new formula for calculation of cobalt-60 percent depth dose

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 ( × − cm 2 .    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  0.3152.   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.