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