Evaluation of the Effects of Different Filters and Helium Bag on the Reduction of Electron Contamination in Photon Beam of Neptun Linac

Introduction:  Skin  sparing  is  one  of  the  most  desirable  characteristics  of  high  energy  photon  beams. However, the photons emerging from the target of linacs are contaminated by secondary  electrons  as  a  result  of  their  interactions  with  air,  collimators,  flattening  filter  and  any  other  objects in their path. This phenomenon tends to increase the skin dose received by the patients. A  practical and simple way to reduce the contribution of electron contamination is to place a sheet of  medium  to  high  Z  material  just  after  the  secondary  collimator.  In  this  study,  filters  having  different thickness and atomic number were applied and their effectiveness on the reduction of  skin dose was evaluated.  Materials and Methods: The filters of different thickness and atomic number were applied. The  percent depth dose values were determined by the direct measurements made in a Scanditronix  water phantom using a PTW 31006 Pin Point chamber having a sensitive volume of 0.015 cm3. A  Perspex  filter  holder  was  made  to  be  installed  on  the  accessory  slot.  A  plastic  bag  containing  helium  was  also  made  using  thin  plastic  sheet  to  study  the  effect  of  the  helium  bag  when  it  replaces the air column between the head of the linac and the phantom. All of the measurements  were  carried  out  for  the  three  field  sizes  of  10×10,  20×20  and  25×25  cm2.  The  setups  were  adjusted for SSD = 100 cm. The ratio of the surface dose to maximum dose (Ds) was used as the  criterion to determine the optimum filter.   Results: The dosimetry results obtained in the water phantom indicated that a 0.4 mm thick Pb  filter is the most effective one. This filter reduces the Ds for the field sizes of 10×10, 20×20 and  25×25 cm2 by 5.7, 7.9 and 9.6%, respectively. Also the simultaneous use of the optimum filter and  He bag is more effective than the filter alone. It reduces the Ds by 6.3, 10.1 and 12.3% for the  field sizes of 10×10, 20×20 and 25×25 cm2, respectively.  Discussion and Conclusion:  Based on the results of this work it is evident that the contribution  of contaminant electrons to dose from the air column between the head and the phantom is much  smaller than it from the secondary electrons arising from the head of the linac. On the other hand,  the electron contamination originating from the air column is almost independent of the field size.  But  the  surface  dose  arising  from  the  secondary  electrons  produced  by  the  head  of  the  linac  depends on the field size, which is increased by increasing the field size.