Computational Anthropomorphic Phantoms for Radiation Protection Dosimetry: Evolution and Prospects

Phantoms for radiation dosimetry, including physical phantoms for radiological use in medicine in particular, were introduced as early as the 1910s [1] while those for radiation protection dosimetry started to evolve in the late 1950s. This innovation was driven partly from the consensus that dose quantity for protection purposes should be assessed in “receptor conditions” instead of the traditional free-air or receptor-free conditions and partly from the availability of digital computers. Early approaches focused on the maximum dose in a body having size and composition compatible to those of the human body. The main factors considered to this end were that doses to a certain volume of interest depend on the relative position of the volume in the body as well as the penetrating power of radiation and that the specific dose distribution in the body cannot be considered in detail in radiation protection practices. Notably, there was an underlying assumption that risk to irradiated tissues or organs in the human body would not be underestimated by taking the maximum dose as the representative value. In determination of the location and size of the maximum dose in the body, computational approaches, particularly the Monte Carlo technique, together with a computational or mathematical phantom play an invaluable role. The first version of a computational phantom was a 30 cm thick slab [2], which was followed by a right circular cylinder 30 cm in diameter and 60 cm in height [3]. In 1960, Hayes and Brucer [4] constructed compartimentalized phantoms resembling the human body. These preceded the famous Fisher and Snyder model [5] developed by some extension and refinement after the request of the Medical Internal Radiation Dose (MIRD) committee of the Society Computational anthropomorphic phantoms are computer models of human anatomy used in the calculation of radiation dose distribution in the human body upon exposure to a radiation source. Depending on the manner to represent human anatomy, they are categorized into two classes: stylized and tomographic phantoms. Stylized phantoms, which have mainly been developed at the Oak Ridge National Laboratory (ORNL), describe human anatomy by using simple mathematical equations of analytical geometry. Several improved stylized phantoms such as male and female adults, pediatric series, and enhanced organ models have been developed following the first hermaphrodite adult stylized phantom, Medical Internal Radiation Dose (MIRD)-5 phantom. Although stylized phantoms have significantly contributed to dosimetry calculation, they provide only approximations of the true anatomical features of the human body and the resulting organ dose distribution. An alternative class of computational phantom, the tomographic phantom, is based upon three-dimensional imaging techniques such as magnetic resonance (MR) imaging and computed tomography (CT). The tomographic phantoms represent the human anatomy with a large number of voxels that are assigned tissue type and organ identity. To date, a total of around 30 tomographic phantoms including male and female adults, pediatric phantoms, and even a pregnant female, have been developed and utilized for realistic radiation dosimetry calculation. They are based on MRI/CT images or sectional color photos from patients, volunteers or cadavers. Several investigators have compared tomographic phantoms with stylized phantoms, and demonstrated the superiority of tomographic phantoms in terms of realistic anatomy and dosimetry calculation. This paper summarizes the history and current status of both stylized and tomographic phantoms, including Korean computational phantoms. Advantages, limitations, and future prospects are also discussed.