New formula for Calculation of Cobalt-60 Percent Depth Dose


1 Associate Professor, Medical Physics Dept., Ahwaz University of Medical Sciences, Ahwaz, Iran.

2 Ph.D. Student in Medical Physics, Mashhad University of Medical Sciences, Mashhad, Iran.


Introduction:  On  the  basis  of  percent  depth  dose  (PDD)  calculation,  the  application  of 
dosimetry  in  radiotherapy  has  an  important  role  to  play  in  reducing  the  chance  of  tumor 
recurrence. The aim of this study is to introduce a new formula for calculating the centeral axis 
percent depth doses of Cobalt-60 beam.  
Materials and Methods: In the present study, based on the British Journal of Radiology (BJR) 
table, nine new formulas are developed and evaluated for depths of 0.5 - 30 cm and fields of 
)4 4( × ) 45 45 ( × − cm2.    To  evaluate  the  agreement  between  the  formulas  and  the  table,  the 
average of the absolute differences between the values was used and the formula with the least 
average was selected as the best fitted formula. The Microsoft Excel 2000 and the Datafit 8.0 soft 
wares were used to perform the calculations.    
 Results: The results of this study indicated that one amongst the nine formulas gave a better 
agreement with the PDDs listed in the table of BJR. The new formula has two parts in terms of 
log (A/P). The first part as a linear function with the depth in the range of 0.5 to 5 cm and the 
other one as a second order polynomial with the depth in the range of 6 to 30 cm. The average of 
the differences between the tabulated and the calculated data using the formula ( Δ ) is equal to 
Discussion  and  Conclusion:  Therefore,  the  calculated  percent  depth  dose  data  based  on  this 
formula has a better ageement with the published data for Cobalt-60 source. This formula could be 
used to calculate the percent depth dose for the depths and the field sizes not listed in the BJR table.  


Main Subjects