Characteristics of bremsstrahlung in electron beams

Abstract

Clinical electron beams contain an admixture of bremsstrahlung produced in structures in the accelerator head, in field-defining cerrobend or lead cutouts, 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 beams (energy 6-22 MeV, diameter 0-5 cm) using measurements in water and calculations. The results show that bremsstrahlung from the accelerator head dominates and increases with field size for electron beams generated by accelerators 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 the 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 effective 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, respectively. The lateral bremsstrahlung dose distribution is more forward-peaked for higher electron energy. The bremsstrahlung components could be described for any machine by a set of simple measurements and can be modeled by an analytical expression.

Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

“…The doses decrease exponentially with off‐axis distance and increase with energy, which can be understood from the characteristics of bremsstrahlung produced by impinging an electron beam on a target. Our measurements for in‐field bremsstrahlung doses at 10 cm depth on the Varian $2300\hspace{0.17em}\mathrm{C}/\mathrm{D}$ are in agreement with those reported by Zhu et al (14) …”

“…The bremsstrahlung photons can be produced in different structures of the accelerator head and in the patient irradiated volume (14) . The scattered electrons can travel through the applicator parts, or emanate from the scattering foils in air, without interacting with the different parts of the applicator (11) …”

Determinação da Dose dos Fótons Contaminantes de Feixes de Elétrons Clínicos usando o Método de Recozimento Simulado Generalizado

“…Fica claro que o conhecimento das componentes dos fótons de freamento, sobretudo a dose dos fó-tons contaminantes, é relevante para o modelamento dos feixes de elétrons clínicos no planejamento do tratamento 6 , toda vez que se garante uma dosimetria acurada do feixe de elétrons 5 . Nesse estudo, nós mostramos como o método de recozimento simulado generalizado é capaz de resolver o modelo semiempírico para a dose dos fó-tons contaminantes de maneira rápida e efetiva.…”

Simulation and experimental verificationof dose distributions of electron beams

“…Methods for separation of the electron and photon dose experimentally are described in (Zhu et al, 2001). From the measurements of the PDDs it is possible to determine the ratio of photon dose relative to the electron dose.…”

“…The head-generated bremsstrahlung contribution for a 9 MeV beam may reach 1.6% of the maximum electron dose, depending on the linac head configuration (Zhu et al, 2001). Within the procedure for spectral reconstruction the bremsstrahlung tail was subtracted using extrapolation of the measured data beyond the electron practical range.…”