Radiotherapy for supradiaphragmatic Hodgkin's disease: determination of the proper fetal shielding conditions using Monte Carlo methodology.

This study aimed to estimate fetal dose from mantle field irradiation with 6 MV photons and to determine the proper fetal shielding conditions. The Monte Carlo N-particle code and mathematical phantoms representing pregnancy at the first, second and third trimesters of gestation were used to calculate fetal dose with or without the presence of a 5-cm-thick lead shield of dimensions 35×35 cm(2). Fetal exposure was calculated for lead thicknesses of 2, 3, 4, 6, 7 and 8 cm. The dependence of fetal dose upon the distance separating the shield from the beam edge and phantom's abdomen was investigated. Dose measurements were performed on a physical phantom using thermoluminescent dosimetry. The radiation dose to an unshielded and shielded fetus was 0.578-0.861% and 0.180-0.641% of the prescribed tumor dose, respectively, depending upon the gestational age. The lead thickness increase from 2 to 5 cm led to a fetal dose reduction up to 23.4%. The use of 5- to 8-cm-thick lead resulted in dose values differing less than 4.5%. The shift of the lead from the closer to the more distant position relative to the field edge increased fetal dose up to 42.5%. The respective increase by changing the distance from the phantom's abdomen was 21.9%. The difference between dose calculations and measurements at specific points was 8.3±3.9%. The presented data may be used for fetal dose assessment with different shielding settings before treatment and, then, for the design and construction of the appropriate shielding device.