Characteristics of bremsstrahlung in electron beams

Clinical electron beams contain an admixture of bremsstrahlung produced in structures in the accelerator head, in field-defining cerrobend or lead cut outs, and in the irradiated patient or water phantom. Accurate knowledge of these components is important for dose calculations and treatment planning . In this study, the bremsstrahlung components are separated for electron b eams (energy 6-22 MeV, diameter 0-5 cm) using measurements in water and cal culations. The results show that bremsstrahlung from the accelerator head d ominates and increases with field size for electron beams generated by acce lerators equipped with scattering foils. The bremsstrahlung from the field- defining cerrobend accounts for 10% to 30% of the total bremsstrahlung and decreases with increasing beam radius. The bremsstrahlung is softer than th e x-ray beams of corresponding nominal energy since the latter are hardened by the flattening filter. For the 6, 12, and 22 MeV electron beams, the ef fective attenuation coefficients in water for the bremsstrahlung are 0.058, 0.050, and 0.043 cm(-1). The depths of maximum dose at 100 cm SSD are 0.8, 1.7, and 3.0 cm. The position of the virtual source of the bremsstrahlung shifts downstream from the nominal source position by 20, 13, 5.6 cm, respe ctively. The lateral bremsstrahlung dose distribution is more forward-peake d for higher electron energy. The bremsstrahlung components could be descri bed for any machine by a set of simple measurements and can be modeled by a n analytical expression. (C) 2001 American Association of Physicists in Med icine.