DOSYMMETRIC ESTIMATES OF99m Tc (MAA) and133 Xe IN NEWBORN LUNGS USING CRISTY-ECKERMAN/ SEGARS REPRESENTATIONS

Abstract

Using the Cristy-Eckerman (C-E)/Segars anatomical representations and the MIRD formalism, the Absorbed doses in lungs of newborn patients scanned with radiopharmaceuticals133 Xe (ventilation) and99m Tc (MAA) (perfusion) are estimated. These representations are phantoms used in Monte Carlo calculations to determine specific absorbed fractions, which, associated with the pharmaceutical residence time, determine the absorbed dose. Concerns about the dosimetric impact of using these ventilation/ perfusion agents, as well as the use of different phantoms, were explored in newborn patients. When the lungs were scanned with99m Tc (MAA), the relative difference in total dose between the C-E/Segars anatomical representations was 1.0 %. When the lungs were scanned with133 Xe, the relative difference in total dose between the anthropomorphic representations of C-E/ Segars was 0.5 %. Regardless of the radiopharmaceutical used for the pulmonary studies of a newborn patient, the substitution of the C-E representation for that of Segars does not reflect very significant changes in the calculation of the absorbed dose in the lungs, where the greatest dosimetric contribution is its self-dose, which is supplied mainly by the electrons produced during the99m Tc and133 Xe decay.