Introduction - Anthropomorphic Phantoms

Mathematical descriptions of human bodies-anthropomorphic phantoms-are widely used in computer calculations of doses delivered to the entire body and to specific organs, and are valuable tools in the design and assessment of shielding against occupational exposure. Human phantoms are also useful in determining doses to other parts of the body when a diagnostic or therapeutic dose is delivered to a specific site. They are useful in radiation transport calculations that synthesize a nuclear medicine gamma camera image in order to infer site-specific doses due to an internal radioactive agent.

The image to the right is a rear view of an adult female phantom. All internal organs and the breasts are shown. The skeleton is rendered as semi-transparent.

The mathematical specifications for phantoms that are available assume a specific age, height, and weight. People, however, exhibit a variety of shapes and sizes. Sometimes, women get pregnant. Calculation of doses to an ensemble of body types, rather than to a single average person, will result in a more realistic assessment of shielding adequacy. Modeling of imaging and other medical applications is best done with phantom models that match the gross parameters of an individual patient. (Some calculations make use of more accurate representations of individuals based on volumetric scans, such as CAT, MIR, and PET. Phantom representations are useful when scan data does not exist, or when the increased fidelity of the model does not justify the additional computing time required when many thousands, or perhaps millions, of voxel cells are present.)

Specific organs need to be considered at times, such as when the dose to a fetus is required. The inclusion of soft organs not of interest is undesirable because the unnecessary complexity increases the Monte Carlo simulation time.

Even when the size of an extant phantom is adequate, the enabling or omission of organs can be tedious and prone to error. In a combinatorial geometry description, for example, one must consider both the organ itself and the surrounding tissue and ensure all volumes of space are defined and belong to one and only one cell. Simply treating the entire body as a single cell and excluding all defined organs yields an extremely complex cell description that will result in greatly increased Monte Carlo execution time (if the code accepts it at all).

The selection of an organ as a source of photons or other radiation or as a tally region, for which a dose is to be calculated, also requires knowledge of the input syntax for the target transport code.

With BodyBuilder, a user can easily build a phantom with a desired size and organ selections without writing a complicated input file for the MCNP radiation transport code.

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Last modified: September, 2004

Kenneth A. Van Riper / email