Mass-energy absorption coefficient and backscatter factor ratios for kilovoltage x-ray beams

For low-energy (up to 150 kV) x-rays, the ratio of mass-energy absorption c oefficients for water to air, (<(mu)over bar>(en)/rho)(w,air), and the back scatter factor B are used in the conversion of air kerma, measured free-in- air, to water kerma on the surface of a water phantom. For clinical radioth erapy, similar conversion factors are needed for the determination of the a bsorbed dose to biological tissues on (or near) the surface of a human body . We have computed the <(mu)over bar>(en)/rho ratios and B factor ratios fo r different biological tissues including muscle, soft tissue, lung, skin an d bone relative to water. The <(mu)over bar>en/rho ratios were obtained by integrating the respective mass-energy absorption coefficients over the in- air primary photon spectra. We have also calculated the <(mu)over bar>(en)/ rho ratios at different depths in a water phantom in order to convert the m easured in-phantom water kerma to the absorbed dose to various biological t issues. The EGS4/DOSIMETER Monte Carlo code system has been used for the si mulation of the energy fluence at different depths in a water phantom irrad iated by a kilovoltage x-ray beam of variable beam quality (HVL: 0.1 mm Al- 5 mm Cu), field size and source-surface distance (SSD). The same code was a lso used in the calculation of the B factor ratios, soft tissue to water an d bone to water. The results show that the B factor for bone differs from t he B factor for water by up to 20% for a 100 kV beam (HVL: 2.65 mm Al) with a 100 cm(2) field. On the other hand, the difference in the B factor betwe en water and soft tissue is insignificant (well within 1% generally). This means that the B factors for water may be directly used to convert the 'in- air' water kerma to surface kerma for human soft tissues.