Acceptance criteria for flattening filter-free photon beam from standard medical electron linear accelerator: AERB task group recommendations

Abstract

Medical electron linear accelerators with the capability of generating unflat photon (flattening filter-free, FFF) beams are also available commercially for clinical applications in radiotherapy. However, the beam characteristics evaluation criteria and parameters are not yet available for such photon beams. Atomic Energy Regulatory Board (AERB) of India constituted a Task Group comprising experts from regulatory agency, advisory body/research and technical institutions, and clinical radiotherapy centers in the country to evolve and recommend the acceptance criteria for the flattening filter-free (FFF) photon beams. The Task Group thoroughly reviewed the literature and inputs of the manufactures/suppliers of the FFF linac and recommended a set of dosimetry parameters for evaluating the characteristics of the unflat photon beam. The recommendations included the evaluation of quality index, degree of unflatness, difference in percentage surface dose between flat and unflat photon beams, percentage depth dose at 10 cm depth, off-axis-ratios and radiation beam penumbra. The recommended parameters were evaluated for FFF photon beams generated by three different models of the linac, and it was observed that recommended evaluation methods are simple and easy to be implemented with the existing dosimetry and quality assurance infrastructure of the linac facilities of the radiotherapy departments. Recommendations were also made for periodic quality control check of the unflat photon beams and constancy evaluation in the beam characteristics.

Dosimetric properties of equivalent‐quality flattening filter‐free (FFF) and flattened photon beams of Versa HD linear accelerator

“…The beam data were confirmed to be within the manufacturer's specifications and also followed the National Task Group recommendations (15) for clinical operations. In the FFF mode, the quality‐matching process was done to achieve %dd(10) equivalent to those of the corresponding flattened beams at 10 cm depth for the 6MVFFF as well as 10MVFFF beam.…”

“…However, when the FFF profile is renormalized, as previously described, a region within a certain percentage of the field can be used to define a parameter for evaluating the off‐axis profile variation for both beam modalities 6 , 10 , 13 , 15 . The off‐axis profile variations of both flattened and FFF beams for different field sizes at changing depths were quantified by using the off‐axis ratio (OAR).…”

Commissioning of TrueBeam<sup>TM</sup> Medical Linear Accelerator: Quantitative and Qualitative Dosimetric Analysis and Comparison of Flattening Filter (FF) and Flattening Filter Free (FFF) Beam

“…Same definition cannot be applied to FFF beam. In order to determine the field size for FFF beam, the AERB Task Group recommended the procedure as follows [3]. Field size shall be defined by collimator settings only.…”

“…The Inflection point is located in h/2 on both sides of the beam profile. Separation between inflection points is defined as the geometrical field size for the FFF beam [3]. Field size for both FF and FFF beam is illustrated.…”

“…This was measured as per the recommended protocol in AERB Task Group; quantification of FFF beam is carried out based on lateral separation of the dose profile at 90%, 75%, and 60% from the central axis [3]. This value is recorded for all FFF beams.…”