MIRD pamphlet no. 15: Radionuclide S values in a revised dosimetric model of the adult head and brain

Current dosimetric models of the brain and head lack the anatomic detail ne eded to provide the physical data necessary for suborgan brain dosimetry. D uring the last decade, several new radiopharmaceuticals have been introduce d for brain imaging. The marked differences of these tracers in tissue spec ificity within the train and their increasing use for diagnostic studies su pport the need for a more anthropomorphic model of the human brain and head for use in estimating regional absorbed dose within the brain and its adja cent structures. Methods: A new brain model has been developed that include s eight subregions: the caudate nuclei, the cerebellum, the cerebral cortex , the lateral ventricles, the lentiform nuclei, the thalami, the third vent ricle and the white matter. This brain model is incorporated within a total revision of the head model presented in MIRD Pamphlet No. 5 Revised. Modif ications include the addition of the eyes, the teeth, the mandible, an uppe r facial region, a neck region and the cerebrospinal fluid within both the cranial and spinal regions. Results: absorbed fractions of energy for photo n and electron sources located in 14 source regions within the new model we re calculated using the EGS4 Monte Carlo radiation transport code for parti cles in the energy range 10 keV-4 MeV. These absorbed fractions were then u sed along with radionuclide decay data to generate S values for 24 radionuc lides that are used in clinical or investigational studies of the brain, 12 radionuclides that localize within the cranium and spinal skeleton and 12 radionuclides that selectively localize in the thyroid gland. Conclusion: A substantial revision to the dosimetric model of the adult head and brain o riginally published in MIRD Pamphlet No. 5 Revised is presented. This revis ion supports suborgan brain dosimetry for a variety of radiopharmaceuticals used in neuroimaging. Dose calculations for the neuroimaging agent I-123-t ropane provide an example of the new model and yield mean brain doses that are consistent with published values. However, the absorbed dose to subregi ons within the brain such as the caudate and lentiform nuclei may exceed th e average brain dose by a factor of up to 5.