A mathematical expression for %DD accurate from Co-60 to 24 MV.

Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean-square accuracy of approximately 1.0% versus measured values. The expression is applicable to beam energies ranging from Co-60 to 24 MV, field sizes from 4 X 4 to 40 X 40 cm2, and depths from 1 cm deeper than dmax to 30 cm. The only information required by this expression that is machine specific is the ionization ratio.