Monte Carlo study of a flattening filter-free 6 MV photon beam using the BEAMnrc code. Ankit Kajaria 1* , Neeraj Sharma 1 , Shiru Sharma 1 , Satyajit Pradhan 2 , Abhijit Mandal 2 , Lalit Aggarwal M 2 1 School of Biomedical Engineering, Indian Institute of Technology (BHU) Varanasi, UP, India 2 Department of Radiotherapy and Radiation Medicine, Institute of Medical Science (BHU), Varanasi, UP, India Abstract Flattening Filter Free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this study is to evaluate the basic dosimetric properties of a flattening filter free 6 MV photon beam. A Monte Carlo simulation model was developed for a 6 MV photon beam based on Varian Clinic 600 unique performance linac operated with/without a flattening filter and dosimetric features including central axis absorbed dose, beam profiles and photon and electron spectra were calculated for flattened and unflattened cases separately. Absolute depth dose calculations showed an increase in dose rate with a factor of more than 2.4 for the unflattened 6 MV photon beam which is dependent on the depth. Percentage Depth Doses (PDDs) values were found to be lower for unflattened beam for all field sizes. The total Scatter Correction Factor (SCP) were found to have less variation with field sizes for unflattened beam indicating that removing of the filter from the beam line can reduce significant amount of head scatter. However surface doses were found to be higher for the unflattened beam due to more contamination electrons and low energy photons in the beam. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV photon beams. Keywords: Unflattened photon beam, Flattening filter, Monte Carlo. Accepted on August 18, 2016 Introduction Conventional clinical accelerators are equipped with a Flattening Filter (FF) which is primarily designed to produce a flat beam profile at a given depth by compensating for the non- uniformity of photon fluence across the field. But flattening filter decreases the X-ray output considerably and produces quality changes within the primary beam by scattering and absorption of primary photons. The requirement to have a flattened beam profile for treatment delivery is no longer necessary when certain type of treatments such as intensity- modulated radiation therapy or intensity-modulated arc therapy is used. In Intensity Modulated Radiation Therapy (IMRT), the patient dose distribution can instead be shaped by the Multileaf Collimator (MLC) to create the desired clinical effect. In principle, the flattening filter can then be removed, and the leaf sequences can be adjusted accordingly to produce fluence distributions similar to those of a beam with a flattening filter. Removal of flattening filter with its associated attenuation from x-ray beam path increases dose rate [1]. The other possible effect is substantial reduction in head scatter, as the flattening filter is the major source of scattered photons. Flattening Filter Free (FFF) beams in radiotherapy thus have the advantage of shorter treatment delivery time and lower out-of field dose compared with conventional flattened beams. This is especially important where large doses per fraction are prescribed, e.g., stereotactic ablative body radiotherapy [2,3] or where patient motion might affect the efficacy of the delivery or both [4]. Monte Carlo (MC) method has become a powerful tool in radiotherapy dose calculations and many studies have been conducted using this method for analysing linac head components and influencing factors on beam characteristics [5-7] .Thus, the effect of flattening filter on photon energy spectra, absolute absorbed dose per initial electron and beam profiles could be studied by this method [8]. In an Monte Carlo (MC) study on Flattening filter free beams, dose rates increase by a factor of 2.31 (6 MV) and 5.45 (18 MV) and out-of-field dose reductions were reported [9]. In a similar study, a significant improvement in out-of-field dose was reported for small field sizes [10]. Above studies outline the potential benefits of removing the flattening filter. It is therefore important to investigate these properties for a typical modern accelerator such as the Varian Clinic 600 unique performance. This study reports on depth-dose dependencies, dose rates, lateral profiles, out-of-field doses, total scatter factors, and photon and electron fluence in a conventional accelerator and a flattening filter-free system. ISSN 0970-938X www.biomedres.info Biomed Res- India 2017 Volume 28 Issue 4 1566 Biomedical Research 2017; 28 (4): 1566-1573