A method of estimating fetal dose during brain radiation therapy

Purpose: To develop a simple method of estimating fetal dose during brain r adiation therapy. Methods and Materials: An anthropomorphic phantom was modified to simulate pregnancy at 12 and 24 weeks of gestation, Fetal dose measurements were car ried out using thermoluminescent dosimeters. Brain radiation therapy was pe rformed with two lateral and opposed fields using 6 MV photons. Three sheet s of lead, 5.1-cm-thick, were positioned over the phantom's abdomen to redu ce fetal exposure. Linear and nonlinear regression analysis was used to inv estigate the dependence of radiation dose to an unshielded and/or shielded fetus upon field size and distance from field isocenter, Results: Formulas describing the exponential decrease of radiation dose to an unshielded and/or shielded fetus with-distance from the field isocenter are presented. All fitted parameters of the above formulas can be easily de rived using a set of graphs showing their correlation with field size. Conclusion: This study describes a method of estimating fetal dose during b rain radiotherapy, accounting for the effects of gestational age, held size and distance from field isocenter, Accurate knowledge of absorbed dose to the fetus before treatment course allows for the selection of the proper if radiation technique in order to achieve the maximum patient benefit with t he least risk to the fetus, (C) 1999 Elsevier Science Inc.