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| Key concepts, terminology and useful conversion factors How to setup radiation exposure scenarios |
What is a radiation exposure scenario? A radiation exposure scenario is a collection of one or more sequential time intervals in which the instantaneous absorbed dose rate is a constant (average) value greater than or equal to zero. For some additional discussion and illustrative plots, see the Radiation Delivery Examples section of the manual. Relationship between dose, dose rate and irradiation time Dose, dose rate and irradiation time are related by the formula {absorbed dose} = {absorbed dose rate}/{irradiation time} AD = ADR/DT Equivalent formulas are: {absorbed dose rate} = {absorbed dose}/{irradiation time} and {irradiation time} = {absorbed dose}/{absorbed dose rate} Units 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. For additional information on the S.I. system of units, including dosimetric units and conversion factors, see the NIST guide to Constants, Units, and Uncertainty.In the VC system of computer codes (including XIO and RIO), all absorbed doses are specified in units of Gy and dose rates are in Gy/h (Gy per hour). The duration of the exposure and other time-related quantities are specified in units of hour, except where indicated otherwise within the VC manual (e.g., see TTO parameter used with the DFDM keyword). Useful Conversion Factors:
Duration of the Exposure Scenario (DOX) For a single dose of radiation delivered at constant dose rate (e.g., see Tutorial 1), the irradiation time (DT) and the duration of the radiation exposure scenario (DOX) are identical. For all other exposure scenarios, DOX is the time when all radiation delivery ends minus the time when radiation delivery begins. The duration of the exposure scenario is always greater than or equal to the irradiation time (i.e., DOX >= DT). Example: In a split-dose irradiation scheme (see Tutorial 2), a 2 Gy dose is followed two hours later by a second 2 Gy dose. Each 2 Gy dose is delivered at 2 Gy/hour. The time to delivery each 2 Gy dose is 2 Gy/(2 Gy/hour) = 1 hour. The duration of the exposure (DOX) equals (1 hour) + (2 hours) + (1 hour) = 4 hours. |
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