Investigation of fetal absorbed dose in V/Q scan in three trimesters of pregnancy using Monte Carlo simulation

Abstract

The ventilation/perfusion (V/Q) single-photon emission computed tomography is the first method of diagnosis for pulmonary embolism in pregnant women. This study aimed to calculate the fetal absorbed dose and compare to recommended values in V/Q scan at three trimesters of pregnancy by Monte Carlo simulation (code MCNPX) using simulated phantoms, based on the adult female MIRD phantom. The collection of pregnant women phantoms (that of Stabin) was designed with changes in the MIRD phantom. Source organs were defined for each of the radiopharmaceuticals used in two scans, Xe-133 and Kr-81m for the lung and bladder and technetium diethylene-triamine-pentaacetate (Tc-99m-DTPA) aerosol for lung ventilation scan. Also, technetium macroaggregated albumin (Tc-99m-MAA) for lung ventilation scan, lung, bladder, and liver. Fetal absorbed dose was calculated and evaluated for the administration radiopharmaceuticals using the MCNP simulation output. For 200 MBq Tc-99m-MAA, fetal absorbed dose was 1.01-1.97 mGy, which is higher than the values recommended by International Commission on Radiological Protection (ICRP). The same fetal absorbed dose was found for activities of 54 and 70 MBq in the third trimester. For Tc-99m-DTPA-aerosol, fetal absorbed dose as a ventilation tracer was within the permitted range. For Xe-133 and Kr-81m, it was negligible. It is concluded that the fetus received the highest absorbed dose in the third trimester of pregnancy. For this reason, in this period of pregnancy, it is recommended to use the lower administration activity and her awareness must be done.