investigation of the field size effect on wedge field isodose curves angle for two energies; 6 & 18 mv, produced by varian 2100c linac

introduction: nowadays,  considerable developments  in  the field  of  radiotherapy have  been  achieved.  they  include  the  advances  made  in  the  equipments  and  treatment  planning  techniques  which  require  highly complex calculations. such achievements have made it possible to treat cancer patients not only  with  higher  radiation  dose  but  also  with  higher  precision  and  consequently  increasing  the  chance  of  curing the cancer. however, the conventional techniques requiring physical wedge are still being used  but with a lesser frequency. one of the wedge parameters needed to be measured is the wedge angle. it is  the angle that the horizontal line creates with the tilted isodose curve at a specific depth and for a certain  field size.   in this study, the variation of wedge angle for different field sizes was evaluated using dosimetric and  mathematical method.  material and methods: for the wedge fields with a dimension of 6×6 to 20×20 cm 2 , the wedge angle  for  two  photon  energies  of  6  and  18  mv  was  measured  by  the  dosimetric  method.  for  these  measurements, the conventional wedges having the nominal wedge angle of 15, 30, 45 & 60 were used.  the theoretical method suggested by saw et al. is also used to indirectly calculate the slope of isodose  curve  by  the  dose  profile  and  percent  depth  dose  data.  the  dose  profile,  percentage  depth  dose  and  isodose curves were drawn for all the field sizes and the tilt of isodose curve at 10 cm depth, according to  international definition, is considered as the wedge angle. the data were obtained using the theoretical  equation of wedge angle and it was compared to the dosimetric data.  results: the result obtained in this work shows that the wedge angle increases with the field size. for a  6×6 cm 2 field size, the calculated wedge angle has the highest difference in comparison to the nominal  wedge angle. the difference is equal to 14.7 degree for a 45° wedge and a 6 mv photon. the highest  difference for a 45° wedge angle, a field size of 10×10 cm 2 and a 6 mv photon is 9.2 degree. comparing  the calculated and measured wedge angles shows a maximum difference of 4 degree for 6 and 18 mv  photon beams.  discussion  and  conclusion:  the  wedge  angle  varies  with  field  size.  in  order  to  get  a  better  dose  distribution in the conventional radiotherapy, it is necessary to use the appropriate wedge angle which  generates the desired slope for the isodose line and for the specific field size.