Investigation of the Field Size Effect on Wedge Field Isodose Curves Angle for Two Energies; 6 & 18 MV, produced by VARIAN 2100C Linac

Document Type: Original Paper

Authors

1 Associated Professor, Medical Physics & radiotherapy Dept., Golestan Hospital, Ahwaz University of Medical Sciences, Ahwaz, Iran

2 Professor, Medical Physics Dept., Ahwaz University of Medical Sciences, Ahwaz, Iran

3 Assistant Professor, Radiotherapy & Oncology Dept., Golestan Hospital, Ahwaz University of Medical Sciences, Ahwaz, Iran

4 M.Sc. Medical Physics Dept., Ahwaz University of Medical Sciences, Ahwaz, Iran

5 M.Sc. Medical Physics Dept., Golestan Hospital, Ahwaz University of Medical Sciences, Ahwaz, Iran

Abstract

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.  

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