Internal photon and electron dosimetry of the newborn patient--a hybrid computational phantom study.

Estimates of radiation absorbed dose to organs of the nuclear medicine patient are a requirement for administered activity optimization and for stochastic risk assessment. Pediatric patients, and in particular the newborn child, represent that portion of the patient population where such optimization studies are most crucial owing to the enhanced tissue radiosensitivities and longer life expectancies of this patient subpopulation. In cases where whole-body CT imaging is not available, phantom-based calculations of radionuclide S values--absorbed dose to a target tissue per nuclear transformation in a source tissue--are required for dose and risk evaluation. In this study, a comprehensive model of electron and photon dosimetry of the reference newborn child is presented based on a high-resolution hybrid-voxel phantom from the University of Florida (UF) patient model series. Values of photon specific absorbed fraction (SAF) were assembled for both the reference male and female newborn using the radiation transport code MCNPX v2.6. Values of electron SAF were assembled in a unique and time-efficient manner whereby the collisional and radiative components of organ dose--for both self- and cross-dose terms--were computed separately. Dose to the newborn skeletal tissues were assessed via fluence-to-dose response functions reported for the first time in this study. Values of photon and electron SAFs were used to assemble a complete set of S values for some 16 radionuclides commonly associated with molecular imaging of the newborn. These values were then compared to those available in the OLINDA/EXM software. S value ratios for organ self-dose ranged from 0.46 to 1.42, while similar ratios for organ cross-dose varied from a low of 0.04 to a high of 3.49. These large discrepancies are due in large part to the simplistic organ modeling in the stylized newborn model used in the OLINDA/EXM software. A comprehensive model of internal dosimetry is presented in this study for the newborn nuclear medicine patient based upon the UF hybrid computational phantom. Photon dose response functions, photon and electron SAFs, and tables of radionuclide S values for the newborn child--both male and female--are given in a series of four electronic annexes available at stacks.iop.org/pmb/57/1433/mmedia. These values can be applied to optimization studies of image quality and stochastic risk for this most vulnerable class of pediatric patients.