Avaliação da dose fetal em gestantes com uso de protetores abdominais em tomografia de crânio via método de Monte Carlo

Abstract

In this study, simulations were performed using the MCNPX 2.7.0 code, allowing the estimation of the dose in the fetal dosimetric structures of interest for the cranial protocol of the computed tomography (CT) exam to which the pregnant patient was subjected. The results showed that the use of a shield over the patient's abdomen resulted in a dose reduction of more than 40% for all fetal structures, for both CT manufacturers evaluated. Although the simulated equipment had the same current (mA), collimation, pitch, and rotation time, they had different kVp values (130 kV for Siemens and 120 kV for GE) and distinct spectra. This high shield effectiveness occurs due to the low energy of the photons reaching the fetus, which originate from the secondary beam, resulting from scattering within the patient and the structures constituting the scenario. For the pregnant patient, there was no significant difference in the effective dose; only the structures covered by the shield showed a dose reduction of more than 10%. As expected, the highest absorbed doses were in structures exposed to the primary beam, which were within the clinical imaging objectives. The simulated patient was 24 weeks pregnant, weighed 63 kg, and was 1.63 meters tall, represented by the Katja phantom. For the fetus, when comparing both manufacturers, Siemens equipment deposited a dose approximately 3% higher than GE equipment. However, for the pregnant patient, the GE equipment deposited a higher dose when comparing the effective dose. This trend was observed in both the shielded and unshielded cases. The statistical errors of the simulation were within acceptable limits (less than 10%) and were higher in structures far from the primary beam and those with fewer voxels.