Answer to Question #5429 Submitted to "Ask the Experts"

What is the official definition of "whole body" when dealing with "whole-body contamination monitors"?

This is a question that may be a bit more complicated than it appears at first look. To my knowledge there is no "official" definition of what constitutes the whole body in regard to whole-body radiation monitoring, but there are some considerations that apply that help to define requirements. Much of what I discuss below may already be quite familiar to you, and I apologize if that is the case.

To begin with, we should distinguish between two types of monitoring systems. The first is usually referred to as a whole-body counting system and is designed primarily to detect penetrating radiation emitted by radionuclides that have been taken into the body, usually by inhalation or ingestion, often with the intent of quantifying the radionuclides responsible for the radiation. The second is a contamination monitor, which is primarily intended to detect radionuclide contamination on the external surface of the body or clothing.
A whole-body counter for internal contamination detects gamma rays, or sometimes x rays, emitted from within the body. These systems use spectrometry-quality detectors, such as sodium iodide or sometimes germanium, in conjunction with multichannel analyzers. The systems usually incorporate software to quantify the amount(s) of specific radionuclides detected under the assumptions of certain predetermined counting efficiencies. When significant radioactivity is measured, the subsequent actions involve possible follow-up monitoring and attempts to quantify the magnitude of the intake that occurred so that the value can be compared with action guidelines and legal intake limits. Within the context of this need to determine the magnitude of the radionuclide intake, an appropriate specification of the whole body can be defined.

In order to make the intake estimate, we must use the measured quantities in appropriate physiological and metabolic models that describe how the activity distributes among the various tissues and organs of the body. Such models include tissues of the respiratory tract and of the gastrointestinal (G.I.) tract, as well as all other internal organs of the body.

Thus, the whole body that is monitored must include a field of view that encompasses all of these. In general, this would require that the portion of the body monitored should extend at least from the neck below the chin to around the mid- to upper parts of the thighs. There is some flexibility as to whether one includes the nasal/pharyngeal (NP) upper region of the respiratory tract in the field of view for whole-body counting. If it is included the user must recognize that if a scan is done shortly following an inhalation exposure, considerable activity could be resident in the NP region; this material is cleared rapidly, however, and the NP burden changes quickly, and as it clears some of the activity is fed to the G.I. tract and redistributed.

This may make proper quantification difficult at early times following exposure. The hands and forearms and feet and lower legs are often excluded from direct view; this can be an advantage when external contamination, often most notable on the hands and feet, might be present.

Note also that, at least at present, the brain does not appear as an organ of interest in any of the models being used and is, therefore, not a necessary part of the "whole body." In some instances, however, when bone-seeking radionuclides are a concern there may be a benefit in including the skull. Often, however, measurements of the skull will be made separately, using an appropriately positioned and collimated detector, after having calibrated the system with a skull phantom.

There are numerous designs of whole-body counting systems, and they do not all view exactly the same portions of the "whole body." Some chair systems have single detectors mounted so as to be fairly constant in distance from the major portions of the body and view a region from the upper neck to about the midthigh.

Others use one or more shielded detectors that can be placed very close to portions of the body, such as the lungs or the G.I. tract to attempt to quantify amounts in these areas.

Some systems use lie-down beds with one or more detectors that scan along the entire length of the body or with stationary detectors and a movable bed. Some use large, stationary, planar arrays of detectors above and below the bed to provide uniform and efficient detection. One type of popular whole-body counting system uses large stationary sodium iodide detectors in a vertical array so that high-efficiency counting can be done on individuals who stand upright in a partially shielded enclosure. In one of these systems the viewing area extends from about eye level to about thigh level.

In some medical applications where the total body potassium is estimated by measuring ⁴⁰K in the body, some practitioners prefer to use a counting system that views the entire body.

The second types of whole-body contamination monitors intended for external surface contamination detection are most often of the stand-in or walk-through types that use large area detectors to provide high sensitivity.

Unlike whole-body counters for internal radioactivity assessment, these systems attempt to look at pretty much all of the body surfaces, often with accommodations specifically for the hands and feet, body parts that are most prone to contamination in many work environments.

The detectors used in many of these systems may be scintillation types or gas-filled detectors and may be sensitive to beta radiation as well as gamma radiation. Their major purpose is to sense the presence of surface contamination. Some semiquantitative activity estimates may be made under the assumptions of certain radionuclide compositions but this is not as important as for the internal counters.

For these systems the "whole body" emphasizes the external surfaces of the body/clothing, with particular inclusion of the extremities.

In the past, all whole-body counting systems for internally deposited radionuclides were calibrated with physical phantoms intended to simulate all or major portions of the body. In a very real sense, the phantom characteristics and the distribution of activity within it define the "whole body" or body part in so far as the system that is calibrated with that phantom. Phantoms have varied greatly, ranging from a simple "jug" type uniform distribution phantom to highly developed anthropomorphic phantoms that include specific body compartments to allow placement of radioactivity as desired.

Phantoms have advanced in sophistication and accuracy and are still commonly used for calibration, but there is a growing ability and desirability to use mathematical phantoms in conjunction with Monte Carlo simulations to determine expected efficiencies for specified activity distributions.

Such techniques add more flexibility in internal dosimetry through "whole body" counting and conceivably provide a relatively simple means whereby detection efficiencies for various body sections or organs can be estimated and used in conjunction with whole-body counting systems that allow placement of detectors to view restricted portions of the body.

In summary, the definition of what constitutes the whole body will vary somewhat depending on the type of system being used and its intended application.

For internal contamination monitoring, the "whole body" must include (with regard to detector view), as a minimum, the respiratory tract, G.I. tract, and internal organs. For external contamination monitoring, the entire surface of the body, extending to the extremities, including fronts and backs of hands and soles of the feet, properly constitutes the "whole body."


George Chabot, PhD, CHP