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Abstract: 26-1

26-1

Pediadric dose evaluation with new 18F-FDG TIAC values in MIRDcalc software

Authors:
Joao Vitor do Carmo Barbosa (CDTN - Centro de Desenvolvimento da Tecnologia Nuclear) ; Bruno Melo Mendes (CDTN - Centro de Desenvolvimento da Tecnologia Nuclear)

Abstract:

Positron emission tomography with 18F-FDG is a valuable procedure in detecting, mapping, and monitoring tumor lesions in pediatric patients [1]. Therefore, accurate dosimetry is essential for evaluating the absorbed doses in children’s organs and tissues due to the use of 18F-FDG. The ICRP 128, provides a compendium of information about absorbed dose in organs, effective dose and biokinetic data for various radiopharmaceuticals to adults and children aged 1, 5, 10 and 15 years [2]. However, applying adult biokinetic data, more specifically the time-integrated activity coefficient (TIAC), in pediatric dosimetry can lead to variations of doses [3], [4]. In the case of 18F-FDG, ICRP 128 provides TIAC values for brain, heart wall, lungs, liver, urinary bladder contents and other organs and tissues, with only the TIAC for urinary bladder contents is age-differentiated. It was observed that one of the main causes to variation in absorbed doses is the change in the organ-to-body mass ratio that occurs over the years, especially in the brain [3], [4]. For example, in newborns, the brain represents about 10% of body mass, while in adults it is about 2%. The objective of this study was to investigate the pediatric absorbed dose reported by MIRDcalc v1.21 software, comparing the use of TIAC provided by ICRP 128 and new TIAC adjusted by gender and age. The adjustment of TIAC values was done by the equation: Extrapolated TIACpediadric organ × (∑Extrapolated TIAC) ÷ ∑ICRP TIAC), where: Extrapolated TIACpediadric organ = [(organ masspediadric ÷ total masspediadric) ÷ (organ massadult ÷ total massadult)]. In the brain, for the new TIAC, the highest absorbed dose coefficients were for newborns, 3.63E-01 mGy.MBq-1 in males and 3.60E-01 mGy.MBq-1 in females. The greatest difference, however, was at 1 year, with 200% increase for males (1.84E-01 mGy.MBq-1) and 190% increase for females (1.77E-01 mGy.MBq-1). The heart wall makes a significant contribution in the early years of life: 7.38E-01 mGy.MBq-1 for newborns and 3.00E-01 mGy.MBq-1 for 1 year, which decreases over the years similar to the brain, but without markable differences for new TIAC value, the most discrepant was for males at 5 years with 26% reduction (1.44E-01 mGy.MBq-1). For lungs and liver, no significant variations were founded for the same gender and age. In other organs and tissues, there reductions around 15% to 20% in the absorbed dose coefficients in newborns, 1 and 5 years were found. This reduction became less pronounced with age, varying around 1% to 5% for 10 and 15 years. The urinary bladder wall will be better evaluated, since, unlike other organs, there were significant discrepancies between the doses provided by ICRP 128 and those calculated in the MIRDcalc with original TIAC. As perspective to future works, the authors intend to use Monte Carlo simulations with the new TIAC values and voxelized pediatric phantoms, available by ICRP 143, as they have more advanced morphological, anthropomorphic and anthropometric characteristics, in studies of evaluation of the risk/benefit of 18F-FDG for pediatric patients.

Acknowledgement: This work was supported by the Centro de Desenvolvimento da Tecnologia Nuclear - CDTN and by Brazilian Institute of Science and Technology for Nuclear Instrumentation and Applications to Industry and Health (INCT/INAIS), CNPq project 406303/2022-3.

Keywords:
 Computational dosimetry, Pediadric dose, 18F-FDG, TIAC, MIRDcalc