title: Effects of Siemens TT-D carbon fiber table top on beam attenuation, and build up region of 6 MV photon beam.
abstract: OBJECTIVE
  This study deals with Monte Carlo simulations of the effects which the 550 TXT carbon fiber couch can have on the relevant parameters of a 6 MV clinical photon beam in three field sizes.
  
  
  BACKGROUND
  According to the reports issued by the International Commission on Radiation Units and Measurements (ICRU), the calculated dose across a high gradient distribution should be within 2% of the relative dose, or within 0.2 cm of the isodose curve position in the target volume. Nowadays, the use of posterior oblique beam has become a common practice. It is clear that, in radiotherapy, the presence of the couch affects the beam intensity and, as a result, the skin dose.
  
  
  METHODS
  Firstly, Siemens linear accelerator validation for 6 MV photon beam was performed, and satisfactory agreement between Monte Carlo and experimental data for various field sizes was observed. Secondly, the couch transmission factor for the reference field size and depth was computed, and the skin dose enhancement by the couch was assessed.
  
  
  RESULTS
  The largest impact of the carbon fiber couch effect was observed for the 5 × 5 cm2field size. Such evaluation has not been reported for this couch before.
  
  
  CONCLUSIONS
  Despite providing minimal attenuation for the primary radiation, the assumption that carbon fiber couches are radiotranslucent is not valid, and the effects of couches of this type on the transmission factor, and on the skin dose should be carefully investigated for each field size and depth.
authors: Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran; Medical Physics Department, Reza Radiation Oncology Center, Mashhad, Iran.
authors: Asma
authors: A
authors: Sheykhoo
authors: Medical Physics Department, Reza Radiation Oncology Center, Mashhad, Iran.
authors: Sara
authors: S
authors: Abdollahi
authors: Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors: Mohammad Hadi
authors: MH
authors: Hadizadeh Yazdi
authors: Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran.
authors: Mahdi
authors: M
authors: Ghorbani
authors: Department of Medical Physics, Faculty of Medicine, Hamadan University of Medical Sciences, Hamadam, Iran; Department of Medical Physics, Royal Adelaide Hospital, Adelaide, South Australia, Australia.
authors: Mohammad
authors: M
authors: Mohammadi
keywords: 550 TXT carbon fiber couch; Beam attenuation; Monte Carlo simulation; Siemens linac; Skin dose