Dosimetric dependence of the dimensional characteristics on a lead shield in electron radiotherapy: a Monte Carlo study

This study investigates the dosimetric dependence of the dimension of a lead (Pb) layer for shielding using clinical electron beams with different energies. Monte Carlo simulations were used to generate phase space files of the 4, 9 and 16 MeV electron beams produced by a Varian 21 EX linear accelerator using the EGSnrc-based BEAMnrc code, and validated by measurements using films. Pb layers with different thicknesses (2, 4, 6 and 8 mm) and diameters (2.5, 3, 3.5 and 4 cm) were placed at the center of an electron field on a Solid Water phantom. Beam profiles were determined at the depth of maximum dose (d(m)) using Monte Carlo simulations. The line doses under the Pb layer at d(m) including the penumbra at the edge of the layer and relative dose at the central beam axis (CAX) were studied with varying thickness and diameter of the layer. It is found that 2 mm of Pb layer is adequate to provide 5 half value layer (HVL) attenuation for the 4 MeV electron beams, and the beam profiles at dm under the Pb layer depend on the diameter but the thickness of the Pb. However, for the 9 and 16 MeV electron beams, the relative dose at the CAX and dm depends on both the thickness and diameter of the Pb layer. For 8 mm thickness of Pb, 4 and 5 HVL attenuation of electron beams with energies of 9 and 16 MeV can be achieved at d(m), respectively. Moreover, the beam profile under the Pb layer at dm depends on (1) the penumbra region at the edge of the Pb layer, (2) the beam attenuation varying with the thickness of the Pb layer, (3) the electron side scatter contributing to the CAX under the Pb layer, and (4) the photon contamination produced by the Pb layer. A parameter called "shielding area factor", defined as the ratio of the length between two points of 50% relative doses in the beam profile at dm to the diameter of the Pb layer, is suggested to the radiation oncology staff to predict the required size and thickness of Pb for shielding a target with known dimension at d(m). The dosimetric data calculated by Monte Carlo simulations in this study is useful to select what thickness and size of Pb are suitable for the protection of critical tissue in electron radiotherapy.