No abstract

As outlined in NCRP Report No. 160, the average value of the effective dose to exposed individual in the United States has increased by a factor of 1.7 over the time period 1982 to 2006, with the contribution of medical exposures correspondingly increasing by a factor of 5.7. at present, medical contributors to the effective dose include computed tomography (50% of total medical exposure), nuclear medicine (25%), interventional fluoroscopy (15%), and conventional radiography and diagnostic fluoroscopy (10%). This increased awareness of medical exposures has led to a graduate shift in the focus of radiation epidemiological studies from traditional occupational and environmental exposures to those focusing on cohorts of medical patients exposed to both diagnostic and therapeutic sources. The assignment of organ doses to patients in either a retrospective or prospective study has increasingly relied on the use of computational anatomical phantoms. In this paper, we review the various methods and approaches used to construction patient phantom models to include anthropometric databases, cadaver imaging, prospective volunteer imaging studies, and retrospective image reviews. Phantom format typesstylized, voxel, and hybrid -as well as phantom morphometric categories -reference, patientdependent, and patient-specific -are next defined and discussed. Specific emphasis is given to hybrid phantoms -those defined through the use of combinations of polygon mesh and NURBS surfaces. The concept of a patient-dependent phantom is reviewed in which phantoms of non-50 th percentile heights and weights are designed from population-based morphometric databases and provided as a larger library of phantoms for patient matching and lookup of refined values of organ dose coefficients and/or radionuclide S values. We close with two brief examples of the use of hybrid phantoms in medical dose reconstruction -diagnostic nuclear medicine for pediatric subjects and interventional fluoroscopy for adult patients.

Objective 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 patient model series. Methods Values of photon specific absorbed fraction (SAF) were assembled for the both the reference male and female newborn using the radiation transport code MCNPX v2.6. Values of electron specific absorbed fraction 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. Results Values of photon and electron specific absorbed fractions 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. Conclusion 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 specific absorbed fractions, and tables of radionuclide S values for the newborn child – both male and female – are given in a series of four electronic annexes. These values can be applied to optimization studies of image quality and stochastic risk for this most vulnerable class of pediatric patients.

Estimates of regional blood volumes (BVs) in humans are needed in dosimetric models of radionuclides and radiopharmaceuticals that decay in the circulation to a significant extent. These values are also needed to refine models of tissue elemental composition in computational human phantoms of both patients and exposed members of the general public. The International Commission on Radiological Protection (ICRP) in its Publication 89 provides reference values for total blood content in the full series of their reference individuals, to include the male and female newborn, 1 year-old, 5 year-old, 10 year-old, 15 year-old, and adult. Furthermore, Publication 89 provides reference values for the percentage distribution of total blood volume in 27 different blood-filled organs and tissues of the reference adult male and adult female. However, no similar distribution values are provided for non-adults. The goal of the present study is to present a volumetric scaling methodology to derive these values for the same organs and tissues at ages younger than the reference adult. Literature data on organ-specific vascular growth in the brain, kidneys, and skeletal tissues are also considered.