Beam quality specification for photon beam dosimetry.

It is argued that %dd(10), the percentage depth dose at 10 cm in a 10 x 10 cm2 photon beam at a SSD of 100 cm, is a better beam quality specifier for radiotherapy beams than the commonly used values of TPR10(20) or nominal accelerating potential (NAP). For radiation dosimetry purposes, TPR10(20) is not an ideal beam quality specifier because (i) stopping-power ratios for the same value of TPR10(20) can vary by up to 0.7% for thick-target bremsstrahlung beams; (ii) the value of TPR10(20) becomes insensitive to beam quality changes for high-energy beams; and (iii) it has little intuitive meaning. In contrast, %dd(10) in a pure photon beam specifies stopping-power ratios within 0.2% for all thick-target bremsstrahlung beams, maintains its sensitivity for high-energy beams, and has a simple physical and clinical meaning. It is shown that for all thick-target bremsstrahlung beams the spr (water/air) = 1.2676-0.002 224[%dd(10)] with a rms deviation of 0.1%. The effects of electron contamination in typical high-energy clinical beams can be corrected for using previously published experimental results or by reducing electron contamination using lead scattering foils.