform a direct comparison of the source model for photon beams with and without flattening filter (FF) and to develop an efficient independent algorithm for planar dose calculation for FF‐free (FFF) intensity‐modulated radiotherapy (IMRT) quality assurance (QA). The source model consisted of a point source modeling the primary photons and extrafocal bivariate Gaussian functions modeling the head scatter, monitor chamber backscatter, and collimator exchange effect. The model parameters were obtained by minimizing the difference between the calculated and measured in‐air output factors (S c). The fluence of IMRT beams was calculated from the source model using a backprojection and integration method. The off‐axis ratio in FFF beams were modeled with a fourth degree polynomial. An analytical kernel consisting of the sum of three Gaussian functions was used to describe the dose deposition process. A convolution‐based method was used to account for the ionization chamber volume averaging effect when commissioning the algorithm. The algorithm was validated by comparing the calculated planar dose distributions of FFF head‐and‐neck IMRT plans with measurements performed with a 2D diode array. Good agreement between the measured and calculated S c was achieved for both FF beams (<0.25%) and FFF beams (<0.10%). The relative contribution of the head‐scattered photons reduced by 34.7% for 6 MV and 49.3% for 10 MV due to the removal of the FF. Superior agreement between the calculated and measured dose distribution was also achieved for FFF IMRT. In the gamma comparison with a 2%/2 mm criterion, the average passing rate was 96.2 ± 1.9% for 6 MV FFF and 95.5 ± 2.6% for 10 MV FFF. The efficient independent planar dose calculation algorithm is easy to implement and can be valuable in FFF IMRT QA.

Dosimetric characterization of a novel dual-energy medical linear accelerator without a flattening filter

Objective To study the dosimetric characteristics of flattened and flattening filter free (FFF) beams with 6 MV and 10 MV photon energy using a novel dual-energy medical linear accelerator (Elekta Versa HDTM), to identify the dosimetric characteristics and advantages of FFF beams, and to provide a basis for their clinical application. Methods The percentage depth dose (PDD), profiles/dose rate of off-axis ratio (OAR), field size, penumbral width, dose out of the fields, collimator scatter factor (Sc), and total scatter factor (Sc, p) were compared between flattened and FFF beams. Results (1) After beam energy matching, the FFF beams had the same beam energy with the flattened beams. The matching error of PDD at a depth of 10 cm was less than 1% between fields.(2) The FFF beams had a smaller variation in dose rate of OAR with the depth than the flattened beams.(3) The FFF beams had smaller variations in field size and penumbral width than the flattened beams. Moreover, the penumbral width of the FFF beams increased with the increasing field size or depth. The FFF beams had a lower dose out of the fields than the flattened beams.(4) The FFF beams had smaller variations in Sc and Sc, p with the field size and depth than the flattened beams. Conclusions Removal of the flattening filter can substantially improve the dose rate, shorten radiotherapy time, and reduce leakage and scattering of the head. The dosimetric advantages make the FFF beams appropriate for clinical treatment. Key words: Linear accelerator; Flattening filter; Flattening filter free

Comparison of Flattening Filter (FF) and Flattening-Filter-Free (FFF) 6 MV photon beam characteristics for small field dosimetry using EGSnrc Monte Carlo code

The present study is focused to compare the characteristics of Varian Clinac 600C/D flattened and unflattened 6MV photon beams for small field dosimetry using EGSnrc Monte Carlo Simulation since the small field dosimetry is considered to be the most crucial and provoking task in the field of radiation dosimetry. A 6MV photon beam of a Varian Clinac 600C/D medical linear accelerator operates with Flattening Filter (FF) and Flattening-Filter-Free (FFF) mode for small field dosimetry were performed using EGSnrc Monte Carlo user codes (BEAMnrc and DOSXYZnrc) in order to calculate the beam characteristics using Educated-trial and error method. These includes: Percentage depth dose, lateral beam profile, dose rate delivery, photon energy spectra, photon beam uniformity, out-of-field dose, surface dose, penumbral dose and output factor for small field dosimetry (0.5×0.5cm2 to 4×4cm2) and are compared with magna-field sizes (5×5cm2 to 40×40cm2) at various depths. The results obtained showed that the optimized beam energy and Full-width-half maximum value for small field dosimetry and magna-field dosimetry was found to be 5.7MeV and 0.13cm for both FF and FFF beams. The depth of dose maxima for small field size deviates minimally for both FF and FFF beams similar to magna-fields. The depths greater than dmax depicts a steeper dose fall off in the exponential region for FFF beams comparing FF beams where its deviations gets increased with the increase in field size. The shape of the lateral beam profiles of FF and FFF beams varies remains similar for the small field sizes less than 4×4cm2 whereas it varies in the case of magna-fields. Dose rate delivery for FFF beams shows an eminent increase with a two-fold factor for both small field dosimetry and magna-field sizes. The surface dose measurements of FFF beams for small field size were found to be higher whereas it gets lower for magna-fields than FF beam. The amount of out-of-field dose reduction gets increased with the increase in field size. It is also observed that the photon energy spectrum gets increased with the increase in field size for FFF beam mode. Finally, the output factors for FFF beams were relatively quite low for small field sizes than FF beams whereas it gets higher for magna-field sizes. From this study, it is concluded that the FFF beams depicted minimal deviations in the treatment field region irrespective to the normal tissue region for small field dosimetry compared to FF beams. The more prominent result observed from the study is that the shape of the beam profile remains similar for FF and FFF beams in the case of smaller field size that leads to more accurate treatment planning in the case of IMRT (Image-Guided Radiation Therapy), IGAT (Image-Guided Adaptive Radiation Therapy), SBRT (Stereotactic Body Radiation Therapy), SRS (Stereotactic Radio Surgery), and Tomotherapy techniques where homogeneous dose is not necessary. On the whole, the determination of dosimetric beam characteristics of Varian linac machine using Monte Carlo simulation provides accurate dose calculation as the clinical golden data.

Comparison of dosimetric and radiobiological parameters on plans for prostate stereotactic body radiotherapy using an endorectal balloon for different dose-calculation algorithms and delivery-beam modes

The purpose of this study was to evaluate the impact of dosimetric and radiobiological parameters on treatment plans by using different dose-calculation algorithms and delivery-beam modes for prostate stereotactic body radiation therapy using an endorectal balloon. For 20 patients with prostate cancer, stereotactic body radiation therapy (SBRT) plans were generated by using a 10-MV photon beam with flattening filter (FF) and flattening-filter-free (FFF) modes. The total treatment dose prescribed was 42.7 Gy in 7 fractions to cover at least 95% of the planning target volume (PTV) with 95% of the prescribed dose. The dose computation was initially performed using an anisotropic analytical algorithm (AAA) in the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) and was then re-calculated using Acuros XB (AXB V. 11.0.34) with the same monitor units and multileaf collimator files. The dosimetric and the radiobiological parameters for the PTV and organs at risk (OARs) were analyzed from the dose-volume histogram. An obvious difference in dosimetric parameters between the AAA and the AXB plans was observed in the PTV and rectum. Doses to the PTV, excluding the maximum dose, were always higher in the AAA plans than in the AXB plans. However, doses to the other OARs were similar in both algorithm plans. In addition, no difference was observed in the dosimetric parameters for different delivery-beam modes when using the same algorithm to generate plans. As a result of the dosimetric parameters, the radiobiological parameters for the two algorithm plans presented an apparent difference in the PTV and the rectum. The average tumor control probability of the AAA plans was higher than that of the AXB plans. The average normal tissue complication probability (NTCP) to rectum was lower in the AXB plans than in the AAA plans. The AAA and the AXB plans yielded very similar NTCPs for the other OARs. In plans using the same algorithms, the NTCPs for delivery-beam modes showed no differences. This study demonstrated that the dosimetric and the radiobiological parameters for the PTV and the rectum affected the dose-calculation algorithms for prostate SBRT using an endorectal balloon. However, the dosimetric and the radiobiological parameters in the AAA and the AXB plans for other OARs were similar. Furthermore, difference between the dosimetric and the radiobiological parameters for different delivery-beam modes were not found when the same algorithm was used to generate the treatment plan.

Accounting for the ion recombination factor in relative dosimetry of flattening filter free photon radiation

Flattening filter free (FFF) linear accelerators (linacs) are increasingly being used to deliver external beam radiotherapy. Advantages of FFF include lower out-of-field dose to the patient due to reduced head scatter, and shorter treatment times due to increased dose rate. The ion recombination factor (kion) is known to increase linearly with dose per pulse in pulsed photon beams. This variation is neglected when performing relative dosimetry in flattened beams, however could be significant in the higher dose rates encountered with FFF. kion has been derived from theory and measured using Jaffe plots and two voltage analysis for three ionisation chambers at a range of depths and points off-axis in 6 and 10 MV FFF beams produced by an Elekta Versa HD linac. The chambers under investigation were the PTW 30013 ‘Farmer’ (0.60 cc), the Exradin A1SL (0.053 cc) and the IBA CC04 (0.04 cc). The effect of neglecting ion recombination corrections in commissioning data was quantified using a 1D gamma analysis (1%/2 mm) on depth dose curves and beam profiles. The largest variation in kion was for a Farmer chamber in a 10 MV FFF beam, where kion varied by up to 1.2% ± 0.15% (95% CL) between different points in a water tank. The Farmer variation with depth was comparable to that measured by Kry et al (2012 J. Appl. Clin. Med. Phys. 13 318–325). Other chambers exhibited negligible changes in kion. It was concluded that neglecting ion recombination in relative measurements at 6 MV FFF would have a negligible effect on dosimetric accuracy. Furthermore kion could be safely neglected when acquiring relative data with small volume chambers for commissioning 10 MV FFF, although may need to be accounted for if relative data are acquired with a Farmer chamber and when routine energy checks are performed.

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.

Beam Characterization of 10-MV Photon Beam from Medical Linear Accelerator without Flattening Filter.

Aim:This work investigated the dosimetric properties of a 10-MV photon beam emitted from a medical linear accelerator (linac) with no flattening filter (FF). The aim of this study is to analyze the radiation fluence and energy emitted from the flattening filter free (FFF) linac using Monte Carlo (MC) simulations.Materials and Methods:The FFF linac was created by removing the FF from a linac in clinical use. Measurements of the depth dose (DD) and the off-axis profile were performed using a three-dimensional water phantom with an ionization chamber. A MC simulation for a 10-MV photon beam from this FFF linac was performed using the BEAMnrc code.Results:The off-axis profiles for the FFF linac exhibited a chevron-like distribution, and the dose outside the irradiation field was found to be lower for the FFF linac than for a linac with an FF (FF linac). The DD curves for the FFF linac included many contaminant electrons in the build-up region.Conclusion:Therefore, for clinical use, a metal filter is additionally required to reduce the effects of the electron contamination. The mean energy of the FFF linac was found to be lower than that of the FF linac owing to the absence of beam hardening caused by the FF.

Identification of the suitable leaf margin for liver stereotactic body radiotherapy with flattening filter-free beams

The purpose of this study is to identify the suitable leaf margin for liver stereotactic body radiotherapy (SBRT) with flattening filter-free (FFF) beams, as compared with that with flattening filter (FF) beams. SBRT treatment planning for 10 patients with liver cancer was performed using 10-MV FFF and FF beams obtained from a Varian TrueBeam (Varian Medical Systems, Palo Alto, CA) linear accelerator. Each plan was generated with the leaf margin to the planning target volume (PTV) ranging from −3 to 5 mm. The prescription dose at D95 (dose covering 95% of the volume) was 48 Gy in 4 fractions to the PTV. The following dosimetric parameters were evaluated quantitatively: homogeneity index (HI), conformity index (CI), gradient index (GI), the normal liver receiving a dose greater than or equal to 20 Gy (V20), and the mean normal liver dose. The HI for FFF and FF beams increased as the leaf margin decreased. The leaf margins that achieved the best CI and GI were 0.1 and −0.3 mm for FFF beams, and 0.1 and −0.9 mm for FF beams. The liver V20 and the mean liver dose reached their minimum values at leaf margins of −0.8 and 0.0 mm for FFF beams, and −0.8 and 0.0 mm for FF beams. The suitable leaf margin for SBRT planning did not differ significantly for FFF and FF beams. Our data showed that, for both FFF and FF beams, a leaf margin of 0 or −1 mm was optimal for liver SBRT planning in terms of both target coverage and normal tissue sparing.

38. Use of flattening filter free beams for stereotactic body radiotherapy treatments: Impact of the size of the planning target volume

Introduction Flattening filter free (FFF) beams substitude flattened filter (FF) beams for stereotactic body radiotherapy (SBRT). There are several treatment modalities available for SBRT: conventional static fields, dynamic conformal arc (DCA) or Volumetric Modulated Arc Therapy (VMAT). We compared SBRT plans in DCA FF, DCA FFF, VMAT FF, and VMAT FFF for different volumes of planning target volume (PTV). The aim was to obtain a consensus of the employment of FFF beams and of the DCA or VMAT technique depending of the size of PTV. Methods Three environments were studied with several concentrics PTV: 11 in an elliptical water equivalent phantom, 6 in lung, 7 in liver. For each PTV, 8 treatments plans were generated: DCA and VMAT in 6 MV and 10 MV FF and FFF. Homogeneity (HI), conformity (CI), gradient (GI) index, healthy tissue mean dose, organ at risk (OAR) dose, number of monitor unit (MU) and beam on time were evaluated. The ratio FFF/FF and VMAT/DCA of these parameters were performed and Spearman test was executed in order to establish a correlation with the size of the PTV. Results Dosimetrics index were improved in FFF but not significantly (median CI: DCA 6 FF = 1.19; DCA 6 FFF = 1.17; VMAT 6 FF = 1.04; VMAT 6 FFF = 1.04). Healthy tissue mean dose was significantly damaged in DCA 10 FFF with the incease of PTV size (phantom: p = 0.0001, rho = 0.91; V20 lung: p = 0.000, rho = 1.00; D700cc liver: p = 0.000, rho = 1.00). The results obtained in liver showed a better dose gradient with VMAT than DCA for extensives PTV (GI: p p = 0.0188, rho = −0.89). Beam on time was significantly shorter with FFF energies (55% in 6 MV, 75% in 10 MV) and DCA technique (55% in 6 MV and 70% in 10 MV). Number of MU was greater in FFF and VMAT technique. Conclusion No significant difference between FFF and FF beams were noted in our SBRT plans. It seems that FFF beams provide a better OAR sparing except for DCA with large PTV volume particularly in 10 FFF. DCA allows shorter beam on time than VMAT and remains immune to the MLC interplay effect.

IPEM topical report 1: guidance on implementing flattening filter free (FFF) radiotherapy

Flattening filter free (FFF) beams are now commonly available with new standard linear accelerators. These beams have recognised clinical advantages in certain circumstances, most notably the reduced beam-on times for high dose per fraction stereotactic treatments. Therefore FFF techniques are quickly being introduced into clinical use. The purpose of this report is to provide practical implementation advice and references for centres implementing FFF beams clinically. In particular UK-specific guidance is given for reference dosimetry and radiation protection.

A comparison of methods for monitoring photon beam energy constancy.

In extension of a previous study, we compared several photon beam energy metrics to determine which was the most sensitive to energy change; in addition to those, we accounted for both the sensitivity of each metric and the uncertainty in determining that metric for both traditional flattening filter (FF) beams (4, 6, 8, and 10 MV) and for flattening filter‐free (FFF) beams (6 and 10 MV) on a Varian TrueBeam. We examined changes in these energy metrics when photon energies were changed to ±5% and ±10% from their nominal energies: 1) an attenuation‐based metric (the percent depth dose at 10 cm depth, PDD(10)) and, 2) profile‐based metrics, including flatness (Flat) and off‐axis ratios (OARs) measured on the orthogonal axes or on the diagonals (diagonal normalized flatness, FDN). Profile‐based metrics were measured near dmax and also near 10 cm depth in water (using a 3D scanner) and with ionization chamber array (ICA). PDD(10) was measured only in water. Changes in PDD, OAR, and FDN were nearly linear to the changes in the bend magnet current (BMI) over the range from −10% to +10% for both FF and FFF beams: a ±10% change in energy resulted in a ±1.5% change in PDD(10) for both FF and FFF beams, and changes in OAR and FDN were >3.0% for FF beams and >2.2% for FFF beams. The uncertainty in determining PDD(10) was estimated to be 0.15% and that for OAR and FDN about 0.07%. This resulted in minimally detectable changes in energy of 2.5% for PDD(10) and 0.5% for OAR and FDN. We found that the OAR‐ or FDN‐ based metrics were the best for detecting energy changes for both FF and FFF beams. The ability of the OAR‐based metrics determined with a water scanner to detect energy changes was equivalent to that using an ionization chamber array. We recommend that OAR be measured either on the orthogonal axes or the diagonals, using an ionization chamber array near the depth of maximum dose, as a sensitive and efficient way to confirm stability of photon beam energy.PACS number(s): 87.55.Qr, 87.56.Fc