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Single-Dose Exposure
Radiation Delivery

Perhaps the simplest of all exposure scenarios is a single dose of radiation delivered at constant dose rate. Figure 1 to the right shows three examples of a single-dose exposure scenario. The delivered dose, the dose rate, and the irradiation time are related by the formula

 {dose rate} = {dose}/{irradiation time}

or equivalently

{irradiation time}={dose}/{dose rate}

Figure 1. Three single-dose radiation exposures
(click on figure to enlarge)Single dose of radiation delivered at constant dose rate

Definition: In the International System of Units (S.I.), the amount of energy deposited in matter is specified in units called the gray (Gy). One Gy equals 1 Joule (J) of energy deposited in 1 kilogram (kg) of matter.  See the NIST guide to Constants, Units, and Uncertainty for other dosimetric units and conversion factors.

In radiobiology, dose rates are conveniently specified in Gy/h or Gy/minute. In radiation therapy, Gy/minute is often preferred (1 Gy/minute = 60 Gy/h). Dose rates in the range from about 1 Gy/h to 100 or 1,000 Gy/h are used in many radiobiological experiments. Much higher dose rates (kGy/h or MGy/h) are used in some applications, e.g., food irradiation or sterilization of medical equipment.
 

Why study dose and dose rate effects? The biological response to radiation often tends to increase with increasing dose rate. That is, rapid radiation delivery usually produces a stronger biological response than an equal amount of radiation delivered over a protracted period of time. Knowledge of the way cells respond to time varying radiation fields is needed to improve the design of radiation therapy for the treatment of cancer (see biological optimization of radiation therapy). Varying the temporal pattern of radiation delivery is also a useful way to probe the kinetics of the molecular and cellular processes underlying mutagenesis, cell killing, neoplastic transformation, and other related endpoints.

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Last updated: 10 June, 2011
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