Five pediatric head and brain mathematical models for use in internal dosimetry

Mathematical models of the head and brain currently used in pediatric neuro imaging dosimetry lack the anatomic detail needed to provide the necessary physical data for suborgan brain dosimetry. To overcome this limitation, th e Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclea r Medicine recently adopted a detailed dosimetric model of the head and bra in for the adult. Methods: New head and brain models have been developed fo r a newborn, 1, 5, 10 and 15 y old for use in internal dosimetry. These mod els are based on the MIRD adult head and brain model and on published head and brain dimensions. They contain the same eight brain subregions and the same head regions as the adult model. These new models were coupled with th e Monte Carlo transport code EGS4, and absorbed fractions of energy were ca lculated for 14 sources of monoenergetic photons and electrons in the energ y range of 10 keV-4 MeV. These absorbed fractions were then used along with radionuclide decay data to generate S values for all ages for Tc-99m, cons idering 12 source and 15 target regions. Results: Explicit transport of pos itrons was also considered with separation of the annihilation photons comp onent to the absorbed fraction of energy in the calculation of S values for positron-emitting radionuclides. No statistically significant differences were found when S values were calculated for positron-emitting radionuclide s under explicit consideration of the annihilation event compared with the traditional assumption of a uniform distribution of 0.511-MeV photons. Conc lusion: The need for electron transport within the suborgan brain regions o f these pediatric phantoms was reflected by the relatively fast decrease of the self-absorbed fraction within many of the brain subregions, with incre asing particle energy. This series of five dosimetric head and brain models will allow more precise dosimetry of radiopharmaceuticals in pediatric nuc lear medicine brain procedures.