Flattening Filter Free Photon Beams Research Articles

Elaboration of a quality assurance programme for flattening filter free photon beams

TrueBeam STx is a new linac that can produce high dose rate photon beams up to 2400 UM/min by removing the flattening filter. Higher dose rates would potentially reduce treatment time, for instance in the case of extracranial stereotactic radiotherapy. A quality assurance programme dedicated to the specificities of FFF beams must be elaborated before any clinical exploitation. Some parameters previously used for flattened beams need to be rethought. The behaviour of detectors in high dose rates must be examined. The following parameters were measured in our centre for 6 MV and 10 MV FFF beams. Quality index (TPR20,10) and recombination factors (ks) were measured with the reference chamber FC65-G. Relative measurements were performed for field sizes from 3 × 3 cm 2 to 40 × 40 cm 2 : scatter factors (Sc,p), percentage depth dose curves and profiles. Several methods to describe the profiles were compared. Quality index values are similar to the results found in literature. ks reach a maximum value of 1.018 for the X10FFF beam. Surface dose is higher andmaximumdose is closer to the surface for FFF than for flattened beams. DS, dmax and Sc,p values show weak variationwith field size. Profiles show a typical peak in the central region. The acquisition method was modified to account for the important signal fluctuations in this central part. The renormalization at the inflection point introduces an uncertainty around 10% on the penumbra measurement. The renormalization at the profile shoulder gives more accurate results. FFF characteristics are reproducible between linacs of different centres. Experience acquired with flattened beams will mostly be preserved. However, some parameters employed to describe the profiles become obsolete for FFF beams. Physicist will have to come to a consensus in order to characterise FFF beams in a complete yet simple manner.

OC-0066: Detector comparison for small field output factor measurements with flattening filter free photon beams

OC-0066: Detector comparison for small field output factor measurements with flattening filter free photon beams

SU-E-T-536: Is BJR Supplement #25 Recommendation for Megavoltage Energy Independent Scatter Factor Still Valid for Flattening Filter Free Photon Beams?

In the process of measuring and validating fundamental dosimetry data prior to the clinical use of a treatment unit, it is prudent to compare measurements with previously published equivalent data. During the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian True Beam 6 MV FFF and 10 MV FFF) we compared measured Phantom Scatter Factors (Sp) with the Normalized Peak Scatter Factors (NPSFs) from the British Journal of Radiology Supplement 25 (BJR #25). The purpose of this work was to determine whether the energy independent NPSFs BJR#25 are valid for comparison with FFF photon beams. All measurements were performed using a Varian TrueBeam linear accelerator with photon energies of 6-MV, 6-MV FFF and 10-MV FFF modes. For all measurements, a Scanditronix CC04 ionization chamber was used. Both water and in-air measurements were made to obtain NPSFs normalized to the 10 × 10 cm2 field size. For measurements in water, the chamber was positioned at 100 cm source-to-axis distance at the depth of dose maximum. For in-air measurements, the chamber was positioned at 100 cm source-to-axis distance with appropriate build-up cap. From BJR #25, NPSFs were obtained for comparison with the measurements. The NPSF agreement between the 6-MV and 6-MV FFF with the BJR#25 were all within ±0.5%. The agreement ranged from 0.996 to 1.004 and 0.995 to 1.002 for 6-MV and 6-MV FFF, respectively. We also found that 10-MV FFF showed very similar trend. The scatter factors reported in BJR #25 are valid for comparison for 6-MV, 6-MV FFF, and 10-MV. Additional investigation is needed to further understand the dosimetric characteristics of FFF mode.

SU-E-T-643: Optimum Beam Parameters for Lung SBRT Volumetric Arc Treatment.

We present an analysis of the variation in acceptable SBRT lung plans with beam parameters. A figure of merit encompassing standard metrics is used for analytical comparison to determine the optimum plan quality. A set of optimization dose-volume constraints was formulated that consistently produced acceptable plans. Plans were normalized to deliver a prescription dose (PD) of 5000 cGy to 95% of the PTV volume. The Conformity Index (CI), Conformity Number (CN), and Gradient Index (GI), and mean GTV dose (MDgtv) were calculated. In SBRT hotspots near the target center are often deemed acceptable. The ratio MDgtv/PD is greater then 1.0 and larger values indicate that more dose is delivered where desired within the PTV. We combine the indices into a single figure of merit, FOM = (1/CI)*CN*(1/GI)*(MDgtv/PD), for which larger values indicate better plan quality dosimetrically. FOM values were normalized to 1.0 for the best plan. Twenty four plans were calculated for 6X, 6X flattening filter free (FFF), 10X, and 10X FFF photon beams. The gantry arc rotations were 0°-180° (180arc), 135°-30° (255arc), and 181°-179° (360arc). The couch angle was either 0° (coplanar) or +/- 15° (non-coplanar). For the normal lung volume there was no significant variation in either mean dose or percent volume receiving 2000 cGy. However, the percent volume receiving 500 cGy varies significantly with energy and couch angle. Ninety six plan quality indices were tabulated. Overall, the 6X FFF non-coplanar beam with a 255 degree arc gave the best the figure of merit; it was 6.5% higher then nearest competitor largely due to superior conformality. Individual plan quality indices were combined into a single figure of merit for various beam parameters that can be used to analytically select the optimum dosimetric plan.

SU-E-T-01: Applications of 6MV FFF Photon Beams in Optimizing Radiobiological Response for Respiratory-Gated Liver SBRT.

To establish a radiobiological basis for gated stereotactic body radiotherapy of primary and metastatic liver cancers using volumetric arc radiotherapy in a flattening filter free (FFF) mode. Human cervical carcinoma, SiHa, non-small cell lung carcinoma, H460, and Chinese hamster V79 cells were irradiated in a water bath with 6MV photons from a Varian TrueBeam linear accelerator. To establish dose-response and its sensitivity to dose rate following acute irradiation, doses of 2, 4, 6, 8 and 10 Gy were delivered in FFF mode at 400 and 1200 MU/min. To investigate whether removal of the flattening filter affects cell response, doses of 5 and 10 Gy were delivered to SiHa and H460 cells in FFF and filtered modes at 400 MU/min. Finally, to assess the effect of protracting dose delivery by gating, a dose of 10 Gy was delivered to SiHa and H460 cells acutely and also over 15, 30 and 60 min. Dose-response over doses examined was independent of dose rate in FFF mode. Differences in cell survival following irradiation in FFF and filtered modes were not significant. However a significant increase in survival for both H460 and SiHa cells was observed for 15 min split-dose irradiation compared to acute irradiation but further increase in irradiation time to 60 min did not affect cell survival. Dose rate and presence of a flattening filter showed no effect on cell survival, however, survival was significantly affected when dose delivery time was protracted to that typical of conformal field therapy. Volumetric arc based gated SBRT may be beneficial for tumor cell kill, though the gating window and duty cycle have to be balanced against the effect of dose delivery protraction. Research Support (Varian Medical Systems).

Surface Dose Investigation of the Flattening Filter-Free Photon Beams

Flattening filter-free (FFF) x-rays can provide more efficient use of photons and a significant increase of dose rate compared with conventional flattened x-rays, features that are especially beneficial for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). The available data on the entrance doses of the FFF photon beams remain limited. The purpose of this study was to investigate the entrance dose of FFF photons in the buildup region and to compare it with that of conventional flattened photons. A Varian TrueBeam linear accelerator has been in full clinical operation with 6-MV and 10-MV FFF and flattened x-ray photons. Entrance dose at the surface was measured using a parallel plate ionization chamber in a solid water phantom with buildup depth = 0~15 mm for 6X and 0~25 mm for 10X. Different field size (FS) patterns were created in the Eclipse Treatment Planning System by multileaf collimator (MLC) rather than jaws (FS = 2 × 2, 3 × 3, 4 × 4, 6 × 6, and 10 × 10 cm(2) by MLC and jaw size = 2.2 × 2.2, 3.2 × 3.2, 4.2 × 4.2, 6 × 6, and 10 × 10 cm(2)). The smallest FS was about four times larger than the ion chamber dimension. All buildup dose measurements were normalized to FS = 10 × 10 cm(2) at the depth of dose maximum (dmax). Good repeatability was demonstrated and surface dose increased linearly with FS for both flattened and FFF photons. The entrance dose of the FFF photons was modestly larger than that of the corresponding flattened photons for both 6X and 10X for different FS ranging from 2 × 2 cm(2) to 10 × 10 cm(2). The FFF photons have a higher entrance dose than that of the corresponding flattened photons for FS smaller than 10 × 10 cm(2). However, the difference is not substantial and may be clinically insignificant.

Chest wall radiotherapy with volumetric modulated arcs and the potential role of flattening filter free photon beams

The goal of the work was to assess the role of RapidArc treatments in chest wall irradiation after mastectomy and determine the potential benefit of flattening filter free beams. Planning CT scans of 10 women requiring post-mastectomy chest wall radiotherapy were included in the study. A dose of 50 Gy in 2 Gy fractions was prescribed. Organs at risk (OARs) delineated were heart, lungs, contralateral breast, and spinal cord. Dose-volume metrics were defined to quantify the quality of concurrent treatment plans assessing target coverage and sparing of OARs. Plans were designed for conformal 3D therapy (3DCRT) or for RapidArc with double partial arcs (RA). RapidArc plans were optimized for both conventional beams as well as for unflattened beams (RAF). The goal for this planning effort was to cover 100% of the planning target volume (PTV) with ≥ 90% of the prescribed dose and to minimize the volume inside the PTV receiving > 105% of the dose. The mean ipsilateral lung dose was required to be lower than 15 Gy and V(20 Gy) < 22%. Contralateral organ irradiation was required to be kept as low as possible. All techniques met planning objectives for PTV and for lung (3DCRT marginally failed for V(20 Gy)). RA plans showed superiority compared to 3DCRT in the medium to high dose region for the ipsilateral lung. Heart irradiation was minimized by RAF plans with ~4.5 Gy and ~15 Gy reduction in maximum dose compared to RA and 3DCRT, respectively. RAF resulted in superior plans compared to RA with respect to contralateral breast and lung with a reduction of ~1.7 Gy and 1.0 Gy in the respective mean doses. RapidArc treatment resulted in acceptable plan quality with superior ipsilateral tissue sparing compared to traditional techniques. Flattening filter free beams, recently made available for clinical use, might provide further healthy tissue sparing, particularly in contralateral organs, suggesting their applicability for large and complex targets.

Fidelity of dose delivery at high dose rate of volumetric modulated arc therapy in a truebeam linac with flattening filter free beams

The purpose of this study is to assess fidelity of radiation delivery between high and low dose rates of the flattening filter free (FFF) modes of a new all-digital design medical linear accelerator (Varian TrueBeam™), particularly for plans optimized for volumetric modulated arc therapy (VMAT). Measurements were made for the two energies of flattening filter free photon beams with a Varian TrueBeam™ linac: 6 MV (6 XFFF) at 400 and 1400 MU/min, and 10 MV (10 XFFF) at 400 and 2400 MU/min. Data acquisition and analysis was performed with both ionization chambers and diode detector system Delta4, for square radiation fields and for 8 VMAT treatment plans optimized for SBRT treatment of lung tumors. For the square fields, a percent dose difference between high and low dose rate of the order of 0.3-0.4% for both photon energies was seen with the ionization chambers, while the contribution to the difference from ion recombination was found to be negligible. For both the VMAT and square-field deliveries, the Delta4 showed the same average percent dose difference between the two dose rates of ~0.8% and ~0.6% for 10 MV and 6 MV, respectively, with the lower dose rate values giving the greater measured dose compared to the high dose rate. Thus, the VMAT deliveries introduced negligible dose differences between high and low dose rate. Finally, reproducibility of dose measurements was good for both energies.

MO‐D‐BRB‐11: Evaluation of the Accuracy of Eclipse AAA Modeling for Flatten Filter Free Beam Used for SBRT

Purpose: To evaluate the Eclipse analytical anisotropic algorithm (AAA) modeling of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, TruebeamSTx, TM, used for Gated SBRT. Materials and Methods: With the new Varian TruebeamSTx, Eclipse AAA was configured by using the measured dosimetric data. The TruebeamSTx can operate 6 and 10 MV photon beams free of the flattening filter (FFF). FFF photon beams are especially suitable for use of SBRT since they can operate at high dose rate (1400MU/min for 6MVFFF, 2400MU/min for 10MVFFF). Gated IMRT, RapidArc and gated RapidArc have routinely been used for SBRT, using the FFF photon beams. The accuracy of treatment plans with field sizes as small as 1×1 sq. cm are evaluated by measurement performed using films, ion chamber, and also with a Delta4 measurement system (ScandiDos). For gated treatments, measurements were done on a Respiratory Gating Platform (Standard Imaging) with motion simulating the actual patient breathing pattern. Results: The dosimetric leaf gap of the HD120MLC was adjusted to 1.7 mm with leaf transmission set to 0.015 so the calculated isodose distribution has the best fit with the measurement. The comparison between Eclipse and measured isodose distribution show an agreement of better than 95% using a gamma index of (3%/3mm). For gated RapidArc, the difference of FWHM of the measured and calculated profiles is found to be less than 0.2 mm with target size larger than 3 cm in diameter, and less than 1 mm for 1 cm diameter target size. Conclusions: The Eclipse AAA (V8.9) is found to be within 1mm accuracy for target volume as small as 1 cm in diameter. No major difference of the isodose distribution produced between the WFF and the FFF beams.

Current status and future perspective of flattening filter free photon beams

Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook. A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection. The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance. New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.

SU-GG-T-271: Dosimetric Properties of Flattening Filter Free Photon Beams from a New Clinical Accelerator

Purpose: To determine the dosimetric properties of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, Trilogy MX. Materials and methods: A new Varian linac consists of 6, 10 and 15 MV photon beams equipped with flattening filters (WFF), and 6 and 10 MV photon beam free of the flattening filter (FFF). Dosimetric data including depth dose, profiles, output factors, phantom scatter factors, and surface doses operated under the FFF mode were examined. Measurements of leaf transmission, leakage radiation, and off axis beam quality were also performed. Treatment plans using the FFF beams were evaluated to determine differences between WFF and FFF beams. Results: Depth of dose maxima for photon beams with WFF and FFF were compared. It was found that for FFF beams, dmax was shallower. The PDDs for the FFF beams were found to be lower than WFF beams. Surface doses were found to be larger for FFF. The flatness for a 10×10 cm2 field is 6.1 % (FFF) versus 2.5 % (WFF) for a 6MV at a depth of 10 cm. Sc,p were found to be higher for FFF beams for field sizes < 10×10 cm2, but lower otherwise. No significant dose distribution differences were observed between IMRT treatment plans using the FFF or WFF beams. Conclusions: The results indicate that the physical characteristic of the FFF photon beams are not dramatically different from the WFF photon beams, except for uniformity across the field. However, FFF mode does result in a softer beam due to reduced beam hardening. The increased dose rate for FFF beams will reduce treatment time. Conflicts of interest: Research funding from Varian Medical Systems.

SU-FF-T-70: A Treatment Planning Study of Liver Cancer Treatments with a Flattening Filter Free Linear Accelerator

Purpose: Our preliminary studies have shown that the removal of the flattening filter from the beam‐line of a medicallinear accelerator increases the dose rate, reduces head scatter and reduces dose outside the treatment volume. Although, in principle, the filter is unnecessary when a multileaf collimator is present, it remains to be seen whether treatments with unflattened beams are clinically feasible. In this study we investigate the application of flattening filter free photon beams for the treatment of small livercancertumors to determine if these beams offer an advantage for respiratory‐gated radiotherapy. Methods and Materials: This is a treatment planning study. We created plans using Eclipse 8.0 (Varian Medical Systems), which we commissioned with measured (6 MV, 18 MV) and Monte Carlo (10 MV) data for a Clinac 21EX linear accelerator, operated with and without a flattening filter. We selected several conventional 3D conformal treatment plans for flattened beams with field sizes not exceeding 10 cm × 10 cm. These were then compared with treatment plans, including 3D conformal plans and IMRT plans, developed with unflattened beams. Results:Dose distributions for treatment volumes and nearby critical structures were typically equivalent for the intensity‐modulated plans with flattened and unflattened beams. The number of monitor units required to deliver the same dose was much lower in the plans with unflattened beams. For example, at 18 MV the total number of monitor units in the intensity‐modulated plans with unflattened beams was about 1/2 of those in the conformal 3D plans with flattened beams. Conclusion: In this study we show that clinically acceptable treatment plans for livercancer can be obtained with unflattened beams. With respiratory‐gated radiotherapy, the high dose rates achieved with unflattened beams may shorten the overall treatment time dramatically. Conflict of Interest: Research sponsored by Varian Medical Systems.