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

Abstract

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.PACS number: 87.53.‐j, 87.53.Bn, 87.53.Ly, 87.55.‐x, 87.55.N‐, 87.56.bc

A novel, yet simple MLC‐based 3D‐crossfire technique for spatially fractionated GRID therapy treatment of deep‐seated bulky tumors

“…Even with relatively shorter beam on times, but similar overall treatment time compared to traditional GRID‐block, our 3D‐MLC crossfire plans could potentially be delivered using image‐guidance procedure. Furthermore, this time can be reduced by using recently adopted flattening filter free (FFF) beams for 3D‐MLC plans. With the use of FFF‐beam for 3D planning, the instantaneous dose rate has increased by approximately a factor of 2.33 with 6X‐FFF beam (1400 MU/min).…”

“…Since MLCs are integral parts of each medical linear accelerator (by now), our technique can be easily adopted to other small radiotherapy clinics with less extensive physics or machine support for GRID therapy patients. Our future work includes the following: generating an MLC‐based GRID template in Eclipse for automation, prospectively quantifying the therapeutic gain and treatment related toxicity by escalating tumor dose to the deep‐seated tumors and potentially using DIBH with FFF‐beams in the management of tumor motion for the MLC‐based GRID therapy patients. Moreover, the potential use of the 3D‐MLC crossfire approach for highly irregular GRID targets will be explored.…”

“…26 Even with relatively shorter beam on times, but similar overall treatment time compared to traditional GRID-block, our 3D-MLC crossfire plans could potentially be delivered using image-guidance procedure. Furthermore, this time can be reduced by using recently adopted flattening filter free (FFF) beams 31,32 for 3D-MLC plans.…”

Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory optimization model

“…In addition to the dose rate increase, it was concluded that FFF beams offer some dosimetric advantages compared to FF beams, such as lower out‐of‐field dose, lower head scatter magnitude, and less spectrum variation for various field sizes . Those features can help simplify the beam model and ultimately improve dose calculation accuracy …”

Monte Carlo Simulation of 6 MV Flattening Filter Free Photon Beam of True Beam STx LINAC at Songklanagarind Hospital

“…Flattening filter in a medical LINAC is used to homogenize beam profiles from the photon beam at a patient or water phantom (Mayles et al 2007). However, in modern radiotherapy techniques such as Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Radiotherapy (VMAT), the flattening filter was removed in order to increase dose rate and reduce the treatment time (Huang et al 2012;Xiao et al 2015).…”