Quantifying effects of lead shielding in electron beams: a Monte Carlo study

Lead shielding in contact with the patient's skin is often encountered in r adiotherapy with electron beams. The influence of the lead shielding on dos e distributions in the patient cannot fully be assessed using modern treatm ent planning systems. In this work the problem of quantifying the effect of lead shielding on dose distributions is addressed. Monte Carlo dose calcul ations were performed in a half-blocked water phantom shielded by lead, usi ng a realistic model for the fluence of an electron linear accelerator. Ele ctron beam energies of 6-20 MeV and lead thicknesses of 1-7 mm are used for 10 x 10 cm(2) and 5 x 5 cm(2) fields. The perturbation of the particle flu ence and dose distributions in water introduced by the lead shielding is qu antified. The effect of oblique electron beams on the dose perturbation is shown. A fictitious clinical example, the shielding of an eye in electron b eam treatment, is used to demonstrate the usefulness of Monte Carlo based t reatment planning algorithms that can incorporate the effects of lead shiel ding.