Flattening filter-free accelerators: a report from the AAPM Therapy Emerging Technology Assessment Work Group.

This report describes the current state of flattening filter-free (FFF) radiotherapy beams implemented on conventional linear accelerators, and is aimed primarily at practicing medical physicists. The Therapy Emerging Technology Assessment Work Group of the American Association of Physicists in Medicine (AAPM) formed a writing group to assess FFF technology. The published literature on FFF technology was reviewed, along with technical specifications provided by vendors. Based on this information, supplemented by the clinical experience of the group members, consensus guidelines and recommendations for implementation of FFF technology were developed. Areas in need of further investigation were identified. Removing the flattening filter increases beam intensity, especially near the central axis. Increased intensity reduces treatment time, especially for high-dose stereotactic radiotherapy/radiosurgery (SRT/SRS). Furthermore, removing the flattening filter reduces out-of-field dose and improves beam modeling accuracy. FFF beams are advantageous for small field (e.g., SRS) treatments and are appropriate for intensity-modulated radiotherapy (IMRT). For conventional 3D radiotherapy of large targets, FFF beams may be disadvantageous compared to flattened beams because of the heterogeneity of FFF beam across the target (unless modulation is employed). For any application, the nonflat beam characteristics and substantially higher dose rates require consideration during the commissioning and quality assurance processes relative to flattened beams, and the appropriate clinical use of the technology needs to be identified. Consideration also needs to be given to these unique characteristics when undertaking facility planning. Several areas still warrant further research and development. Recommendations pertinent to FFF technology, including acceptance testing, commissioning, quality assurance, radiation safety, and facility planning, are presented. Examples of clinical applications are provided. Several of the areas in which future research and development are needed are also indicated

Dosimetric effects of removing the flattening filter in radiotherapy treatment units

The aim of this work was to investigate the dosimetric effects of removing the flattening filter from conventional C-arm medical linear accelerators. In conventional linear accelerators used for radiotherapy, a flattening filter is positioned in the beam line to provide a uniform lateral dose profile at a specified depth in water. However, for some radiotherapy treatments, a uniform lateral dose profile is not necessary, e.g. stereotactic treatments with small fields or treatments with intensity modulated fields. In this work, a comprehensive set of measurements and Monte Carlo simulations for a modified Elekta Precise linear accelerator, operating with and without a flattening filter, were performed and the differences were evaluated. For an Elekta Precise linac, it was found that by removing the flattening filter the dose could be delivered approximately twice as fast as when the flattening filter is in the beam line, under certain conditions. The scatter produced in the treatment head was reduced by 30 %–45 % when the flattening filter was removed and the variation of scattered radiation with field size was also reduced. Removal of the flattening filter resulted in a softer photon energy spectra which leads to a steeper absorbed dose fall-off with depth and less lateral variation across the field. By increasing the acceleration potential of the linac, the depth–dose profiles become more similar to those of the equivalent conventional photon beam and thus the output will also be increased. The suitability of two beam quality measures, TPR20,10 and %dd(10)x, in predicting water to air mass collision stopping-power ratios sw,air for flattening filter-free photon beams was also investigated. These quality measures are used in reference dosimetry for the determination of absorbed dose in water. It was shown that the relationship between TPR20,10 and sw,air used in a current international code of practice for reference dosimetry, overestimates the stopping-power ratio by approximately 0.3 % for flattening filter-free photon beams, while the relationship between %dd(10)x and sw,air, used in the North American code of practice is more accurate. A new beam quality metric, consisting of both TPR20,10 and TPR10,5 was evaluated. It was found that this new beam quality specifier more accurately predicted stopping power ratios for flattening filter-free photon beams. A beam quality specifier defined by the first two moments (describing the mean and variance) of the spectral distribution was also investigated and found to accurately predict stopping-power ratios for beams without a flattening filter

Unflattened Radiotherapy beams; characterisation, optimisation and application

The goal of this thesis was to investigate flattening filter free (FFF) beams produced by medical linear accelerators for radiotherapy applications, from their initial setup through to their clinical implementation. This was split into four sections that comprise the main experimental chapters of this thesis. The characteristics of FFF beams both matched (by tuning beam quality to the equivalent cFF beam) and unmatched were compared to conventional flattening filter (cFF) beams. The characterisation of FFF beams highlighted inconsistencies with the current parameters used for the description and quality assurance (QA) of cFF beams. New methods suitable for the QA of both cFF and FFF beams were investigated and proposed. The use of Monte Carlo (MC) modelling was investigated to determine how to model an FFF beam and facilitate further investigations. Treatment planning studies were performed for lung stereotactic ablative radiotherapy (SABR), pelvic SABR and Head and Neck volumetric modulated arc therapy (VMAT). The planning work concluded that clinically acceptable plans were achievable through the use of FFF beams and provides a solid basis for clinical implementation. The work overall provides a comprehensive set of practical data and methods to support the use of FFF beams in clinical practice

The effects of backscattered radiation into beam monitor chamber: Study of 6 and 18 MV conventional and removed flattening filter clinical accelerator

In some linear accelerators (Linac), the collected charges in beam monitor chamber (BMC) is partly caused by the backscattered particles from the accelerator components downstream the BMC that influence the Linac output factors. In the intensity modulated radiation therapy technique, the desired dose distribution can be achieved through an unflattened beam. Although removing the flattening filter provides some advantages, the amount of backscatter radiation into BMC can be changed. In this study, contribution of backscattered particles into the BMC response of a Varian 2300 C/D Linac with and without a flattening filter was determined for 6, 18 MV photon beams. The experimental procedure included telescopic method and calculation procedure consisted of Monte Carlo simulation (MCNPX, version 2.4.0), were used to investigate the contribution of backscattered particles into the BMC performance. Our results showed a 2.3 and 3 increase in backscatter for a 0.5 � 0.5 cm2 field compared to a 40 � 40 cm2 field for 6 MV and 18 M V, respectively. The energy deposition from backscattered radiation is mainly caused by backscattered electrons. Removing the flattening filter did not change the BMC performance for a conventional Linac with a flattening filter. However, this result was not valid for small fields (e.g. 0.5 � 0.5 cm2, 18 MV). The corrected backscatter factors is necessary to taking into account the contribution of backscattered radiation in the monitor chamber response for small fields in the case of the free flattening filter Linacs (18 MV)

Calculated beam quality correction factors for ionization chambers in MV photon beams

The beam quality correction factor, , which corrects for the difference in the ionization chamber response between the reference and clinical beam quality, is an integral part of radiation therapy dosimetry. The uncertainty of is one of the most significant sources of uncertainty in the dose determination. To improve the accuracy of available data, four partners calculated factors for 10 ionization chamber models in linear accelerator beams with accelerator voltages ranging from 6 MV to 25 MV, including flattening-filter-free (FFF) beams. The software used in the calculations were EGSnrc and PENELOPE, and the ICRU report 90 cross section data for water and graphite were included in the simulations. Volume averaging correction factors were calculated to correct for the dose averaging in the chamber cavities. A comparison calculation between partners showed a good agreement, as did comparison with literature. The values from TRS-398 were higher than our values for each chamber where data was available. The values for the FFF beams did not follow the same , relation as beams with flattening filter (values for 10 MV FFF beams were below fits made to other data on average by 0.3%), although our FFF sources were only for Varian linacs.Peer reviewe

Absorbed dose in the build-up region in modern megavoltage photon radiotherapy

In megavoltage photon radiotherapy, accurate knowledge of the absorbed dose in the build-up region is essential in the clinical management of skin toxicity and superficial target coverage. These build-up doses strongly depend on a wide range of treatment parameters and evolve, consequently, in close relation with treatment technology. Commercial treatment planning systems and multiple dosimetric techniques, however, fail to accurately predict buildup doses. In this respect, this dissertation investigated the impact of recent evolutions in megavoltage photon radiotherapy on the absorbed dose and calculation accuracy in the build-up region. To that purpose, we first compared and optimized the performance of two transmission scanners for radiochromic film dosimetry in the high dose gradient of the build-up region. The Nikon Coolscan 9000ED scanner (Nikon Co., Tokyo, Japan) was identified as the preferential tool for high-gradient dosimetry, mainly due to its high sensitivity resulting in more contrast in the digitized image. The main limitation of the Nikon digitizer is its film size restriction to 6.2x20 cm². Secondly, we focussed on flattening filter-free linear accelerators, which currently face swift implementation into clinical practice because of their increased dose rate and reduction in head scatter compared to conventional designs. Our study demonstrated that build-up doses in flattening filter-free beams provided by standard linear accelerators slightly exceed build-up doses in conventional beams, indicating that the shift of the X-ray spectrum to lower energies in flattening filter-free mode is nearly compensated by the reduction of electron contamination and head scatter. In literature, the reduction in electron contamination in flattening-filter free mode is often anticipated to improve build-up calculation accuracy, as treatment planning systems generally use simple electron contamination models. For the XVMC code implemented in the Monaco treatment planning system however, considerable differences between build-up measurements and calculations were detected in both flattened and unflattened beams, emphasizing the importance of other factors that negatively affect build-up calculation accuracy. Thirdly, this dissertation investigated the dosimetric impact of patient positioning devices, which are increasingly used to ensure set-up accuracy while delivering highly conformal dose distributions. The presence of the components of the AIO patient positioning device (Orfit Industries, Wijnegem, Belgium) was found to cause considerable and clinically relevant build-up dose increases. Integration of the device CT data into treatment planning allowed to accurately model the most important attenuation effects, but failed to accurately predict build-up doses. Finally, we investigated the potential of modern volumetric arc techniques (VMAT) to intentionally reduce the superficial dose to the hair follicles and prevent temporary alopecia in whole brain radiotherapy. Compared to the standard opposed fields techniques, VMAT was shown to considerably reduce the subcutaneously absorbed dose (20.5% on average), mainly on the top and the back of the skull (41.8% on average). Accordingly, the mean hair loss in the study subjects in a prospective phase II trial was the least pronounced on the top and the back of the skull