Monte Carlo study of fetal dosimetry parameters for 6 MV photon beam

Abstract

Background: Because of the adverse effects of ionizing radiation on fetuses, prior to radiotherapy of pregnant patients, fetal dose should be estimated. Fetal dose has been studied by several authors in different depths in phantoms with various abdomen thicknesses (AT). In this study, the effect of maternal AT and depth in fetal dosimetry was investigated, using peripheral dose (PD) distribution evaluations.

Methods: A BEAMnrc model of Oncor linac using out of beam components was used for dose calculations in out of field border. A 6 MV photon beam was used to irradiate a chest phantom. Measurements were done using EBT2 radiochromic film in a RW3 phantom as abdomen. The followings were measured for different ATs: depth PD profiles at two distances from the field's edge, and in-plane PD profiles at two depths.

Results: The results of this study show that PD is depth dependent near the field’s edge. The increase in AT does not change PD depth of maximum and its distribution as a function of distance from the field’s edge.

Conclusions: It is concluded that estimating the maximum fetal dose, using a flat phantom i.e. without taking into account the abdomen thickness, is possible. Also, an in-plane profile measured at any depth can represent the dose variation as a function of distance. However in order to estimate the maximum PD the depth of D_max in out of field should be used for in-plane profile measurement.