Monte Carlo Study Of Dosimetric Properties Of Flattening Filter Free6MV X Ray From Trilogy Linear Accelerator

The flattening filter is an important part of the conventional medcial linear accelerator.The flattening filter modified the original non-uniform dose distribution into thehomogeneous dose distribution to meet the demands of conventional radiotherapy andintensity-modulated radiotherapy when the X ray passed. For intensity-modulated radiationtherapy treatments, optimal fluence maps produced by planning systems typically are not flatand optimal treatments,in principle,can be delivered with the multileaf collimator (MLC)movement. Actually, the MLC modulated the fluence produced by the accelerator fromuniform into non-uniform in this process. Of course,the MLC also could modulated theoriginal non-uniform fluence into non-uniform which was the intensity-modulated radiationtherapy need. In order to obtain the original non-uniform distribution fluence produced by theaccelerator, the flattening filter can be removed, then there will be a huge impact on thecharacteristics of the X ray. With regards to this, in this shudy the dosimetric properties of6MV X ray from Trilogy linear accelerator with and without the flattening filter were simulatedusing the Monte Carlo code and the impact of the flattening filter on the dosimetric propertieswere discussed.A water phantom of50cm×50cm×50cm was built, calculated the depth dose（DD）,off-axis ratio（OAR） at1.5cm、5cm、10cm and20cm depths, total scatter factor (Sc,p)andthe central axis dose of the6-MV X ray from a Varian Trilogy linear accelerator with andwithout flattening filter by the Monte Carlo code BEAMnrc and DOSXYZnrc. The field sizesused in this study were1cm、2cm、3cm、4cm、6cm、8cm、10cm、12cm、15cm、20cm、25cm、30cm、and40cm. The photon energy spectrum was analyzed by theBEAMDP. The physical quantities of the two conditions have been compared.In this study, for the accelerator without flattening filter, the photon energy spectrummoved to the left and became softer, the photon fluence was increased. The depth dose wasincreased in the buildup area,and reduced past buildup area.The field flatness became worseand it was obvious with the increase of field sizes. The dose at the field edge became lower. The dose rate was significant inceased. The relationship between total scatter factors(Sc,p)and field sizes was not obvious.In summary，the output properties of the flattening filter free accelerator changedsignificantly and it may has some benefits or some unknown situations in clinical treatment.As well as needed to further research to support.