ABSTRACT
The International Commission on Radiological Protection (ICRP) has created a task group on dose calculations, which, among other objectives, should replace the currently used mathematical MIRD phantoms by voxel phantoms. Voxel phantoms are based on digital images recorded from scanning of real persons by computed tomography or magnetic resonance imaging (MRI). Compared to the mathematical MIRD phantoms, voxel phantoms are true to the natural representations of a human body. Connected to a radiation transport code, voxel phantoms serve as virtual humans for which equivalent dose to organs and tissues from exposure to ionizing radiation can be calculated. The principal database for the construction of the FAX (Female Adult voXel) phantom consisted of 151 CT images recorded from scanning of trunk and head of a female patient, whose body weight and height were close to the corresponding data recommended by the ICRP in Publication 89. All 22 organs and tissues at risk, except for the red bone marrow and the osteogenic cells on the endosteal surface of bone ('bone surface'), have been segmented manually with a technique recently developed at the Departamento de Energia Nuclear of the UFPE in Recife, Brazil. After segmentation the volumes of the organs and tissues have been adjusted to agree with the organ and tissue masses recommended by ICRP for the Reference Adult Female in Publication 89. Comparisons have been made with the organ and tissue masses of the mathematical EVA phantom, as well as with the corresponding data for other female voxel phantoms. The three-dimensional matrix of the segmented images has eventually been connected to the EGS4 Monte Carlo code. Effective dose conversion coefficients have been calculated for exposures to photons, and compared to data determined for the mathematical MIRD-type phantoms, as well as for other voxel phantoms.
Subject(s)
Monte Carlo Method , Phantoms, Imaging , Radiation Dosage , Radiometry , Whole-Body Irradiation/standards , Adult , Bone Marrow/radiation effects , Bone and Bones/radiation effects , Female , Humans , Models, Anatomic , Radiation Protection , Radiotherapy Planning, Computer-Assisted , Relative Biological Effectiveness , Tissue DistributionABSTRACT
Recent progress in computer speed and medical imaging has made possible the development of a new family of anthropomorphic models, based on a volume elements (voxels) approach to phantom design. Such phantoms can represent details of the anatomical structures of the human body more realistically. Tomographic images (CT or MRI) contain the basic information for the construction of voxel-based phantoms. Use of voxel-based phantoms has its most significant application in the planning of individual patients therapy. To be implemented, results must be obtained in a reasonably short period of time. The segmentation of organs and tissues is a critical step in this process. This article presents a new approach in the construction of voxel-based phantoms that was implemented to simplify the segmentation process of organs and tissues, reducing the time used in this procedure. A voxel-based head and neck phantom, called MCvoxEL, was built using this new approach. The volumes and masses of the segmented organs and tissues were compared with data published by other investigators.