For more than 50 years attening filters have been inserted into the beam path oflinacs to produce a uniform energy fluence distribution of the photon beam and make it suitable for clinical use. Recently, linacs without flattening fifilter (Flattening FilterFree - FFF) are increasingly used in radiotherapy because of its benefifits, e.g. high dose rate (2000 MU/min), reduced scattered and leakage radiation. Hypofractionated radiotherapy is interested in the high dose rate of FFF beams to shorten the treatment delivery time (TDT) especially the FFF beams have acceptable flatness at small fifieldsizes. Radiotherapy techniques that deliver intensity-modulated beams (IMBs), e.g.Tomotherapy, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), deal with the non-uniformity of the FFF beam profifile and produce homogeneous dose to the target as FF beams do. Siemens modified the Artiste linac in order to enable photon beam delivery with and without a flattening fifilter. The VMAT version developed by Siemens for Artiste linacs as a novel radiation technique is a modulated arc therapy (mARC). mARC technique is available with single, double and multiple complete or partial arcs. The aims of the current study were the determination of the main characteristics of 7 MV and 11 MV FFF photon beams in comparison with their corresponding 6 MV and 10 MV FF photon beams from Artiste digital linacs. Furthermore, IMRT planning comparisons using FF and FFF photon beams were performed using an Oncentra planning system. The performance of various mARC techniques were estimated and compared with Step and Shoot (S&S) IMRT by using a RayStation planning system. The mARC plans created by FF and FFF beams were evaluated to know which technique is the best. All the treatment plans were created for simple and complex shaped target volumes. The treatment plans are compared using two parameters - plan quality and treatment effi ciency. In addition to the planning study, the plan quality assurance of IMRT and mARC plans were performed using two difffferent volumetric quality assurance devices, Delta4 and Octavius 4D. Removal of the flattening fifilter causes changes in the dosimetric features of photon beams. IMRT plans with and without flattening fifilter were clinically acceptable where both plans have similar quality. In comparison with IMRT-FF, IMRT-FFF plansrequire more MUs and for some clinical cases require longer TDT. mARC technique can deliver dose distributions that are comparable to S&S-IMRT and could be an alternative with a potential to improve the effi ciency of the IMRT treatment delivery.:Abstract Abbreviation list 1 Introduction 2 Theory 2.1 Linac head configuration 2.2 Flattening fifilter disadvantages 2.3 Flattening fifilter free beams 2.4 Intensity modulated radiation therapy 2.5 Multi leaf collimator 2.6 Step and shoot IMRT 2.7 Dynamic delivery 2.8 Intensity modulated arc therapy 2.9 Modulated arc therapy 2.10 Verification of IMRT and mARC treatment plans 3 Materials and Methods 3.1 Materials 3.1.1 Linear accelerator 3.1.2 Dosimetric tools 3.1.3 Dosimetric verification systems 3.1.4 Treatment planning systems 3.2 Methods 3.2.1 Dosimetric parameters of FF and FFF beams 3.2.2 Comparison of IMRT-FF and IMRT-FFF 3.2.3 mARC planning study 3.2.4 Planning comparison parameters 3.2.5 Dosimetric verification 4 Results 4.1 Dosimetric characteristics of FF and FFF beams 4.1.1 Dose rate 4.1.2 Dose profile 4.1.3 Depth dose curve 4.1.4 Dose in buildup region 4.2 Comparison of IMRT-FF and IMRT-FFF 4.2.1 Plan quality 4.2.2Treatment e ciency 4.3 mARC 4.3.1 Final gantry spacing (F.G.S) 4.3.2 mARC and IMRT 4.3.3 Comparison of mARC 10 MV FF and 11 MV FFF plans 4.3.4 Plan verifications 5 Discussion 5.1 Dosimetric parameters of FF and FFF beams 5.2 IMRT-FF and IMRT-FFF 5.3 mARC Summary Bibliography Selbstandigkeitserklarung Curriculum Vitae Acknowledgement