R(50) AS A BEAM QUALITY SPECIFIER FOR SELECTING STOPPING-POWER RATIOSAND REFERENCE DEPTHS FOR ELECTRON DOSIMETRY

For electron beam reference dosimetry in radiotherapy, it is shown tha t by choosing the reference depth as d(ref)=0.6R(50)-0.1 cm, where R(5 0) is the half-value depth in centimeters, the Spencer-Attix water-to- air stopping-power ratio at d(ref) is given by ((L) over bar/rho)(air) (w)=1.2534-0.1487 (R(50))(0.2144). This is derived from data for ((L) over bar/rho)(air)(w) obtained from realistic Monte Carlo simulations for 24 clinical beams. The rms deviation of this expression from the M onte Carlo calculations is 0.16%, with a maximum deviation of 0.26%. T his approach fully takes into account the spectral differences between real electron beams of the same R(50) and allows an absorbed-dose cal ibration at a standards laboratory to be easily and accurately transfe rred to a reference clinical beam. Using a single parameter to specify ((L) over bar/rho)(air)(w), rather than the two parameters (R(50) and depth) needed when the reference depth is chosen as the depth of dose maximum, has the potential to greatly simplify electron beam dosimetr y protocols and allows the use of a similar formalism for photon and e lectron beam dosimetry. For use in converting a depth-ionization curve into a depth-dose curve, a somewhat less accurate but general express ion for ((L) over bar/rho)(air)(w) as a function of R(50) and depth is presented.